// Copyright 2009 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.

// Package testing provides support for automated testing of Go packages.
// It is intended to be used in concert with the "go test" command, which automates
// execution of any function of the form
//
//	func TestXxx(*testing.T)
//
// where Xxx does not start with a lowercase letter. The function name
// serves to identify the test routine.
//
// Within these functions, use [T.Error], [T.Fail] or related methods to signal failure.
//
// To write a new test suite, create a file that
// contains the TestXxx functions as described here,
// and give that file a name ending in "_test.go".
// The file will be excluded from regular
// package builds but will be included when the "go test" command is run.
//
// The test file can be in the same package as the one being tested,
// or in a corresponding package with the suffix "_test".
//
// If the test file is in the same package, it may refer to unexported
// identifiers within the package, as in this example:
//
//	package abs
//
//	import "testing"
//
//	func TestAbs(t *testing.T) {
//	    got := Abs(-1)
//	    if got != 1 {
//	        t.Errorf("Abs(-1) = %d; want 1", got)
//	    }
//	}
//
// If the file is in a separate "_test" package, the package being tested
// must be imported explicitly and only its exported identifiers may be used.
// This is known as "black box" testing.
//
//	package abs_test
//
//	import (
//		"testing"
//
//		"path_to_pkg/abs"
//	)
//
//	func TestAbs(t *testing.T) {
//	    got := abs.Abs(-1)
//	    if got != 1 {
//	        t.Errorf("Abs(-1) = %d; want 1", got)
//	    }
//	}
//
// For more detail, run [go help test] and [go help testflag].
//
// # Benchmarks
//
// Functions of the form
//
//	func BenchmarkXxx(*testing.B)
//
// are considered benchmarks, and are executed by the "go test" command when
// its -bench flag is provided. Benchmarks are run sequentially.
//
// For a description of the testing flags, see [go help testflag].
//
// A sample benchmark function looks like this:
//
//	func BenchmarkRandInt(b *testing.B) {
//	    for b.Loop() {
//	        rand.Int()
//	    }
//	}
//
// The output
//
//	BenchmarkRandInt-8   	68453040	        17.8 ns/op
//
// means that the body of the loop ran 68453040 times at a speed of 17.8 ns per loop.
//
// Only the body of the loop is timed, so benchmarks may do expensive
// setup before calling b.Loop, which will not be counted toward the
// benchmark measurement:
//
//	func BenchmarkBigLen(b *testing.B) {
//	    big := NewBig()
//	    for b.Loop() {
//	        big.Len()
//	    }
//	}
//
// If a benchmark needs to test performance in a parallel setting, it may use
// the RunParallel helper function; such benchmarks are intended to be used with
// the go test -cpu flag:
//
//	func BenchmarkTemplateParallel(b *testing.B) {
//	    templ := template.Must(template.New("test").Parse("Hello, {{.}}!"))
//	    b.RunParallel(func(pb *testing.PB) {
//	        var buf bytes.Buffer
//	        for pb.Next() {
//	            buf.Reset()
//	            templ.Execute(&buf, "World")
//	        }
//	    })
//	}
//
// A detailed specification of the benchmark results format is given
// in https://go.dev/design/14313-benchmark-format.
//
// There are standard tools for working with benchmark results at
// [golang.org/x/perf/cmd].
// In particular, [golang.org/x/perf/cmd/benchstat] performs
// statistically robust A/B comparisons.
//
// # b.N-style benchmarks
//
// Prior to the introduction of [B.Loop], benchmarks were written in a
// different style using B.N. For example:
//
//	func BenchmarkRandInt(b *testing.B) {
//	    for range b.N {
//	        rand.Int()
//	    }
//	}
//
// In this style of benchmark, the benchmark function must run
// the target code b.N times. The benchmark function is called
// multiple times with b.N adjusted until the benchmark function
// lasts long enough to be timed reliably. This also means any setup
// done before the loop may be run several times.
//
// If a benchmark needs some expensive setup before running, the timer
// should be explicitly reset:
//
//	func BenchmarkBigLen(b *testing.B) {
//	    big := NewBig()
//	    b.ResetTimer()
//	    for range b.N {
//	        big.Len()
//	    }
//	}
//
// New benchmarks should prefer using [B.Loop], which is more robust
// and more efficient.
//
// # Examples
//
// The package also runs and verifies example code. Example functions may
// include a concluding line comment that begins with "Output:" and is compared with
// the standard output of the function when the tests are run. (The comparison
// ignores leading and trailing space.) These are examples of an example:
//
//	func ExampleHello() {
//	    fmt.Println("hello")
//	    // Output: hello
//	}
//
//	func ExampleSalutations() {
//	    fmt.Println("hello, and")
//	    fmt.Println("goodbye")
//	    // Output:
//	    // hello, and
//	    // goodbye
//	}
//
// The comment prefix "Unordered output:" is like "Output:", but matches any
// line order:
//
//	func ExamplePerm() {
//	    for _, value := range Perm(5) {
//	        fmt.Println(value)
//	    }
//	    // Unordered output: 4
//	    // 2
//	    // 1
//	    // 3
//	    // 0
//	}
//
// Example functions without output comments are compiled but not executed.
//
// The naming convention to declare examples for the package, a function F, a type T and
// method M on type T are:
//
//	func Example() { ... }
//	func ExampleF() { ... }
//	func ExampleT() { ... }
//	func ExampleT_M() { ... }
//
// Multiple example functions for a package/type/function/method may be provided by
// appending a distinct suffix to the name. The suffix must start with a
// lower-case letter.
//
//	func Example_suffix() { ... }
//	func ExampleF_suffix() { ... }
//	func ExampleT_suffix() { ... }
//	func ExampleT_M_suffix() { ... }
//
// The entire test file is presented as the example when it contains a single
// example function, at least one other function, type, variable, or constant
// declaration, and no test or benchmark functions.
//
// # Fuzzing
//
// 'go test' and the testing package support fuzzing, a testing technique where
// a function is called with randomly generated inputs to find bugs not
// anticipated by unit tests.
//
// Functions of the form
//
//	func FuzzXxx(*testing.F)
//
// are considered fuzz tests.
//
// For example:
//
//	func FuzzHex(f *testing.F) {
//	  for _, seed := range [][]byte{{}, {0}, {9}, {0xa}, {0xf}, {1, 2, 3, 4}} {
//	    f.Add(seed)
//	  }
//	  f.Fuzz(func(t *testing.T, in []byte) {
//	    enc := hex.EncodeToString(in)
//	    out, err := hex.DecodeString(enc)
//	    if err != nil {
//	      t.Fatalf("%v: decode: %v", in, err)
//	    }
//	    if !bytes.Equal(in, out) {
//	      t.Fatalf("%v: not equal after round trip: %v", in, out)
//	    }
//	  })
//	}
//
// A fuzz test maintains a seed corpus, or a set of inputs which are run by
// default, and can seed input generation. Seed inputs may be registered by
// calling [F.Add] or by storing files in the directory testdata/fuzz/<Name>
// (where <Name> is the name of the fuzz test) within the package containing
// the fuzz test. Seed inputs are optional, but the fuzzing engine may find
// bugs more efficiently when provided with a set of small seed inputs with good
// code coverage. These seed inputs can also serve as regression tests for bugs
// identified through fuzzing.
//
// The function passed to [F.Fuzz] within the fuzz test is considered the fuzz
// target. A fuzz target must accept a [*T] parameter, followed by one or more
// parameters for random inputs. The types of arguments passed to [F.Add] must
// be identical to the types of these parameters. The fuzz target may signal
// that it's found a problem the same way tests do: by calling [T.Fail] (or any
// method that calls it like [T.Error] or [T.Fatal]) or by panicking.
//
// When fuzzing is enabled (by setting the -fuzz flag to a regular expression
// that matches a specific fuzz test), the fuzz target is called with arguments
// generated by repeatedly making random changes to the seed inputs. On
// supported platforms, 'go test' compiles the test executable with fuzzing
// coverage instrumentation. The fuzzing engine uses that instrumentation to
// find and cache inputs that expand coverage, increasing the likelihood of
// finding bugs. If the fuzz target fails for a given input, the fuzzing engine
// writes the inputs that caused the failure to a file in the directory
// testdata/fuzz/<Name> within the package directory. This file later serves as
// a seed input. If the file can't be written at that location (for example,
// because the directory is read-only), the fuzzing engine writes the file to
// the fuzz cache directory within the build cache instead.
//
// When fuzzing is disabled, the fuzz target is called with the seed inputs
// registered with [F.Add] and seed inputs from testdata/fuzz/<Name>. In this
// mode, the fuzz test acts much like a regular test, with subtests started
// with [F.Fuzz] instead of [T.Run].
//
// See https://go.dev/doc/fuzz for documentation about fuzzing.
//
// # Skipping
//
// Tests or benchmarks may be skipped at run time with a call to
// [T.Skip] or [B.Skip]:
//
//	func TestTimeConsuming(t *testing.T) {
//	    if testing.Short() {
//	        t.Skip("skipping test in short mode.")
//	    }
//	    ...
//	}
//
// The [T.Skip] method can be used in a fuzz target if the input is invalid,
// but should not be considered a failing input. For example:
//
//	func FuzzJSONMarshaling(f *testing.F) {
//	    f.Fuzz(func(t *testing.T, b []byte) {
//	        var v interface{}
//	        if err := json.Unmarshal(b, &v); err != nil {
//	            t.Skip()
//	        }
//	        if _, err := json.Marshal(v); err != nil {
//	            t.Errorf("Marshal: %v", err)
//	        }
//	    })
//	}
//
// # Subtests and Sub-benchmarks
//
// The [T.Run] and [B.Run] methods allow defining subtests and sub-benchmarks,
// without having to define separate functions for each. This enables uses
// like table-driven benchmarks and creating hierarchical tests.
// It also provides a way to share common setup and tear-down code:
//
//	func TestFoo(t *testing.T) {
//	    // <setup code>
//	    t.Run("A=1", func(t *testing.T) { ... })
//	    t.Run("A=2", func(t *testing.T) { ... })
//	    t.Run("B=1", func(t *testing.T) { ... })
//	    // <tear-down code>
//	}
//
// Each subtest and sub-benchmark has a unique name: the combination of the name
// of the top-level test and the sequence of names passed to Run, separated by
// slashes, with an optional trailing sequence number for disambiguation.
//
// The argument to the -run, -bench, and -fuzz command-line flags is an unanchored regular
// expression that matches the test's name. For tests with multiple slash-separated
// elements, such as subtests, the argument is itself slash-separated, with
// expressions matching each name element in turn. Because it is unanchored, an
// empty expression matches any string.
// For example, using "matching" to mean "whose name contains":
//
//	go test -run ''        # Run all tests.
//	go test -run Foo       # Run top-level tests matching "Foo", such as "TestFooBar".
//	go test -run Foo/A=    # For top-level tests matching "Foo", run subtests matching "A=".
//	go test -run /A=1      # For all top-level tests, run subtests matching "A=1".
//	go test -fuzz FuzzFoo  # Fuzz the target matching "FuzzFoo"
//
// The -run argument can also be used to run a specific value in the seed
// corpus, for debugging. For example:
//
//	go test -run=FuzzFoo/9ddb952d9814
//
// The -fuzz and -run flags can both be set, in order to fuzz a target but
// skip the execution of all other tests.
//
// Subtests can also be used to control parallelism. A parent test will only
// complete once all of its subtests complete. In this example, all tests are
// run in parallel with each other, and only with each other, regardless of
// other top-level tests that may be defined:
//
//	func TestGroupedParallel(t *testing.T) {
//	    for _, tc := range tests {
//	        t.Run(tc.Name, func(t *testing.T) {
//	            t.Parallel()
//	            ...
//	        })
//	    }
//	}
//
// Run does not return until parallel subtests have completed, providing a way
// to clean up after a group of parallel tests:
//
//	func TestTeardownParallel(t *testing.T) {
//	    // This Run will not return until the parallel tests finish.
//	    t.Run("group", func(t *testing.T) {
//	        t.Run("Test1", parallelTest1)
//	        t.Run("Test2", parallelTest2)
//	        t.Run("Test3", parallelTest3)
//	    })
//	    // <tear-down code>
//	}
//
// # Main
//
// It is sometimes necessary for a test or benchmark program to do extra setup or teardown
// before or after it executes. It is also sometimes necessary to control
// which code runs on the main thread. To support these and other cases,
// if a test file contains a function:
//
//	func TestMain(m *testing.M)
//
// then the generated test will call TestMain(m) instead of running the tests or benchmarks
// directly. TestMain runs in the main goroutine and can do whatever setup
// and teardown is necessary around a call to m.Run. m.Run will return an exit
// code that may be passed to [os.Exit]. If TestMain returns, the test wrapper
// will pass the result of m.Run to [os.Exit] itself.
//
// When TestMain is called, flag.Parse has not been run. If TestMain depends on
// command-line flags, including those of the testing package, it should call
// [flag.Parse] explicitly. Command line flags are always parsed by the time test
// or benchmark functions run.
//
// A simple implementation of TestMain is:
//
//	func TestMain(m *testing.M) {
//		// call flag.Parse() here if TestMain uses flags
//		m.Run()
//	}
//
// TestMain is a low-level primitive and should not be necessary for casual
// testing needs, where ordinary test functions suffice.
//
// [go help test]: https://pkg.go.dev/cmd/go#hdr-Test_packages
// [go help testflag]: https://pkg.go.dev/cmd/go#hdr-Testing_flags
package testing

