2 files changed, 452 insertions, 0 deletions
diff --git a/src/internal/godebug/godebug.go b/src/internal/godebug/godebug.go
new file mode 100644
index 0000000..36bfeac
--- /dev/null
+++ b/src/internal/godebug/godebug.go
@@ -0,0 +1,290 @@
+// Copyright 2021 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 godebug makes the settings in the $GODEBUG environment variable
+// available to other packages. These settings are often used for compatibility
+// tweaks, when we need to change a default behavior but want to let users
+// opt back in to the original. For example GODEBUG=http2server=0 disables
+// HTTP/2 support in the net/http server.
+//
+// In typical usage, code should declare a Setting as a global
+// and then call Value each time the current setting value is needed:
+//
+//	var http2server = godebug.New("http2server")
+//
+//	func ServeConn(c net.Conn) {
+//		if http2server.Value() == "0" {
+//			disallow HTTP/2
+//			...
+//		}
+//		...
+//	}
+//
+// Each time a non-default setting causes a change in program behavior,
+// code should call [Setting.IncNonDefault] to increment a counter that can
+// be reported by [runtime/metrics.Read].
+// Note that counters used with IncNonDefault must be added to
+// various tables in other packages. See the [Setting.IncNonDefault]
+// documentation for details.
+package godebug
+
+// Note: Be careful about new imports here. Any package
+// that internal/godebug imports cannot itself import internal/godebug,
+// meaning it cannot introduce a GODEBUG setting of its own.
+// We keep imports to the absolute bare minimum.
+import (
+	"internal/bisect"
+	"internal/godebugs"
+	"sync"
+	"sync/atomic"
+	"unsafe"
+	_ "unsafe" // go:linkname
+)
+
+// A Setting is a single setting in the $GODEBUG environment variable.
+type Setting struct {
+	name string
+	once sync.Once
+	*setting
+}
+
+type setting struct {
+	value          atomic.Pointer[value]
+	nonDefaultOnce sync.Once
+	nonDefault     atomic.Uint64
+	info           *godebugs.Info
+}
+
+type value struct {
+	text   string
+	bisect *bisect.Matcher
+}
+
+// New returns a new Setting for the $GODEBUG setting with the given name.
+//
+// GODEBUGs meant for use by end users must be listed in ../godebugs/table.go,
+// which is used for generating and checking various documentation.
+// If the name is not listed in that table, New will succeed but calling Value
+// on the returned Setting will panic.
+// To disable that panic for access to an undocumented setting,
+// prefix the name with a #, as in godebug.New("#gofsystrace").
+// The # is a signal to New but not part of the key used in $GODEBUG.
+func New(name string) *Setting {
+	return &Setting{name: name}
+}
+
+// Name returns the name of the setting.
+func (s *Setting) Name() string {
+	if s.name != "" && s.name[0] == '#' {
+		return s.name[1:]
+	}
+	return s.name
+}
+
+// Undocumented reports whether this is an undocumented setting.
+func (s *Setting) Undocumented() bool {
+	return s.name != "" && s.name[0] == '#'
+}
+
+// String returns a printable form for the setting: name=value.
+func (s *Setting) String() string {
+	return s.Name() + "=" + s.Value()
+}
+
+// IncNonDefault increments the non-default behavior counter
+// associated with the given setting.
+// This counter is exposed in the runtime/metrics value
+// /godebug/non-default-behavior/<name>:events.
+//
+// Note that Value must be called at least once before IncNonDefault.
+func (s *Setting) IncNonDefault() {
+	s.nonDefaultOnce.Do(s.register)
+	s.nonDefault.Add(1)
+}
+
+func (s *Setting) register() {
+	if s.info == nil || s.info.Opaque {
+		panic("godebug: unexpected IncNonDefault of " + s.name)
+	}
+	registerMetric("/godebug/non-default-behavior/"+s.Name()+":events", s.nonDefault.Load)
+}
+
+// cache is a cache of all the GODEBUG settings,
+// a locked map[string]*atomic.Pointer[string].
