2 files changed, 203 insertions, 0 deletions
diff --git a/src/internal/godebug/godebug.go b/src/internal/godebug/godebug.go
new file mode 100644
index 0000000..dbcd980
--- /dev/null
+++ b/src/internal/godebug/godebug.go
@@ -0,0 +1,164 @@
+// 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
+//			...
+//		}
+//		...
+//	}
+package godebug
+
+import (
+	"sync"
+	"sync/atomic"
+	_ "unsafe" // go:linkname
+)
+
+// A Setting is a single setting in the $GODEBUG environment variable.
+type Setting struct {
+	name  string
+	once  sync.Once
+	value *atomic.Pointer[string]
+}
+
+// New returns a new Setting for the $GODEBUG setting with the given name.
+func New(name string) *Setting {
+	return &Setting{name: name}
+}
+
+// Name returns the name of the setting.
+func (s *Setting) Name() string {
+	return s.name
+}
+
+// String returns a printable form for the setting: name=value.
+func (s *Setting) String() string {
+	return s.name + "=" + s.Value()
+}
+
+// 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 string
+
+// 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() {
+		v, ok := cache.Load(s.name)
+		if !ok {
+			p := new(atomic.Pointer[string])
+			p.Store(&empty)
+			v, _ = cache.LoadOrStore(s.name, p)
+		}
+		s.value = v.(*atomic.Pointer[string])
+	})
+	return *s.value.Load()
+}
+
+// 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))
+
+func init() {
+	setUpdate(update)
+}
+
+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, v any) bool {
+		if !did[name.(string)] {
+			v.(*atomic.Pointer[string]).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.
+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, value := s[i+1:eq], s[eq+1:end]
+				if !did[name] {
+					did[name] = true
+					v, ok := cache.Load(name)
+					if !ok {
+						p := new(atomic.Pointer[string])
+						p.Store(&empty)
+						v, _ = cache.LoadOrStore(name, p)
+					}
+					v.(*atomic.Pointer[string]).Store(&value)
+				}
+			}
+			eq = -1
+			end = i
+		} else if s[i] == '=' {
+			eq = i
+		}
+	}
+}
diff --git a/src/internal/godebug/godebug_test.go b/src/internal/godebug/godebug_test.go
new file mode 100644
index 0000000..319229d
--- /dev/null
+++ b/src/internal/godebug/godebug_test.go
@@ -0,0 +1,39 @@
+// 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 (
+	. "internal/godebug"
+	"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)
+		}
+	}
+}