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// 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