+//
+// All Settings with the same name share a single
+// *atomic.Pointer[string], so that when GODEBUG
+// changes only that single atomic string pointer
+// needs to be updated.
+//
+// A name appears in the values map either if it is the
+// name of a Setting for which Value has been called
+// at least once, or if the name has ever appeared in
+// a name=value pair in the $GODEBUG environment variable.
+// Once entered into the map, the name is never removed.
+var cache sync.Map // name string -> value *atomic.Pointer[string]
+
+var empty value
+
+// Value returns the current value for the GODEBUG setting s.
+//
+// Value maintains an internal cache that is synchronized
+// with changes to the $GODEBUG environment variable,
+// making Value efficient to call as frequently as needed.
+// Clients should therefore typically not attempt their own
+// caching of Value's result.
+func (s *Setting) Value() string {
+	s.once.Do(func() {
+		s.setting = lookup(s.Name())
+		if s.info == nil && !s.Undocumented() {
+			panic("godebug: Value of name not listed in godebugs.All: " + s.name)
+		}
+	})
+	v := *s.value.Load()
+	if v.bisect != nil && !v.bisect.Stack(&stderr) {
+		return ""
+	}
+	return v.text
+}
+
+// lookup returns the unique *setting value for the given name.
+func lookup(name string) *setting {
+	if v, ok := cache.Load(name); ok {
+		return v.(*setting)
+	}
+	s := new(setting)
+	s.info = godebugs.Lookup(name)
+	s.value.Store(&empty)
+	if v, loaded := cache.LoadOrStore(name, s); loaded {
+		// Lost race: someone else created it. Use theirs.
+		return v.(*setting)
+	}
+
+	return s
+}
+
+// setUpdate is provided by package runtime.
+// It calls update(def, env), where def is the default GODEBUG setting
+// and env is the current value of the $GODEBUG environment variable.
+// After that first call, the runtime calls update(def, env)
+// again each time the environment variable changes
+// (due to use of os.Setenv, for example).
+//
+//go:linkname setUpdate
+func setUpdate(update func(string, string))
+
+// registerMetric is provided by package runtime.
+// It forwards registrations to runtime/metrics.
+//
+//go:linkname registerMetric
+func registerMetric(name string, read func() uint64)
+
+// setNewIncNonDefault is provided by package runtime.
+// The runtime can do
+//
+//	inc := newNonDefaultInc(name)
+//
+// instead of
+//
+//	inc := godebug.New(name).IncNonDefault
+//
+// since it cannot import godebug.
+//
+//go:linkname setNewIncNonDefault
+func setNewIncNonDefault(newIncNonDefault func(string) func())
+
+func init() {
+	setUpdate(update)
+	setNewIncNonDefault(newIncNonDefault)
+}
+
+func newIncNonDefault(name string) func() {
+	s := New(name)
+	s.Value()
+	return s.IncNonDefault
+}
+
+var updateMu sync.Mutex
+
+// update records an updated GODEBUG setting.
+// def is the default GODEBUG setting for the running binary,
+// and env is the current value of the $GODEBUG environment variable.
+func update(def, env string) {
+	updateMu.Lock()
+	defer updateMu.Unlock()
+
+	// Update all the cached values, creating new ones as needed.
+	// We parse the environment variable first, so that any settings it has
+	// are already locked in place (did[name] = true) before we consider
+	// the defaults.
+	did := make(map[string]bool)
+	parse(did, env)
+	parse(did, def)
+
+	// Clear any cached values that are no longer present.
+	cache.Range(func(name, s any) bool {
+		if !did[name.(string)] {
+			s.(*setting).value.Store(&empty)
+		}
+		return true
+	})
+}
+
+// parse parses the GODEBUG setting string s,
+// which has the form k=v,k2=v2,k3=v3.