import (
	"bytes"
	"context"
	"errors"
	"flag"
	"fmt"
	"internal/race"
	"io"
	"math/rand"
	"os"
	"path/filepath"
	"reflect"
	"runtime"
	"runtime/debug"
	"runtime/trace"
	"slices"
	"strconv"
	"strings"
	"sync"
	"sync/atomic"
	"time"
	"unicode"
	"unicode/utf8"
	_ "unsafe" // for linkname
)

var initRan bool

var (
	parallelStart atomic.Int64 // number of parallel tests started
	parallelStop  atomic.Int64 // number of parallel tests stopped
)

// Init registers testing flags. These flags are automatically registered by
// the "go test" command before running test functions, so Init is only needed
// when calling functions such as Benchmark without using "go test".
//
// Init is not safe to call concurrently. It has no effect if it was already called.
func Init() {
	if initRan {
		return
	}
	initRan = true
	// The short flag requests that tests run more quickly, but its functionality
	// is provided by test writers themselves. The testing package is just its
	// home. The all.bash installation script sets it to make installation more
	// efficient, but by default the flag is off so a plain "go test" will do a
	// full test of the package.
	short = flag.Bool("test.short", false, "run smaller test suite to save time")

	// The failfast flag requests that test execution stop after the first test failure.
	failFast = flag.Bool("test.failfast", false, "do not start new tests after the first test failure")

	// The directory in which to create profile files and the like. When run from
	// "go test", the binary always runs in the source directory for the package;
	// this flag lets "go test" tell the binary to write the files in the directory where
	// the "go test" command is run.
	outputDir = flag.String("test.outputdir", "", "write profiles to `dir`")
	// Report as tests are run; default is silent for success.
	flag.Var(&chatty, "test.v", "verbose: print additional output")
	count = flag.Uint("test.count", 1, "run tests and benchmarks `n` times")
	coverProfile = flag.String("test.coverprofile", "", "write a coverage profile to `file`")
	gocoverdir = flag.String("test.gocoverdir", "", "write coverage intermediate files to this directory")
	matchList = flag.String("test.list", "", "list tests, examples, and benchmarks matching `regexp` then exit")
	match = flag.String("test.run", "", "run only tests and examples matching `regexp`")
	skip = flag.String("test.skip", "", "do not list or run tests matching `regexp`")
	memProfile = flag.String("test.memprofile", "", "write an allocation profile to `file`")
	memProfileRate = flag.Int("test.memprofilerate", 0, "set memory allocation profiling `rate` (see runtime.MemProfileRate)")
	cpuProfile = flag.String("test.cpuprofile", "", "write a cpu profile to `file`")
	blockProfile = flag.String("test.blockprofile", "", "write a goroutine blocking profile to `file`")
	blockProfileRate = flag.Int("test.blockprofilerate", 1, "set blocking profile `rate` (see runtime.SetBlockProfileRate)")
	mutexProfile = flag.String("test.mutexprofile", "", "write a mutex contention profile to the named file after execution")
	mutexProfileFraction = flag.Int("test.mutexprofilefraction", 1, "if >= 0, calls runtime.SetMutexProfileFraction()")
	panicOnExit0 = flag.Bool("test.paniconexit0", false, "panic on call to os.Exit(0)")
	traceFile = flag.String("test.trace", "", "write an execution trace to `file`")
	timeout = flag.Duration("test.timeout", 0, "panic test binary after duration `d` (default 0, timeout disabled)")
	cpuListStr = flag.String("test.cpu", "", "comma-separated `list` of cpu counts to run each test with")
	parallel = flag.Int("test.parallel", runtime.GOMAXPROCS(0), "run at most `n` tests in parallel")
	testlog = flag.String("test.testlogfile", "", "write test action log to `file` (for use only by cmd/go)")
	shuffle = flag.String("test.shuffle", "off", "randomize the execution order of tests and benchmarks")
	fullPath = flag.Bool("test.fullpath", false, "show full file names in error messages")

	initBenchmarkFlags()
	initFuzzFlags()
}

var (
	// Flags, registered during Init.
	short                *bool
	failFast             *bool
	outputDir            *string
	chatty               chattyFlag
	count                *uint
	coverProfile         *string
	gocoverdir           *string
	matchList            *string
	match                *string
	skip                 *string
	memProfile           *string
	memProfileRate       *int
	cpuProfile           *string
	blockProfile         *string
	blockProfileRate     *int
	mutexProfile         *string
	mutexProfileFraction *int
	panicOnExit0         *bool
	traceFile            *string
	timeout              *time.Duration
	cpuListStr           *string
	parallel             *int
	shuffle              *string
	testlog              *string
	fullPath             *bool

	haveExamples bool // are there examples?

	cpuList     []int
	testlogFile *os.File

	numFailed atomic.Uint32 // number of test failures

	running sync.Map // map[string]time.Time of running, unpaused tests
)

type chattyFlag struct {
	on   bool // -v is set in some form
	json bool // -v=test2json is set, to make output better for test2json
}

func (*chattyFlag) IsBoolFlag() bool { return true }

func (f *chattyFlag) Set(arg string) error {
	switch arg {
	default:
		return fmt.Errorf("invalid flag -test.v=%s", arg)
	case "true", "test2json":
		f.on = true
		f.json = arg == "test2json"
	case "false":
		f.on = false
		f.json = false
	}
	return nil
}

func (f *chattyFlag) String() string {
	if f.json {
		return "test2json"
	}
	if f.on {
		return "true"
	}
	return "false"
}

func (f *chattyFlag) Get() any {
	if f.json {
		return "test2json"
	}
	return f.on
}

const marker = byte(0x16) // ^V for framing

func (f *chattyFlag) prefix() string {
	if f.json {
		return string(marker)
	}
	return ""
}

type chattyPrinter struct {
	w          io.Writer
	lastNameMu sync.Mutex // guards lastName
	lastName   string     // last printed test name in chatty mode
	json       bool       // -v=json output mode
}

func newChattyPrinter(w io.Writer) *chattyPrinter {
	return &chattyPrinter{w: w, json: chatty.json}
}

// prefix is like chatty.prefix but using p.json instead of chatty.json.
// Using p.json allows tests to check the json behavior without modifying
// the global variable. For convenience, we allow p == nil and treat
// that as not in json mode (because it's not chatty at all).
func (p *chattyPrinter) prefix() string {
	if p != nil && p.json {
		return string(marker)
	}
	return ""
}

// Updatef prints a message about the status of the named test to w.
//
// The formatted message must include the test name itself.
func (p *chattyPrinter) Updatef(testName, format string, args ...any) {
	p.lastNameMu.Lock()
	defer p.lastNameMu.Unlock()

	// Since the message already implies an association with a specific new test,
	// we don't need to check what the old test name was or log an extra NAME line
	// for it. (We're updating it anyway, and the current message already includes
	// the test name.)
	p.lastName = testName
	fmt.Fprintf(p.w, p.prefix()+format, args...)
}

// Printf prints a message, generated by the named test, that does not
// necessarily mention that tests's name itself.
func (p *chattyPrinter) Printf(testName, format string, args ...any) {
	p.lastNameMu.Lock()
	defer p.lastNameMu.Unlock()

	if p.lastName == "" {
		p.lastName = testName
	} else if p.lastName != testName {
		fmt.Fprintf(p.w, "%s=== NAME  %s\n", p.prefix(), testName)
		p.lastName = testName
	}

	fmt.Fprintf(p.w, format, args...)
}

// The maximum number of stack frames to go through when skipping helper functions for
// the purpose of decorating log messages.
const maxStackLen = 50

// common holds the elements common between T and B and
// captures common methods such as Errorf.
type common struct {
	mu          sync.RWMutex         // guards this group of fields
	output      []byte               // Output generated by test or benchmark.
	w           io.Writer            // For flushToParent.
	o           *outputWriter        // Writes output.
	ran         bool                 // Test or benchmark (or one of its subtests) was executed.
	failed      bool                 // Test or benchmark has failed.
	skipped     bool                 // Test or benchmark has been skipped.
	done        bool                 // Test is finished and all subtests have completed.
	helperPCs   map[uintptr]struct{} // functions to be skipped when writing file/line info
	helperNames map[string]struct{}  // helperPCs converted to function names
	cleanups    []func()             // optional functions to be called at the end of the test
	cleanupName string               // Name of the cleanup function.
	cleanupPc   []uintptr            // The stack trace at the point where Cleanup was called.
	finished    bool                 // Test function has completed.
	inFuzzFn    bool                 // Whether the fuzz target, if this is one, is running.
	isSynctest  bool

	chatty         *chattyPrinter // A copy of chattyPrinter, if the chatty flag is set.
	bench          bool           // Whether the current test is a benchmark.
	hasSub         atomic.Bool    // whether there are sub-benchmarks.
	cleanupStarted atomic.Bool    // Registered cleanup callbacks have started to execute
	runner         string         // Function name of tRunner running the test.
	isParallel     bool           // Whether the test is parallel.

