Source File
godebug.go
Belonging Package
internal/godebug
// 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 (
_ // 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 ( string) *Setting {
return &Setting{name: }
}
// Name returns the name of the setting.
func ( *Setting) () string {
return .name
}
// String returns a printable form for the setting: name=value.
func ( *Setting) () string {
return .name + "=" + .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 ( *Setting) () string {
.once.Do(func() {
, := cache.Load(.name)
if ! {
:= new(atomic.Pointer[string])
.Store(&empty)
, _ = cache.LoadOrStore(.name, )
}
.value = .(*atomic.Pointer[string])
})
return *.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( 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(, 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.
:= make(map[string]bool)
parse(, )
parse(, )
// Clear any cached values that are no longer present.
cache.Range(func(, any) bool {
if ![.(string)] {
.(*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( map[string]bool, 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.
:= len()
:= -1
for := - 1; >= -1; -- {
if == -1 || [] == ',' {
if >= 0 {
, := [+1:], [+1:]
if ![] {
[] = true
, := cache.Load()
if ! {
:= new(atomic.Pointer[string])
.Store(&empty)
, _ = cache.LoadOrStore(, )
}
.(*atomic.Pointer[string]).Store(&)
}
}
= -1
=
} else if [] == '=' {
=
}
}
}
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