+// Later settings override earlier ones.
+// Parse only updates settings k=v for which did[k] = false.
+// It also sets did[k] = true for settings that it updates.
+// Each value v can also have the form v#pattern,
+// in which case the GODEBUG is only enabled for call stacks
+// matching pattern, for use with golang.org/x/tools/cmd/bisect.
+func parse(did map[string]bool, s string) {
+	// Scan the string backward so that later settings are used
+	// and earlier settings are ignored.
+	// Note that a forward scan would cause cached values
+	// to temporarily use the ignored value before being
+	// updated to the "correct" one.
+	end := len(s)
+	eq := -1
+	for i := end - 1; i >= -1; i-- {
+		if i == -1 || s[i] == ',' {
+			if eq >= 0 {
+				name, arg := s[i+1:eq], s[eq+1:end]
+				if !did[name] {
+					did[name] = true
+					v := &value{text: arg}
+					for j := 0; j < len(arg); j++ {
+						if arg[j] == '#' {
+							v.text = arg[:j]
+							v.bisect, _ = bisect.New(arg[j+1:])
+							break
+						}
+					}
+					lookup(name).value.Store(v)
+				}
+			}
+			eq = -1
+			end = i
+		} else if s[i] == '=' {
+			eq = i
+		}
+	}
+}
+
+type runtimeStderr struct{}
+
+var stderr runtimeStderr
+
+func (*runtimeStderr) Write(b []byte) (int, error) {
+	if len(b) > 0 {
+		write(2, unsafe.Pointer(&b[0]), int32(len(b)))
+	}
+	return len(b), nil
+}
+
+// Since we cannot import os or syscall, use the runtime's write function
+// to print to standard error.
+//
+//go:linkname write runtime.write
+func write(fd uintptr, p unsafe.Pointer, n int32) int32
diff --git a/src/internal/godebug/godebug_test.go b/src/internal/godebug/godebug_test.go
new file mode 100644
index 0000000..1ed0a36
--- /dev/null
+++ b/src/internal/godebug/godebug_test.go
@@ -0,0 +1,162 @@
+// Copyright 2021 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 godebug_test
+
+import (
+	"fmt"
+	. "internal/godebug"
+	"internal/race"
+	"internal/testenv"
+	"os"
+	"os/exec"
+	"reflect"
+	"runtime/metrics"
+	"sort"
+	"strings"
+	"testing"
+)
+
+func TestGet(t *testing.T) {
+	foo := New("#foo")
+	tests := []struct {
+		godebug string
+		setting *Setting
+		want    string
+	}{
+		{"", New("#"), ""},
+		{"", foo, ""},
+		{"foo=bar", foo, "bar"},
+		{"foo=bar,after=x", foo, "bar"},
+		{"before=x,foo=bar,after=x", foo, "bar"},
+		{"before=x,foo=bar", foo, "bar"},
+		{",,,foo=bar,,,", foo, "bar"},
+		{"foodecoy=wrong,foo=bar", foo, "bar"},
+		{"foo=", foo, ""},
+		{"foo", foo, ""},
+		{",foo", foo, ""},
+		{"foo=bar,baz", New("#loooooooong"), ""},
+	}
+	for _, tt := range tests {
+		t.Setenv("GODEBUG", tt.godebug)
+		got := tt.setting.Value()
+		if got != tt.want {
+			t.Errorf("get(%q, %q) = %q; want %q", tt.godebug, tt.setting.Name(), got, tt.want)
+		}
+	}
+}
+
+func TestMetrics(t *testing.T) {
+	const name = "http2client" // must be a real name so runtime will accept it
+
+	var m [1]metrics.Sample
+	m[0].Name = "/godebug/non-default-behavior/" + name + ":events"
+	metrics.Read(m[:])
+	if kind := m[0].Value.Kind(); kind != metrics.KindUint64 {
+		t.Fatalf("NonDefault kind = %v, want uint64", kind)
+	}
+
+	s := New(name)
+	s.Value()
+	s.IncNonDefault()
+	s.IncNonDefault()
+	s.IncNonDefault()
+	metrics.Read(m[:])
+	if kind := m[0].Value.Kind(); kind != metrics.KindUint64 {
+		t.Fatalf("NonDefault kind = %v, want uint64", kind)
+	}
+	if count := m[0].Value.Uint64(); count != 3 {
+		t.Fatalf("NonDefault value = %d, want 3", count)
+	}
+}
+
+// TestPanicNilRace checks for a race in the runtime caused by use of runtime
+// atomics (not visible to usual race detection) to install the counter for
+// non-default panic(nil) semantics.  For #64649.