	parent   *common
	level    int               // Nesting depth of test or benchmark.
	creator  []uintptr         // If level > 0, the stack trace at the point where the parent called t.Run.
	name     string            // Name of test or benchmark.
	start    highPrecisionTime // Time test or benchmark started
	duration time.Duration
	barrier  chan bool // To signal parallel subtests they may start. Nil when T.Parallel is not present (B) or not usable (when fuzzing).
	signal   chan bool // To signal a test is done.
	sub      []*T      // Queue of subtests to be run in parallel.

	lastRaceErrors  atomic.Int64 // Max value of race.Errors seen during the test or its subtests.
	raceErrorLogged atomic.Bool

	tempDirMu  sync.Mutex
	tempDir    string
	tempDirErr error
	tempDirSeq int32

	ctx       context.Context
	cancelCtx context.CancelFunc
}

// Short reports whether the -test.short flag is set.
func Short() bool {
	if short == nil {
		panic("testing: Short called before Init")
	}
	// Catch code that calls this from TestMain without first calling flag.Parse.
	if !flag.Parsed() {
		panic("testing: Short called before Parse")
	}

	return *short
}

// testBinary is set by cmd/go to "1" if this is a binary built by "go test".
// The value is set to "1" by a -X option to cmd/link. We assume that
// because this is possible, the compiler will not optimize testBinary
// into a constant on the basis that it is an unexported package-scope
// variable that is never changed. If the compiler ever starts implementing
// such an optimization, we will need some technique to mark this variable
// as "changed by a cmd/link -X option".
var testBinary = "0"

// Testing reports whether the current code is being run in a test.
// This will report true in programs created by "go test",
// false in programs created by "go build".
func Testing() bool {
	return testBinary == "1"
}

// CoverMode reports what the test coverage mode is set to. The
// values are "set", "count", or "atomic". The return value will be
// empty if test coverage is not enabled.
func CoverMode() string {
	return cover.mode
}

// Verbose reports whether the -test.v flag is set.
func Verbose() bool {
	// Same as in Short.
	if !flag.Parsed() {
		panic("testing: Verbose called before Parse")
	}
	return chatty.on
}

func (c *common) checkFuzzFn(name string) {
	if c.inFuzzFn {
		panic(fmt.Sprintf("testing: f.%s was called inside the fuzz target, use t.%s instead", name, name))
	}
}

// frameSkip searches, starting after skip frames, for the first caller frame
// in a function not marked as a helper and returns that frame.
// The search stops if it finds a tRunner function that
// was the entry point into the test and the test is not a subtest.
// This function must be called with c.mu held.
func (c *common) frameSkip(skip int) runtime.Frame {
	// If the search continues into the parent test, we'll have to hold
	// its mu temporarily. If we then return, we need to unlock it.
	shouldUnlock := false
	defer func() {
		if shouldUnlock {
			c.mu.Unlock()
		}
	}()
	var pc [maxStackLen]uintptr
	// Skip two extra frames to account for this function
	// and runtime.Callers itself.
	n := runtime.Callers(skip+2, pc[:])
	if n == 0 {
		panic("testing: zero callers found")
	}
	frames := runtime.CallersFrames(pc[:n])
	var firstFrame, prevFrame, frame runtime.Frame
	for more := true; more; prevFrame = frame {
		frame, more = frames.Next()
		if frame.Function == "runtime.gopanic" {
			continue
		}
		if frame.Function == c.cleanupName {
			frames = runtime.CallersFrames(c.cleanupPc)
			continue
		}
		if firstFrame.PC == 0 {
			firstFrame = frame
		}
		if frame.Function == c.runner {
			// We've gone up all the way to the tRunner calling
			// the test function (so the user must have
			// called tb.Helper from inside that test function).
			// If this is a top-level test, only skip up to the test function itself.
			// If we're in a subtest, continue searching in the parent test,
			// starting from the point of the call to Run which created this subtest.
			if c.level > 1 {
				frames = runtime.CallersFrames(c.creator)
				parent := c.parent
				// We're no longer looking at the current c after this point,
				// so we should unlock its mu, unless it's the original receiver,
				// in which case our caller doesn't expect us to do that.
				if shouldUnlock {
					c.mu.Unlock()
				}
				c = parent
				// Remember to unlock c.mu when we no longer need it, either
				// because we went up another nesting level, or because we
				// returned.
				shouldUnlock = true
				c.mu.Lock()
				continue
			}
			return prevFrame
		}
		// If more helper PCs have been added since we last did the conversion
		if c.helperNames == nil {
			c.helperNames = make(map[string]struct{})
			for pc := range c.helperPCs {
				c.helperNames[pcToName(pc)] = struct{}{}
			}
		}
		if _, ok := c.helperNames[frame.Function]; !ok {
			// Found a frame that wasn't inside a helper function.
			return frame
		}
	}
	return firstFrame
}

// flushToParent writes c.output to the parent after first writing the header
// with the given format and arguments.
func (c *common) flushToParent(testName, format string, args ...any) {
	p := c.parent
	p.mu.Lock()
	defer p.mu.Unlock()

	c.mu.Lock()
	defer c.mu.Unlock()

	if len(c.output) > 0 {
		// Add the current c.output to the print,
		// and then arrange for the print to replace c.output.
		// (This displays the logged output after the --- FAIL line.)
		format += "%s"
		args = append(args[:len(args):len(args)], c.output)
		c.output = c.output[:0]
	}

	if c.chatty != nil && (p.w == c.chatty.w || c.chatty.json) {
		// We're flushing to the actual output, so track that this output is
		// associated with a specific test (and, specifically, that the next output
		// is *not* associated with that test).
		//
		// Moreover, if c.output is non-empty it is important that this write be
		// atomic with respect to the output of other tests, so that we don't end up
		// with confusing '=== NAME' lines in the middle of our '--- PASS' block.
		// Neither humans nor cmd/test2json can parse those easily.
		// (See https://go.dev/issue/40771.)
		//
		// If test2json is used, we never flush to parent tests,
		// so that the json stream shows subtests as they finish.
		// (See https://go.dev/issue/29811.)
		c.chatty.Updatef(testName, format, args...)
	} else {
		// We're flushing to the output buffer of the parent test, which will
		// itself follow a test-name header when it is finally flushed to stdout.
		fmt.Fprintf(p.w, c.chatty.prefix()+format, args...)
	}
}

type indenter struct {
	c *common
}

const indent = "    "

func (w indenter) Write(b []byte) (n int, err error) {
	n = len(b)
	for len(b) > 0 {
		end := bytes.IndexByte(b, '\n')
		if end == -1 {
			end = len(b)
		} else {
			end++
		}
		// An indent of 4 spaces will neatly align the dashes with the status
		// indicator of the parent.
		line := b[:end]
		if line[0] == marker {
			w.c.output = append(w.c.output, marker)
			line = line[1:]
		}
		w.c.output = append(w.c.output, indent...)
		w.c.output = append(w.c.output, line...)
		b = b[end:]
	}
	return
}

// fmtDuration returns a string representing d in the form "87.00s".
func fmtDuration(d time.Duration) string {
	return fmt.Sprintf("%.2fs", d.Seconds())
}

// TB is the interface common to [T], [B], and [F].
type TB interface {
	Attr(key, value string)
	Cleanup(func())
	Error(args ...any)
	Errorf(format string, args ...any)
	Fail()
	FailNow()
	Failed() bool
	Fatal(args ...any)
	Fatalf(format string, args ...any)
	Helper()
	Log(args ...any)
	Logf(format string, args ...any)
	Name() string
	Setenv(key, value string)
	Chdir(dir string)
	Skip(args ...any)
	SkipNow()
	Skipf(format string, args ...any)
	Skipped() bool
	TempDir() string
	Context() context.Context
	Output() io.Writer

	// A private method to prevent users implementing the
	// interface and so future additions to it will not
	// violate Go 1 compatibility.
	private()
}

var (
	_ TB = (*T)(nil)
	_ TB = (*B)(nil)
)

// T is a type passed to Test functions to manage test state and support formatted test logs.
//
// A test ends when its Test function returns or calls any of the methods
// [T.FailNow], [T.Fatal], [T.Fatalf], [T.SkipNow], [T.Skip], or [T.Skipf]. Those methods, as well as
// the [T.Parallel] method, must be called only from the goroutine running the
// Test function.
//
// The other reporting methods, such as the variations of [T.Log] and [T.Error],
// may be called simultaneously from multiple goroutines.
type T struct {
	common
	denyParallel bool
	tstate       *testState // For running tests and subtests.
}

func (c *common) private() {}

// Name returns the name of the running (sub-) test or benchmark.
//
// The name will include the name of the test along with the names of
// any nested sub-tests. If two sibling sub-tests have the same name,
// Name will append a suffix to guarantee the returned name is unique.
func (c *common) Name() string {
	return c.name
}

func (c *common) setRan() {
	if c.parent != nil {
		c.parent.setRan()
	}
	c.mu.Lock()
	defer c.mu.Unlock()
	c.ran = true
}

// Fail marks the function as having failed but continues execution.
func (c *common) Fail() {
	if c.parent != nil {
		c.parent.Fail()
	}
	c.mu.Lock()
	defer c.mu.Unlock()
	// c.done needs to be locked to synchronize checks to c.done in parent tests.
	if c.done {
		panic("Fail in goroutine after " + c.name + " has completed")
	}
	c.failed = true
}

// Failed reports whether the function has failed.
func (c *common) Failed() bool {
	c.mu.RLock()
	defer c.mu.RUnlock()

	if !c.done && int64(race.Errors()) > c.lastRaceErrors.Load() {
		c.mu.RUnlock()
		c.checkRaces()
		c.mu.RLock()
	}

	return c.failed
}

// FailNow marks the function as having failed and stops its execution
// by calling [runtime.Goexit] (which then runs all deferred calls in the
// current goroutine).
// Execution will continue at the next test or benchmark.
// FailNow must be called from the goroutine running the
// test or benchmark function, not from other goroutines
// created during the test. Calling FailNow does not stop
// those other goroutines.
func (c *common) FailNow() {
	c.checkFuzzFn("FailNow")
	c.Fail()

	// Calling runtime.Goexit will exit the goroutine, which
	// will run the deferred functions in this goroutine,
	// which will eventually run the deferred lines in tRunner,
	// which will signal to the test loop that this test is done.
	//
	// A previous version of this code said:
	//
	//	c.duration = ...
	//	c.signal <- c.self
	//	runtime.Goexit()
	//
	// This previous version duplicated code (those lines are in
	// tRunner no matter what), but worse the goroutine teardown
	// implicit in runtime.Goexit was not guaranteed to complete
	// before the test exited. If a test deferred an important cleanup
	// function (like removing temporary files), there was no guarantee
	// it would run on a test failure. Because we send on c.signal during
	// a top-of-stack deferred function now, we know that the send
	// only happens after any other stacked defers have completed.
	c.mu.Lock()
	c.finished = true
	c.mu.Unlock()
	runtime.Goexit()
}

// log generates the output. It is always at the same stack depth. log inserts
// indentation and the final newline if necessary. It prefixes the string
// with the file and line of the call site.
func (c *common) log(s string) {
	s = strings.TrimSuffix(s, "\n")

	// Second and subsequent lines are indented 4 spaces. This is in addition to
	// the indentation provided by outputWriter.
	s = strings.ReplaceAll(s, "\n", "\n"+indent)
	s += "\n"

	n := c.destination()
	if n == nil {
		// The test and all its parents are done. The log cannot be output.
		panic("Log in goroutine after " + c.name + " has completed: " + s)
	}

	// Prefix with the call site. It is located by skipping 3 functions:
	// callSite + log + public function
	s = n.callSite(3) + s

	// Output buffered logs.
	n.flushPartial()

	n.o.Write([]byte(s))
}

// destination selects the test to which output should be appended. It returns the
// test if it is incomplete. Otherwise, it finds its closest incomplete parent.
func (c *common) destination() *common {
	c.mu.Lock()
	defer c.mu.Unlock()

	if !c.done && !c.isSynctest {
		return c
	}
	for parent := c.parent; parent != nil; parent = parent.parent {
		parent.mu.Lock()
		defer parent.mu.Unlock()
		if !parent.done {
			return parent
		}
	}
	return nil
}

// callSite retrieves and formats the file and line of the call site.
func (c *common) callSite(skip int) string {
	c.mu.Lock()
	defer c.mu.Unlock()

	frame := c.frameSkip(skip)
	file := frame.File
	line := frame.Line
	if file != "" {
		if *fullPath {
			// If relative path, truncate file name at last file name separator.
		} else {
			file = filepath.Base(file)
		}
	} else {
		file = "???"
	}
	if line == 0 {
		line = 1
	}

	return fmt.Sprintf("%s:%d: ", file, line)
}

// flushPartial checks the buffer for partial logs and outputs them.
func (c *common) flushPartial() {
	partial := func() bool {
		c.mu.Lock()
		defer c.mu.Unlock()
		return (c.o != nil) && (len(c.o.partial) > 0)
	}

	if partial() {
		c.o.Write([]byte("\n"))
	}
}

// Output returns a Writer that writes to the same test output stream as TB.Log.
// The output is indented like TB.Log lines, but Output does not
// add source locations or newlines. The output is internally line
// buffered, and a call to TB.Log or the end of the test will implicitly
// flush the buffer, followed by a newline. After a test function and all its
// parents return, neither Output nor the Write method may be called.
func (c *common) Output() io.Writer {
	c.checkFuzzFn("Output")
	n := c.destination()
	if n == nil {
		panic("Output called after " + c.name + " has completed")
	}
	return n.o
}

// setOutputWriter initializes an outputWriter and sets it as a common field.
func (c *common) setOutputWriter() {
	c.o = &outputWriter{c: c}
}

// outputWriter buffers, formats and writes log messages.
type outputWriter struct {
	c       *common
	partial []byte // incomplete ('\n'-free) suffix of last Write
}

// Write writes a log message to the test's output stream, properly formatted and
// indented. It may not be called after a test function and all its parents return.
func (o *outputWriter) Write(p []byte) (int, error) {
	// o can be nil if this is called from a top-level *TB that is no longer active.
	// Just ignore the message in that case.
	if o == nil || o.c == nil {
		return 0, nil
	}
	if o.c.destination() == nil {
		panic("Write called after " + o.c.name + " has completed")
	}

	o.c.mu.Lock()
	defer o.c.mu.Unlock()

	// The last element is a partial line.
	lines := bytes.SplitAfter(p, []byte("\n"))
	last := len(lines) - 1 // Inv: 0 <= last
	for i, line := range lines[:last] {
		// Emit partial line from previous call.
		if i == 0 && len(o.partial) > 0 {
			line = slices.Concat(o.partial, line)
			o.partial = o.partial[:0]
		}
		o.writeLine(line)
	}
	// Save partial line for next call.
	o.partial = append(o.partial, lines[last]...)

	return len(p), nil
}

// writeLine generates the output for a given line.
func (o *outputWriter) writeLine(b []byte) {
	if !o.c.done && (o.c.chatty != nil) {
		if o.c.bench {
			// Benchmarks don't print === CONT, so we should skip the test
			// printer and just print straight to stdout.
			fmt.Printf("%s%s", indent, b)
		} else {
			o.c.chatty.Printf(o.c.name, "%s%s", indent, b)
		}
		return
	}
	o.c.output = append(o.c.output, indent...)
	o.c.output = append(o.c.output, b...)
}

// Log formats its arguments using default formatting, analogous to [fmt.Println],
// and records the text in the error log. For tests, the text will be printed only if
// the test fails or the -test.v flag is set. For benchmarks, the text is always
// printed to avoid having performance depend on the value of the -test.v flag.
// It is an error to call Log after a test or benchmark returns.
func (c *common) Log(args ...any) {
	c.checkFuzzFn("Log")
	c.log(fmt.Sprintln(args...))
}

// Logf formats its arguments according to the format, analogous to [fmt.Printf], and
// records the text in the error log. A final newline is added if not provided. For
// tests, the text will be printed only if the test fails or the -test.v flag is
// set. For benchmarks, the text is always printed to avoid having performance
// depend on the value of the -test.v flag.
// It is an error to call Logf after a test or benchmark returns.
func (c *common) Logf(format string, args ...any) {
	c.checkFuzzFn("Logf")
	c.log(fmt.Sprintf(format, args...))
}

// Error is equivalent to Log followed by Fail.
func (c *common) Error(args ...any) {
	c.checkFuzzFn("Error")
	c.log(fmt.Sprintln(args...))
	c.Fail()
}

// Errorf is equivalent to Logf followed by Fail.
func (c *common) Errorf(format string, args ...any) {
	c.checkFuzzFn("Errorf")
	c.log(fmt.Sprintf(format, args...))
	c.Fail()
}

// Fatal is equivalent to Log followed by FailNow.
func (c *common) Fatal(args ...any) {
	c.checkFuzzFn("Fatal")
	c.log(fmt.Sprintln(args...))
	c.FailNow()
}

// Fatalf is equivalent to Logf followed by FailNow.
func (c *common) Fatalf(format string, args ...any) {
	c.checkFuzzFn("Fatalf")
	c.log(fmt.Sprintf(format, args...))
	c.FailNow()
}

// Skip is equivalent to Log followed by SkipNow.
func (c *common) Skip(args ...any) {
	c.checkFuzzFn("Skip")
	c.log(fmt.Sprintln(args...))
	c.SkipNow()
}

// Skipf is equivalent to Logf followed by SkipNow.
func (c *common) Skipf(format string, args ...any) {
	c.checkFuzzFn("Skipf")
	c.log(fmt.Sprintf(format, args...))
	c.SkipNow()
}

// SkipNow marks the test as having been skipped and stops its execution
// by calling [runtime.Goexit].
// If a test fails (see Error, Errorf, Fail) and is then skipped,
// it is still considered to have failed.
// Execution will continue at the next test or benchmark. See also FailNow.
// SkipNow must be called from the goroutine running the test, not from
// other goroutines created during the test. Calling SkipNow does not stop
// those other goroutines.
func (c *common) SkipNow() {
	c.checkFuzzFn("SkipNow")
	c.mu.Lock()
	c.skipped = true
	c.finished = true
	c.mu.Unlock()
	runtime.Goexit()
}

// Skipped reports whether the test was skipped.
func (c *common) Skipped() bool {
	c.mu.RLock()
	defer c.mu.RUnlock()
	return c.skipped
}

// Helper marks the calling function as a test helper function.
// When printing file and line information, that function will be skipped.
// Helper may be called simultaneously from multiple goroutines.
func (c *common) Helper() {
	c.mu.Lock()
	defer c.mu.Unlock()
	if c.helperPCs == nil {
		c.helperPCs = make(map[uintptr]struct{})
	}
	// repeating code from callerName here to save walking a stack frame
	var pc [1]uintptr
	n := runtime.Callers(2, pc[:]) // skip runtime.Callers + Helper
	if n == 0 {
		panic("testing: zero callers found")
	}
	if _, found := c.helperPCs[pc[0]]; !found {
		c.helperPCs[pc[0]] = struct{}{}
		c.helperNames = nil // map will be recreated next time it is needed
	}
}

// Cleanup registers a function to be called when the test (or subtest) and all its
// subtests complete. Cleanup functions will be called in last added,
// first called order.
func (c *common) Cleanup(f func()) {
	c.checkFuzzFn("Cleanup")
	var pc [maxStackLen]uintptr
	// Skip two extra frames to account for this function and runtime.Callers itself.
	n := runtime.Callers(2, pc[:])
	cleanupPc := pc[:n]

	fn := func() {
		defer func() {
			c.mu.Lock()
			defer c.mu.Unlock()
			c.cleanupName = ""
			c.cleanupPc = nil
		}()

		name := callerName(0)
		c.mu.Lock()
		c.cleanupName = name
		c.cleanupPc = cleanupPc
		c.mu.Unlock()

		f()
	}

	c.mu.Lock()
	defer c.mu.Unlock()
	c.cleanups = append(c.cleanups, fn)
}

// TempDir returns a temporary directory for the test to use.
// The directory is automatically removed when the test and
// all its subtests complete.
// Each subsequent call to TempDir returns a unique directory;
// if the directory creation fails, TempDir terminates the test by calling Fatal.
func (c *common) TempDir() string {
	c.checkFuzzFn("TempDir")
	// Use a single parent directory for all the temporary directories
	// created by a test, each numbered sequentially.
	c.tempDirMu.Lock()
	var nonExistent bool
	if c.tempDir == "" { // Usually the case with js/wasm
		nonExistent = true
	} else {
		_, err := os.Stat(c.tempDir)
		nonExistent = os.IsNotExist(err)
		if err != nil && !nonExistent {
			c.Fatalf("TempDir: %v", err)
		}
	}

	if nonExistent {
		c.Helper()

		pattern := c.Name()
		// Limit length of file names on disk.
		// Invalid runes from slicing are dropped by strings.Map below.
		pattern = pattern[:min(len(pattern), 64)]

		// Drop unusual characters (such as path separators or
		// characters interacting with globs) from the directory name to
		// avoid surprising os.MkdirTemp behavior.
		mapper := func(r rune) rune {
			if r < utf8.RuneSelf {
				const allowed = "!#$%&()+,-.=@^_{}~ "
				if '0' <= r && r <= '9' ||
					'a' <= r && r <= 'z' ||
					'A' <= r && r <= 'Z' {
					return r
				}
				if strings.ContainsRune(allowed, r) {
					return r
				}
			} else if unicode.IsLetter(r) || unicode.IsNumber(r) {
				return r
			}
			return -1
		}
		pattern = strings.Map(mapper, pattern)
		c.tempDir, c.tempDirErr = os.MkdirTemp("", pattern)
		if c.tempDirErr == nil {
			c.Cleanup(func() {
				if err := removeAll(c.tempDir); err != nil {
					c.Errorf("TempDir RemoveAll cleanup: %v", err)
				}
			})
		}
	}

	if c.tempDirErr == nil {
		c.tempDirSeq++
	}
	seq := c.tempDirSeq
	c.tempDirMu.Unlock()

	if c.tempDirErr != nil {
		c.Fatalf("TempDir: %v", c.tempDirErr)
	}

	dir := fmt.Sprintf("%s%c%03d", c.tempDir, os.PathSeparator, seq)
	if err := os.Mkdir(dir, 0o777); err != nil {
		c.Fatalf("TempDir: %v", err)
	}
	return dir
}