+func TestPanicNilRace(t *testing.T) {
+	if !race.Enabled {
+		t.Skip("Skipping test intended for use with -race.")
+	}
+	if os.Getenv("GODEBUG") != "panicnil=1" {
+		cmd := testenv.CleanCmdEnv(testenv.Command(t, os.Args[0], "-test.run=^TestPanicNilRace$", "-test.v", "-test.parallel=2", "-test.count=1"))
+		cmd.Env = append(cmd.Env, "GODEBUG=panicnil=1")
+		out, err := cmd.CombinedOutput()
+		t.Logf("output:\n%s", out)
+
+		if err != nil {
+			t.Errorf("Was not expecting a crash")
+		}
+		return
+	}
+
+	test := func(t *testing.T) {
+		t.Parallel()
+		defer func() {
+			recover()
+		}()
+		panic(nil)
+	}
+	t.Run("One", test)
+	t.Run("Two", test)
+}
+
+func TestCmdBisect(t *testing.T) {
+	testenv.MustHaveGoBuild(t)
+	out, err := exec.Command("go", "run", "cmd/vendor/golang.org/x/tools/cmd/bisect", "GODEBUG=buggy=1#PATTERN", os.Args[0], "-test.run=^TestBisectTestCase$").CombinedOutput()
+	if err != nil {
+		t.Fatalf("exec bisect: %v\n%s", err, out)
+	}
+
+	var want []string
+	src, err := os.ReadFile("godebug_test.go")
+	for i, line := range strings.Split(string(src), "\n") {
+		if strings.Contains(line, "BISECT"+" "+"BUG") {
+			want = append(want, fmt.Sprintf("godebug_test.go:%d", i+1))
+		}
+	}
+	sort.Strings(want)
+
+	var have []string
+	for _, line := range strings.Split(string(out), "\n") {
+		if strings.Contains(line, "godebug_test.go:") {
+			have = append(have, line[strings.LastIndex(line, "godebug_test.go:"):])
+		}
+	}
+	sort.Strings(have)
+
+	if !reflect.DeepEqual(have, want) {
+		t.Errorf("bad bisect output:\nhave %v\nwant %v\ncomplete output:\n%s", have, want, string(out))
+	}
+}
+
+// This test does nothing by itself, but you can run
+//
+//	bisect 'GODEBUG=buggy=1#PATTERN' go test -run='^TestBisectTestCase$'
+//
+// to see that the GODEBUG bisect support is working.
+// TestCmdBisect above does exactly that.
+func TestBisectTestCase(t *testing.T) {
+	s := New("#buggy")
+	for i := 0; i < 10; i++ {
+		a := s.Value() == "1"
+		b := s.Value() == "1"
+		c := s.Value() == "1" // BISECT BUG
+		d := s.Value() == "1" // BISECT BUG
+		e := s.Value() == "1" // BISECT BUG
+
+		if a {
+			t.Log("ok")
+		}
+		if b {
+			t.Log("ok")
+		}
+		if c {
+			t.Error("bug")
+		}
+		if d &&
+			e {
+			t.Error("bug")
+		}
+	}
+}