// removeAll is like os.RemoveAll, but retries Windows "Access is denied."
// errors up to an arbitrary timeout.
//
// Those errors have been known to occur spuriously on at least the
// windows-amd64-2012 builder (https://go.dev/issue/50051), and can only occur
// legitimately if the test leaves behind a temp file that either is still open
// or the test otherwise lacks permission to delete. In the case of legitimate
// failures, a failing test may take a bit longer to fail, but once the test is
// fixed the extra latency will go away.
func removeAll(path string) error {
	const arbitraryTimeout = 2 * time.Second
	var (
		start     time.Time
		nextSleep = 1 * time.Millisecond
	)
	for {
		err := os.RemoveAll(path)
		if !isWindowsRetryable(err) {
			return err
		}
		if start.IsZero() {
			start = time.Now()
		} else if d := time.Since(start) + nextSleep; d >= arbitraryTimeout {
			return err
		}
		time.Sleep(nextSleep)
		nextSleep += time.Duration(rand.Int63n(int64(nextSleep)))
	}
}

// Setenv calls [os.Setenv] and uses Cleanup to
// restore the environment variable to its original value
// after the test.
//
// Because Setenv affects the whole process, it cannot be used
// in parallel tests or tests with parallel ancestors.
func (c *common) Setenv(key, value string) {
	c.checkFuzzFn("Setenv")
	prevValue, ok := os.LookupEnv(key)

	if err := os.Setenv(key, value); err != nil {
		c.Fatalf("cannot set environment variable: %v", err)
	}

	if ok {
		c.Cleanup(func() {
			os.Setenv(key, prevValue)
		})
	} else {
		c.Cleanup(func() {
			os.Unsetenv(key)
		})
	}
}

// Chdir calls [os.Chdir] and uses Cleanup to restore the current
// working directory to its original value after the test. On Unix, it
// also sets PWD environment variable for the duration of the test.
//
// Because Chdir affects the whole process, it cannot be used
// in parallel tests or tests with parallel ancestors.
func (c *common) Chdir(dir string) {
	c.checkFuzzFn("Chdir")
	oldwd, err := os.Open(".")
	if err != nil {
		c.Fatal(err)
	}
	if err := os.Chdir(dir); err != nil {
		c.Fatal(err)
	}
	// On POSIX platforms, PWD represents “an absolute pathname of the
	// current working directory.” Since we are changing the working
	// directory, we should also set or update PWD to reflect that.
	switch runtime.GOOS {
	case "windows", "plan9":
		// Windows and Plan 9 do not use the PWD variable.
	default:
		if !filepath.IsAbs(dir) {
			dir, err = os.Getwd()
			if err != nil {
				c.Fatal(err)
			}
		}
		c.Setenv("PWD", dir)
	}
	c.Cleanup(func() {
		err := oldwd.Chdir()
		oldwd.Close()
		if err != nil {
			// It's not safe to continue with tests if we can't
			// get back to the original working directory. Since
			// we are holding a dirfd, this is highly unlikely.
			panic("testing.Chdir: " + err.Error())
		}
	})
}

// Context returns a context that is canceled just before
// Cleanup-registered functions are called.
//
// Cleanup functions can wait for any resources
// that shut down on [context.Context.Done] before the test or benchmark completes.
func (c *common) Context() context.Context {
	c.checkFuzzFn("Context")
	return c.ctx
}

// Attr emits a test attribute associated with this test.
//
// The key must not contain whitespace.
// The value must not contain newlines or carriage returns.
//
// The meaning of different attribute keys is left up to
// continuous integration systems and test frameworks.
//
// Test attributes are emitted immediately in the test log,
// but they are intended to be treated as unordered.
func (c *common) Attr(key, value string) {
	if strings.ContainsFunc(key, unicode.IsSpace) {
		c.Errorf("disallowed whitespace in attribute key %q", key)
		return
	}
	if strings.ContainsAny(value, "\r\n") {
		c.Errorf("disallowed newline in attribute value %q", value)
		return
	}
	if c.chatty == nil {
		return
	}
	c.chatty.Updatef(c.name, "=== ATTR  %s %v %v\n", c.name, key, value)
}

// panicHandling controls the panic handling used by runCleanup.
type panicHandling int

const (
	normalPanic panicHandling = iota
	recoverAndReturnPanic
)

// runCleanup is called at the end of the test.
// If ph is recoverAndReturnPanic, it will catch panics, and return the
// recovered value if any.
func (c *common) runCleanup(ph panicHandling) (panicVal any) {
	c.cleanupStarted.Store(true)
	defer c.cleanupStarted.Store(false)

	if ph == recoverAndReturnPanic {
		defer func() {
			panicVal = recover()
		}()
	}

	// Make sure that if a cleanup function panics,
	// we still run the remaining cleanup functions.
	defer func() {
		c.mu.Lock()
		recur := len(c.cleanups) > 0
		c.mu.Unlock()
		if recur {
			c.runCleanup(normalPanic)
		}
	}()

	if c.cancelCtx != nil {
		c.cancelCtx()
	}

	for {
		var cleanup func()
		c.mu.Lock()
		if len(c.cleanups) > 0 {
			last := len(c.cleanups) - 1
			cleanup = c.cleanups[last]
			c.cleanups = c.cleanups[:last]
		}
		c.mu.Unlock()
		if cleanup == nil {
			return nil
		}
		cleanup()
	}
}

// resetRaces updates c.parent's count of data race errors (or the global count,
// if c has no parent), and updates c.lastRaceErrors to match.
//
// Any races that occurred prior to this call to resetRaces will
// not be attributed to c.
func (c *common) resetRaces() {
	if c.parent == nil {
		c.lastRaceErrors.Store(int64(race.Errors()))
	} else {
		c.lastRaceErrors.Store(c.parent.checkRaces())
	}
}

// checkRaces checks whether the global count of data race errors has increased
// since c's count was last reset.
//
// If so, it marks c as having failed due to those races (logging an error for
// the first such race), and updates the race counts for the parents of c so
// that if they are currently suspended (such as in a call to T.Run) they will
// not log separate errors for the race(s).
//
// Note that multiple tests may be marked as failed due to the same race if they
// are executing in parallel.
func (c *common) checkRaces() (raceErrors int64) {
	raceErrors = int64(race.Errors())
	for {
		last := c.lastRaceErrors.Load()
		if raceErrors <= last {
			// All races have already been reported.
			return raceErrors
		}
		if c.lastRaceErrors.CompareAndSwap(last, raceErrors) {
			break
		}
	}

	if c.raceErrorLogged.CompareAndSwap(false, true) {
		// This is the first race we've encountered for this test.
		// Mark the test as failed, and log the reason why only once.
		// (Note that the race detector itself will still write a goroutine
		// dump for any further races it detects.)
		c.Errorf("race detected during execution of test")
	}

	// Update the parent(s) of this test so that they don't re-report the race.
	parent := c.parent
	for parent != nil {
		for {
			last := parent.lastRaceErrors.Load()
			if raceErrors <= last {
				// This race was already reported by another (likely parallel) subtest.
				return raceErrors
			}
			if parent.lastRaceErrors.CompareAndSwap(last, raceErrors) {
				break
			}
		}
		parent = parent.parent
	}

	return raceErrors
}

// callerName gives the function name (qualified with a package path)
// for the caller after skip frames (where 0 means the current function).
func callerName(skip int) string {
	var pc [1]uintptr
	n := runtime.Callers(skip+2, pc[:]) // skip + runtime.Callers + callerName
	if n == 0 {
		panic("testing: zero callers found")
	}
	return pcToName(pc[0])
}

func pcToName(pc uintptr) string {
	pcs := []uintptr{pc}
	frames := runtime.CallersFrames(pcs)
	frame, _ := frames.Next()
	return frame.Function
}

const parallelConflict = `testing: test using t.Setenv or t.Chdir can not use t.Parallel`

// Parallel signals that this test is to be run in parallel with (and only with)
// other parallel tests. When a test is run multiple times due to use of
// -test.count or -test.cpu, multiple instances of a single test never run in
// parallel with each other.
func (t *T) Parallel() {
	if t.isParallel {
		panic("testing: t.Parallel called multiple times")
	}
	if t.isSynctest {
		panic("testing: t.Parallel called inside synctest bubble")
	}
	if t.denyParallel {
		panic(parallelConflict)
	}
	if t.parent.barrier == nil {
		// T.Parallel has no effect when fuzzing.
		// Multiple processes may run in parallel, but only one input can run at a
		// time per process so we can attribute crashes to specific inputs.
		return
	}

	t.isParallel = true

	// We don't want to include the time we spend waiting for serial tests
	// in the test duration. Record the elapsed time thus far and reset the
	// timer afterwards.
	t.duration += highPrecisionTimeSince(t.start)

	// Add to the list of tests to be released by the parent.
	t.parent.sub = append(t.parent.sub, t)

	// Report any races during execution of this test up to this point.
	//
	// We will assume that any races that occur between here and the point where
	// we unblock are not caused by this subtest. That assumption usually holds,
	// although it can be wrong if the test spawns a goroutine that races in the
	// background while the rest of the test is blocked on the call to Parallel.
	// If that happens, we will misattribute the background race to some other
	// test, or to no test at all — but that false-negative is so unlikely that it
	// is not worth adding race-report noise for the common case where the test is
	// completely suspended during the call to Parallel.
	t.checkRaces()

	if t.chatty != nil {
		t.chatty.Updatef(t.name, "=== PAUSE %s\n", t.name)
	}
	running.Delete(t.name)

	t.signal <- true   // Release calling test.
	<-t.parent.barrier // Wait for the parent test to complete.
	t.tstate.waitParallel()
	parallelStart.Add(1)

	if t.chatty != nil {
		t.chatty.Updatef(t.name, "=== CONT  %s\n", t.name)
	}
	running.Store(t.name, highPrecisionTimeNow())
	t.start = highPrecisionTimeNow()

	// Reset the local race counter to ignore any races that happened while this
	// goroutine was blocked, such as in the parent test or in other parallel
	// subtests.
	//
	// (Note that we don't call parent.checkRaces here:
	// if other parallel subtests have already introduced races, we want to
	// let them report those races instead of attributing them to the parent.)
	t.lastRaceErrors.Store(int64(race.Errors()))
}

func (t *T) checkParallel() {
	// Non-parallel subtests that have parallel ancestors may still
	// run in parallel with other tests: they are only non-parallel
	// with respect to the other subtests of the same parent.
	// Since calls like SetEnv or Chdir affects the whole process, we need
	// to deny those if the current test or any parent is parallel.
	for c := &t.common; c != nil; c = c.parent {
		if c.isParallel {
			panic(parallelConflict)
		}
	}

	t.denyParallel = true
}

// Setenv calls os.Setenv(key, value) and uses Cleanup to
// restore the environment variable to its original value
// after the test.
//
// Because Setenv affects the whole process, it cannot be used
// in parallel tests or tests with parallel ancestors.
func (t *T) Setenv(key, value string) {
	t.checkParallel()
	t.common.Setenv(key, value)
}

// Chdir calls [os.Chdir] and uses Cleanup to restore the current
// working directory to its original value after the test. On Unix, it
// also sets PWD environment variable for the duration of the test.
//
// Because Chdir affects the whole process, it cannot be used
// in parallel tests or tests with parallel ancestors.
func (t *T) Chdir(dir string) {
	t.checkParallel()
	t.common.Chdir(dir)
}

// InternalTest is an internal type but exported because it is cross-package;
// it is part of the implementation of the "go test" command.
type InternalTest struct {
	Name string
	F    func(*T)
}

var errNilPanicOrGoexit = errors.New("test executed panic(nil) or runtime.Goexit")

func tRunner(t *T, fn func(t *T)) {
	t.runner = callerName(0)

	// When this goroutine is done, either because fn(t)
	// returned normally or because a test failure triggered
	// a call to runtime.Goexit, record the duration and send
	// a signal saying that the test is done.
	defer func() {
		t.checkRaces()

		// TODO(#61034): This is the wrong place for this check.
		if t.Failed() {
			numFailed.Add(1)
		}

		// Check if the test panicked or Goexited inappropriately.
		//
		// If this happens in a normal test, print output but continue panicking.
		// tRunner is called in its own goroutine, so this terminates the process.
		//
		// If this happens while fuzzing, recover from the panic and treat it like a
		// normal failure. It's important that the process keeps running in order to
		// find short inputs that cause panics.
		err := recover()
		signal := true

		t.mu.RLock()
		finished := t.finished
		t.mu.RUnlock()
		if !finished && err == nil {
			err = errNilPanicOrGoexit
			for p := t.parent; p != nil; p = p.parent {
				p.mu.RLock()
				finished = p.finished
				p.mu.RUnlock()
				if finished {
					if !t.isParallel {
						t.Errorf("%v: subtest may have called FailNow on a parent test", err)
						err = nil
					}
					signal = false
					break
				}
			}
		}

		if err != nil && t.tstate.isFuzzing {
			prefix := "panic: "
			if err == errNilPanicOrGoexit {
				prefix = ""
			}
			t.Errorf("%s%s\n%s\n", prefix, err, string(debug.Stack()))
			t.mu.Lock()
			t.finished = true
			t.mu.Unlock()
			err = nil
		}

		// Use a deferred call to ensure that we report that the test is
		// complete even if a cleanup function calls t.FailNow. See issue 41355.
		didPanic := false
		defer func() {
			// Only report that the test is complete if it doesn't panic,
			// as otherwise the test binary can exit before the panic is
			// reported to the user. See issue 41479.
			if didPanic {
				return
			}
			if err != nil {
				panic(err)
			}
			running.Delete(t.name)
			if t.isParallel {
				parallelStop.Add(1)
			}
			t.signal <- signal
		}()

		doPanic := func(err any) {
			t.Fail()
			if r := t.runCleanup(recoverAndReturnPanic); r != nil {
				t.Logf("cleanup panicked with %v", r)
			}
			// Flush the output log up to the root before dying.
			// Skip this if this *T is a synctest bubble, because we're not a subtest.
			for root := &t.common; !root.isSynctest && root.parent != nil; root = root.parent {
				root.mu.Lock()
				root.duration += highPrecisionTimeSince(root.start)
				d := root.duration
				root.mu.Unlock()
				// Output buffered logs.
				root.flushPartial()
				root.flushToParent(root.name, "--- FAIL: %s (%s)\n", root.name, fmtDuration(d))
				if r := root.parent.runCleanup(recoverAndReturnPanic); r != nil {
					fmt.Fprintf(root.parent.w, "cleanup panicked with %v", r)
				}
			}
			didPanic = true
			panic(err)
		}
		if err != nil {
			doPanic(err)
		}

		t.duration += highPrecisionTimeSince(t.start)

		if len(t.sub) > 0 {
			// Run parallel subtests.

			// Decrease the running count for this test and mark it as no longer running.
			t.tstate.release()
			running.Delete(t.name)

			// Release the parallel subtests.
			close(t.barrier)
			// Wait for subtests to complete.
			for _, sub := range t.sub {
				<-sub.signal
			}

			// Run any cleanup callbacks, marking the test as running
			// in case the cleanup hangs.
			cleanupStart := highPrecisionTimeNow()
			running.Store(t.name, cleanupStart)
			err := t.runCleanup(recoverAndReturnPanic)
			t.duration += highPrecisionTimeSince(cleanupStart)
			if err != nil {
				doPanic(err)
			}
			t.checkRaces()
			if !t.isParallel {
				// Reacquire the count for sequential tests. See comment in Run.
				t.tstate.waitParallel()
			}
		} else if t.isParallel {
			// Only release the count for this test if it was run as a parallel
			// test. See comment in Run method.
			t.tstate.release()
		}
		// Output buffered logs.
		for root := &t.common; root.parent != nil; root = root.parent {
			root.flushPartial()
		}
		t.report() // Report after all subtests have finished.

		// Do not lock t.done to allow race detector to detect race in case
		// the user does not appropriately synchronize a goroutine.
		t.done = true
		if t.parent != nil && !t.hasSub.Load() {
			t.setRan()
		}
	}()
	defer func() {
		if len(t.sub) == 0 {
			t.runCleanup(normalPanic)
		}
	}()

	t.start = highPrecisionTimeNow()
	t.resetRaces()
	fn(t)

	// code beyond here will not be executed when FailNow is invoked
	t.mu.Lock()
	t.finished = true
	t.mu.Unlock()
}

// Run runs f as a subtest of t called name. It runs f in a separate goroutine
// and blocks until f returns or calls t.Parallel to become a parallel test.
// Run reports whether f succeeded (or at least did not fail before calling t.Parallel).
//
// Run may be called simultaneously from multiple goroutines, but all such calls
// must return before the outer test function for t returns.
func (t *T) Run(name string, f func(t *T)) bool {
	if t.isSynctest {
		panic("testing: t.Run called inside synctest bubble")
	}
	if t.cleanupStarted.Load() {
		panic("testing: t.Run called during t.Cleanup")
	}

	t.hasSub.Store(true)
	testName, ok, _ := t.tstate.match.fullName(&t.common, name)
	if !ok || shouldFailFast() {
		return true
	}
	// Record the stack trace at the point of this call so that if the subtest
	// function - which runs in a separate stack - is marked as a helper, we can
	// continue walking the stack into the parent test.
	var pc [maxStackLen]uintptr
	n := runtime.Callers(2, pc[:])

	// There's no reason to inherit this context from parent. The user's code can't observe
	// the difference between the background context and the one from the parent test.
	ctx, cancelCtx := context.WithCancel(context.Background())
	t = &T{
		common: common{
			barrier:   make(chan bool),
			signal:    make(chan bool, 1),
			name:      testName,
			parent:    &t.common,
			level:     t.level + 1,
			creator:   pc[:n],
			chatty:    t.chatty,
			ctx:       ctx,
			cancelCtx: cancelCtx,
		},
		tstate: t.tstate,
	}
	t.w = indenter{&t.common}
	t.setOutputWriter()

	if t.chatty != nil {
		t.chatty.Updatef(t.name, "=== RUN   %s\n", t.name)
	}
	running.Store(t.name, highPrecisionTimeNow())

	// Instead of reducing the running count of this test before calling the
	// tRunner and increasing it afterwards, we rely on tRunner keeping the
	// count correct. This ensures that a sequence of sequential tests runs
	// without being preempted, even when their parent is a parallel test. This
	// may especially reduce surprises if *parallel == 1.
	go tRunner(t, f)

	// The parent goroutine will block until the subtest either finishes or calls
	// Parallel, but in general we don't know whether the parent goroutine is the
	// top-level test function or some other goroutine it has spawned.
	// To avoid confusing false-negatives, we leave the parent in the running map
	// even though in the typical case it is blocked.

	if !<-t.signal {
		// At this point, it is likely that FailNow was called on one of the
		// parent tests by one of the subtests. Continue aborting up the chain.
		runtime.Goexit()
	}

	if t.chatty != nil && t.chatty.json {
		t.chatty.Updatef(t.parent.name, "=== NAME  %s\n", t.parent.name)
	}
	return !t.failed
}

// testingSynctestTest runs f within a synctest bubble.
// It is called by synctest.Test, from within an already-created bubble.
//
//go:linkname testingSynctestTest testing/synctest.testingSynctestTest
func testingSynctestTest(t *T, f func(*T)) (ok bool) {
	if t.cleanupStarted.Load() {
		panic("testing: synctest.Run called during t.Cleanup")
	}

	var pc [maxStackLen]uintptr
	n := runtime.Callers(2, pc[:])

	ctx, cancelCtx := context.WithCancel(context.Background())
	t2 := &T{
		common: common{
			barrier:    make(chan bool),
			signal:     make(chan bool, 1),
			name:       t.name,
			parent:     &t.common,
			level:      t.level + 1,
			creator:    pc[:n],
			chatty:     t.chatty,
			ctx:        ctx,
			cancelCtx:  cancelCtx,
			isSynctest: true,
		},
		tstate: t.tstate,
	}

	go tRunner(t2, f)
	if !<-t2.signal {
		// At this point, it is likely that FailNow was called on one of the
		// parent tests by one of the subtests. Continue aborting up the chain.
		runtime.Goexit()
	}
	return !t2.failed
}

// Deadline reports the time at which the test binary will have
// exceeded the timeout specified by the -timeout flag.
//
// The ok result is false if the -timeout flag indicates “no timeout” (0).
func (t *T) Deadline() (deadline time.Time, ok bool) {
	if t.isSynctest {
		// There's no point in returning a real-clock deadline to
		// a test using a fake clock. We could return "no timeout",
		// but panicking makes it easier for users to catch the error.
		panic("testing: t.Deadline called inside synctest bubble")
	}
	deadline = t.tstate.deadline
	return deadline, !deadline.IsZero()
}

// testState holds all fields that are common to all tests. This includes
// synchronization primitives to run at most *parallel tests.
type testState struct {
	match    *matcher
	deadline time.Time

	// isFuzzing is true in the state used when generating random inputs
	// for fuzz targets. isFuzzing is false when running normal tests and
	// when running fuzz tests as unit tests (without -fuzz or when -fuzz
	// does not match).
	isFuzzing bool

	mu sync.Mutex

	// Channel used to signal tests that are ready to be run in parallel.
	startParallel chan bool

	// running is the number of tests currently running in parallel.
	// This does not include tests that are waiting for subtests to complete.
	running int

	// numWaiting is the number tests waiting to be run in parallel.
	numWaiting int

	// maxParallel is a copy of the parallel flag.
	maxParallel int
}

func newTestState(maxParallel int, m *matcher) *testState {
	return &testState{
		match:         m,
		startParallel: make(chan bool),
		maxParallel:   maxParallel,
		running:       1, // Set the count to 1 for the main (sequential) test.
	}
}

func (s *testState) waitParallel() {
	s.mu.Lock()
	if s.running < s.maxParallel {
		s.running++
		s.mu.Unlock()
		return
	}
	s.numWaiting++
	s.mu.Unlock()
	<-s.startParallel
}

func (s *testState) release() {
	s.mu.Lock()
	if s.numWaiting == 0 {
		s.running--
		s.mu.Unlock()
		return
	}
	s.numWaiting--
	s.mu.Unlock()
	s.startParallel <- true // Pick a waiting test to be run.
}

// No one should be using func Main anymore.
// See the doc comment on func Main and use MainStart instead.
var errMain = errors.New("testing: unexpected use of func Main")

type matchStringOnly func(pat, str string) (bool, error)

func (f matchStringOnly) MatchString(pat, str string) (bool, error)   { return f(pat, str) }
func (f matchStringOnly) StartCPUProfile(w io.Writer) error           { return errMain }
func (f matchStringOnly) StopCPUProfile()                             {}
func (f matchStringOnly) WriteProfileTo(string, io.Writer, int) error { return errMain }
func (f matchStringOnly) ImportPath() string                          { return "" }
func (f matchStringOnly) StartTestLog(io.Writer)                      {}
func (f matchStringOnly) StopTestLog() error                          { return errMain }
func (f matchStringOnly) SetPanicOnExit0(bool)                        {}
func (f matchStringOnly) CoordinateFuzzing(time.Duration, int64, time.Duration, int64, int, []corpusEntry, []reflect.Type, string, string) error {
	return errMain
}
func (f matchStringOnly) RunFuzzWorker(func(corpusEntry) error) error { return errMain }
func (f matchStringOnly) ReadCorpus(string, []reflect.Type) ([]corpusEntry, error) {
	return nil, errMain
}
func (f matchStringOnly) CheckCorpus([]any, []reflect.Type) error { return nil }
func (f matchStringOnly) ResetCoverage()                          {}
func (f matchStringOnly) SnapshotCoverage()                       {}

func (f matchStringOnly) InitRuntimeCoverage() (mode string, tearDown func(string, string) (string, error), snapcov func() float64) {
	return
}

// Main is an internal function, part of the implementation of the "go test" command.
// It was exported because it is cross-package and predates "internal" packages.
// It is no longer used by "go test" but preserved, as much as possible, for other
// systems that simulate "go test" using Main, but Main sometimes cannot be updated as
// new functionality is added to the testing package.
// Systems simulating "go test" should be updated to use [MainStart].
func Main(matchString func(pat, str string) (bool, error), tests []InternalTest, benchmarks []InternalBenchmark, examples []InternalExample) {
	os.Exit(MainStart(matchStringOnly(matchString), tests, benchmarks, nil, examples).Run())
}

// M is a type passed to a TestMain function to run the actual tests.
type M struct {
	deps        testDeps
	tests       []InternalTest
	benchmarks  []InternalBenchmark
	fuzzTargets []InternalFuzzTarget
	examples    []InternalExample

	timer     *time.Timer
	afterOnce sync.Once

	numRun int

	// value to pass to os.Exit, the outer test func main
	// harness calls os.Exit with this code. See #34129.
	exitCode int
}

// testDeps is an internal interface of functionality that is
// passed into this package by a test's generated main package.
// The canonical implementation of this interface is
// testing/internal/testdeps's TestDeps.
type testDeps interface {
	ImportPath() string
	MatchString(pat, str string) (bool, error)
	SetPanicOnExit0(bool)
	StartCPUProfile(io.Writer) error
	StopCPUProfile()
	StartTestLog(io.Writer)
	StopTestLog() error
	WriteProfileTo(string, io.Writer, int) error
	CoordinateFuzzing(time.Duration, int64, time.Duration, int64, int, []corpusEntry, []reflect.Type, string, string) error
	RunFuzzWorker(func(corpusEntry) error) error
	ReadCorpus(string, []reflect.Type) ([]corpusEntry, error)
	CheckCorpus([]any, []reflect.Type) error
	ResetCoverage()
	SnapshotCoverage()
	InitRuntimeCoverage() (mode string, tearDown func(coverprofile string, gocoverdir string) (string, error), snapcov func() float64)
}

// MainStart is meant for use by tests generated by 'go test'.
// It is not meant to be called directly and is not subject to the Go 1 compatibility document.
// It may change signature from release to release.
func MainStart(deps testDeps, tests []InternalTest, benchmarks []InternalBenchmark, fuzzTargets []InternalFuzzTarget, examples []InternalExample) *M {
	registerCover(deps.InitRuntimeCoverage())
	Init()
	return &M{
		deps:        deps,
		tests:       tests,
		benchmarks:  benchmarks,
		fuzzTargets: fuzzTargets,
		examples:    examples,
	}
}

var (
	testingTesting bool
	realStderr     *os.File
)

// Run runs the tests. It returns an exit code to pass to os.Exit.
// The exit code is zero when all tests pass, and non-zero for any kind
// of failure. For machine readable test results, parse the output of
// 'go test -json'.
func (m *M) Run() (code int) {
	defer func() {
		code = m.exitCode
	}()

	// Count the number of calls to m.Run.
	// We only ever expected 1, but we didn't enforce that,
	// and now there are tests in the wild that call m.Run multiple times.
	// Sigh. go.dev/issue/23129.
	m.numRun++

	// TestMain may have already called flag.Parse.
	if !flag.Parsed() {
		flag.Parse()
	}

	if chatty.json {
		// With -v=json, stdout and stderr are pointing to the same pipe,
		// which is leading into test2json. In general, operating systems
		// do a good job of ensuring that writes to the same pipe through
		// different file descriptors are delivered whole, so that writing
		// AAA to stdout and BBB to stderr simultaneously produces
		// AAABBB or BBBAAA on the pipe, not something like AABBBA.
		// However, the exception to this is when the pipe fills: in that
		// case, Go's use of non-blocking I/O means that writing AAA
		// or BBB might be split across multiple system calls, making it
		// entirely possible to get output like AABBBA. The same problem
		// happens inside the operating system kernel if we switch to
		// blocking I/O on the pipe. This interleaved output can do things
		// like print unrelated messages in the middle of a TestFoo line,
		// which confuses test2json. Setting os.Stderr = os.Stdout will make
		// them share a single pfd, which will hold a lock for each program
		// write, preventing any interleaving.
		//
		// It might be nice to set Stderr = Stdout always, or perhaps if
		// we can tell they are the same file, but for now -v=json is
		// a very clear signal. Making the two files the same may cause
		// surprises if programs close os.Stdout but expect to be able
		// to continue to write to os.Stderr, but it's hard to see why a
		// test would think it could take over global state that way.
		//
		// This fix only helps programs where the output is coming directly
		// from Go code. It does not help programs in which a subprocess is
		// writing to stderr or stdout at the same time that a Go test is writing output.
		// It also does not help when the output is coming from the runtime,
		// such as when using the print/println functions, since that code writes
		// directly to fd 2 without any locking.
		// We keep realStderr around to prevent fd 2 from being closed.
		//
		// See go.dev/issue/33419.
		realStderr = os.Stderr
		os.Stderr = os.Stdout
	}

	if *parallel < 1 {
		fmt.Fprintln(os.Stderr, "testing: -parallel can only be given a positive integer")
		flag.Usage()
		m.exitCode = 2
		return
	}
	if *matchFuzz != "" && *fuzzCacheDir == "" {
		fmt.Fprintln(os.Stderr, "testing: -test.fuzzcachedir must be set if -test.fuzz is set")
		flag.Usage()
		m.exitCode = 2
		return
	}

	if *matchList != "" {
		listTests(m.deps.MatchString, m.tests, m.benchmarks, m.fuzzTargets, m.examples)
		m.exitCode = 0
		return
	}

	if *shuffle != "off" {
		var n int64
		var err error
		if *shuffle == "on" {
			n = time.Now().UnixNano()
		} else {
			n, err = strconv.ParseInt(*shuffle, 10, 64)
			if err != nil {
				fmt.Fprintln(os.Stderr, `testing: -shuffle should be "off", "on", or a valid integer:`, err)
				m.exitCode = 2
				return
			}
		}
		fmt.Println("-test.shuffle", n)
		rng := rand.New(rand.NewSource(n))
		rng.Shuffle(len(m.tests), func(i, j int) { m.tests[i], m.tests[j] = m.tests[j], m.tests[i] })
		rng.Shuffle(len(m.benchmarks), func(i, j int) { m.benchmarks[i], m.benchmarks[j] = m.benchmarks[j], m.benchmarks[i] })
	}

	parseCpuList()

	m.before()
	defer m.after()

	// Run tests, examples, and benchmarks unless this is a fuzz worker process.
	// Workers start after this is done by their parent process, and they should
	// not repeat this work.
	if !*isFuzzWorker {
		deadline := m.startAlarm()
		haveExamples = len(m.examples) > 0
		testRan, testOk := runTests(m.deps.MatchString, m.tests, deadline)
		fuzzTargetsRan, fuzzTargetsOk := runFuzzTests(m.deps, m.fuzzTargets, deadline)
		exampleRan, exampleOk := runExamples(m.deps.MatchString, m.examples)
		m.stopAlarm()
		if !testRan && !exampleRan && !fuzzTargetsRan && *matchBenchmarks == "" && *matchFuzz == "" {
			fmt.Fprintln(os.Stderr, "testing: warning: no tests to run")
			if testingTesting && *match != "^$" {
				// If this happens during testing of package testing it could be that
				// package testing's own logic for when to run a test is broken,
				// in which case every test will run nothing and succeed,
				// with no obvious way to detect this problem (since no tests are running).
				// So make 'no tests to run' a hard failure when testing package testing itself.
				fmt.Print(chatty.prefix(), "FAIL: package testing must run tests\n")
				testOk = false
			}
		}
		anyFailed := !testOk || !exampleOk || !fuzzTargetsOk || !runBenchmarks(m.deps.ImportPath(), m.deps.MatchString, m.benchmarks)
		if !anyFailed && race.Errors() > 0 {
			fmt.Print(chatty.prefix(), "testing: race detected outside of test execution\n")
			anyFailed = true
		}
		if anyFailed {
			fmt.Print(chatty.prefix(), "FAIL\n")
			m.exitCode = 1
			return
		}
	}

	fuzzingOk := runFuzzing(m.deps, m.fuzzTargets)
	if !fuzzingOk {
		fmt.Print(chatty.prefix(), "FAIL\n")
		if *isFuzzWorker {
			m.exitCode = fuzzWorkerExitCode
		} else {
			m.exitCode = 1
		}
		return
	}

	m.exitCode = 0
	if !*isFuzzWorker {
		fmt.Print(chatty.prefix(), "PASS\n")
	}
	return
}

func (t *T) report() {
	if t.parent == nil {
		return
	}
	if t.isSynctest {
		return // t.parent will handle reporting
	}
	dstr := fmtDuration(t.duration)
	format := "--- %s: %s (%s)\n"
	if t.Failed() {
		t.flushToParent(t.name, format, "FAIL", t.name, dstr)
	} else if t.chatty != nil {
		if t.Skipped() {
			t.flushToParent(t.name, format, "SKIP", t.name, dstr)
		} else {
			t.flushToParent(t.name, format, "PASS", t.name, dstr)
		}
	}
}

func listTests(matchString func(pat, str string) (bool, error), tests []InternalTest, benchmarks []InternalBenchmark, fuzzTargets []InternalFuzzTarget, examples []InternalExample) {
	if _, err := matchString(*matchList, "non-empty"); err != nil {
		fmt.Fprintf(os.Stderr, "testing: invalid regexp in -test.list (%q): %s\n", *matchList, err)
		os.Exit(1)
	}

	for _, test := range tests {
		if ok, _ := matchString(*matchList, test.Name); ok {
			fmt.Println(test.Name)
		}
	}
	for _, bench := range benchmarks {
		if ok, _ := matchString(*matchList, bench.Name); ok {
			fmt.Println(bench.Name)
		}
	}
	for _, fuzzTarget := range fuzzTargets {
		if ok, _ := matchString(*matchList, fuzzTarget.Name); ok {
			fmt.Println(fuzzTarget.Name)
		}
	}
	for _, example := range examples {
		if ok, _ := matchString(*matchList, example.Name); ok {
			fmt.Println(example.Name)
		}
	}
}

// RunTests is an internal function but exported because it is cross-package;
// it is part of the implementation of the "go test" command.
func RunTests(matchString func(pat, str string) (bool, error), tests []InternalTest) (ok bool) {
	var deadline time.Time
	if *timeout > 0 {
		deadline = time.Now().Add(*timeout)
	}
	ran, ok := runTests(matchString, tests, deadline)
	if !ran && !haveExamples {
		fmt.Fprintln(os.Stderr, "testing: warning: no tests to run")
	}
	return ok
}

func runTests(matchString func(pat, str string) (bool, error), tests []InternalTest, deadline time.Time) (ran, ok bool) {
	ok = true
	for _, procs := range cpuList {
		runtime.GOMAXPROCS(procs)
		for i := uint(0); i < *count; i++ {
			if shouldFailFast() {
				break
			}
			if i > 0 && !ran {
				// There were no tests to run on the first
				// iteration. This won't change, so no reason
				// to keep trying.
				break
			}
			ctx, cancelCtx := context.WithCancel(context.Background())
			tstate := newTestState(*parallel, newMatcher(matchString, *match, "-test.run", *skip))
			tstate.deadline = deadline
			t := &T{
				common: common{
					signal:    make(chan bool, 1),
					barrier:   make(chan bool),
					w:         os.Stdout,
					ctx:       ctx,
					cancelCtx: cancelCtx,
				},
				tstate: tstate,
			}
			if Verbose() {
				t.chatty = newChattyPrinter(t.w)
			}
			tRunner(t, func(t *T) {
				for _, test := range tests {
					t.Run(test.Name, test.F)
				}
			})
			select {
			case <-t.signal:
			default:
				panic("internal error: tRunner exited without sending on t.signal")
			}
			ok = ok && !t.Failed()
			ran = ran || t.ran
		}
	}
	return ran, ok
}

// before runs before all testing.
func (m *M) before() {
	if *memProfileRate > 0 {
		runtime.MemProfileRate = *memProfileRate
	}
	if *cpuProfile != "" {
		f, err := os.Create(toOutputDir(*cpuProfile))
		if err != nil {
			fmt.Fprintf(os.Stderr, "testing: %s\n", err)
			return
		}
		if err := m.deps.StartCPUProfile(f); err != nil {
			fmt.Fprintf(os.Stderr, "testing: can't start cpu profile: %s\n", err)
			f.Close()
			return
		}
		// Could save f so after can call f.Close; not worth the effort.
	}
	if *traceFile != "" {
		f, err := os.Create(toOutputDir(*traceFile))
		if err != nil {
			fmt.Fprintf(os.Stderr, "testing: %s\n", err)
			return
		}
		if err := trace.Start(f); err != nil {
			fmt.Fprintf(os.Stderr, "testing: can't start tracing: %s\n", err)
			f.Close()
			return
		}
		// Could save f so after can call f.Close; not worth the effort.
	}
	if *blockProfile != "" && *blockProfileRate >= 0 {
		runtime.SetBlockProfileRate(*blockProfileRate)
	}
	if *mutexProfile != "" && *mutexProfileFraction >= 0 {
		runtime.SetMutexProfileFraction(*mutexProfileFraction)
	}
	if *coverProfile != "" && CoverMode() == "" {
		fmt.Fprintf(os.Stderr, "testing: cannot use -test.coverprofile because test binary was not built with coverage enabled\n")
		os.Exit(2)
	}
	if *gocoverdir != "" && CoverMode() == "" {
		fmt.Fprintf(os.Stderr, "testing: cannot use -test.gocoverdir because test binary was not built with coverage enabled\n")
		os.Exit(2)
	}
	if *testlog != "" {
		// Note: Not using toOutputDir.
		// This file is for use by cmd/go, not users.
		var f *os.File
		var err error
		if m.numRun == 1 {
			f, err = os.Create(*testlog)
		} else {
			f, err = os.OpenFile(*testlog, os.O_WRONLY, 0)
			if err == nil {
				f.Seek(0, io.SeekEnd)
			}
		}
		if err != nil {
			fmt.Fprintf(os.Stderr, "testing: %s\n", err)
			os.Exit(2)
		}
		m.deps.StartTestLog(f)
		testlogFile = f
	}
	if *panicOnExit0 {
		m.deps.SetPanicOnExit0(true)
	}
}

// after runs after all testing.
func (m *M) after() {
	m.afterOnce.Do(func() {
		m.writeProfiles()
	})

	// Restore PanicOnExit0 after every run, because we set it to true before
	// every run. Otherwise, if m.Run is called multiple times the behavior of
	// os.Exit(0) will not be restored after the second run.
	if *panicOnExit0 {
		m.deps.SetPanicOnExit0(false)
	}
}

func (m *M) writeProfiles() {
	if *testlog != "" {
		if err := m.deps.StopTestLog(); err != nil {
			fmt.Fprintf(os.Stderr, "testing: can't write %s: %s\n", *testlog, err)
			os.Exit(2)
		}
		if err := testlogFile.Close(); err != nil {
			fmt.Fprintf(os.Stderr, "testing: can't write %s: %s\n", *testlog, err)
			os.Exit(2)
		}
	}
	if *cpuProfile != "" {
		m.deps.StopCPUProfile() // flushes profile to disk
	}
	if *traceFile != "" {
		trace.Stop() // flushes trace to disk
	}
	if *memProfile != "" {
		f, err := os.Create(toOutputDir(*memProfile))
		if err != nil {
			fmt.Fprintf(os.Stderr, "testing: %s\n", err)
			os.Exit(2)
		}
		runtime.GC() // materialize all statistics
		if err = m.deps.WriteProfileTo("allocs", f, 0); err != nil {
			fmt.Fprintf(os.Stderr, "testing: can't write %s: %s\n", *memProfile, err)
			os.Exit(2)
		}
		f.Close()
	}
	if *blockProfile != "" && *blockProfileRate >= 0 {
		f, err := os.Create(toOutputDir(*blockProfile))
		if err != nil {
			fmt.Fprintf(os.Stderr, "testing: %s\n", err)
			os.Exit(2)
		}
		if err = m.deps.WriteProfileTo("block", f, 0); err != nil {
			fmt.Fprintf(os.Stderr, "testing: can't write %s: %s\n", *blockProfile, err)
			os.Exit(2)
		}
		f.Close()
	}
	if *mutexProfile != "" && *mutexProfileFraction >= 0 {
		f, err := os.Create(toOutputDir(*mutexProfile))
		if err != nil {
			fmt.Fprintf(os.Stderr, "testing: %s\n", err)
			os.Exit(2)
		}
		if err = m.deps.WriteProfileTo("mutex", f, 0); err != nil {
			fmt.Fprintf(os.Stderr, "testing: can't write %s: %s\n", *mutexProfile, err)
			os.Exit(2)
		}
		f.Close()
	}
	if CoverMode() != "" {
		coverReport()
	}
}

// toOutputDir returns the file name relocated, if required, to outputDir.
// Simple implementation to avoid pulling in path/filepath.
func toOutputDir(path string) string {
	if *outputDir == "" || path == "" {
		return path
	}
	// On Windows, it's clumsy, but we can be almost always correct
	// by just looking for a drive letter and a colon.
	// Absolute paths always have a drive letter (ignoring UNC).
	// Problem: if path == "C:A" and outputdir == "C:\Go" it's unclear
	// what to do, but even then path/filepath doesn't help.
	// TODO: Worth doing better? Probably not, because we're here only
	// under the management of go test.
	if runtime.GOOS == "windows" && len(path) >= 2 {
		letter, colon := path[0], path[1]
		if ('a' <= letter && letter <= 'z' || 'A' <= letter && letter <= 'Z') && colon == ':' {
			// If path starts with a drive letter we're stuck with it regardless.
			return path
		}
	}
	if os.IsPathSeparator(path[0]) {
		return path
	}
	return fmt.Sprintf("%s%c%s", *outputDir, os.PathSeparator, path)
}

// startAlarm starts an alarm if requested.
func (m *M) startAlarm() time.Time {
	if *timeout <= 0 {
		return time.Time{}
	}

	deadline := time.Now().Add(*timeout)
	m.timer = time.AfterFunc(*timeout, func() {
		m.after()
		debug.SetTraceback("all")
		extra := ""

		if list := runningList(); len(list) > 0 {
			var b strings.Builder
			b.WriteString("\nrunning tests:")
			for _, name := range list {
				b.WriteString("\n\t")
				b.WriteString(name)
			}
			extra = b.String()
		}
		panic(fmt.Sprintf("test timed out after %v%s", *timeout, extra))
	})
	return deadline
}

// runningList returns the list of running tests.
func runningList() []string {
	var list []string
	running.Range(func(k, v any) bool {
		list = append(list, fmt.Sprintf("%s (%v)", k.(string), highPrecisionTimeSince(v.(highPrecisionTime)).Round(time.Second)))
		return true
	})
	slices.Sort(list)
	return list
}

// stopAlarm turns off the alarm.
func (m *M) stopAlarm() {
	if *timeout > 0 {
		m.timer.Stop()
	}
}

func parseCpuList() {
	for val := range strings.SplitSeq(*cpuListStr, ",") {
		val = strings.TrimSpace(val)
		if val == "" {
			continue
		}
		cpu, err := strconv.Atoi(val)
		if err != nil || cpu <= 0 {
			fmt.Fprintf(os.Stderr, "testing: invalid value %q for -test.cpu\n", val)
			os.Exit(1)
		}
		cpuList = append(cpuList, cpu)
	}
	if cpuList == nil {
		cpuList = append(cpuList, runtime.GOMAXPROCS(-1))
	}
}

func shouldFailFast() bool {
	return *failFast && numFailed.Load() > 0
}
