internal/godebug: remove dependency on os

The immediate reason is that we want to use godebug from math/rand,
and math/rand importing godebug importing os causes an import cycle
in package testing.

More generally, the new approach to backward compatibility outlined
in discussion #55090 will require using this package from other similarly
sensitive places, perhaps even package os itself. Best to remove all
dependencies.

Preparation for #54880.

Change-Id: Ia01657a2d90e707a8121a336c9db3b7247c0198f
Reviewed-on: https://go-review.googlesource.com/c/go/+/439418
Auto-Submit: Russ Cox <rsc@golang.org>
Run-TryBot: Russ Cox <rsc@golang.org>
Reviewed-by: Austin Clements <austin@google.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>

src/go/build/deps_test.go

@@ -52,9 +52,13 @@ var depsRules = `
 	internal/goarch, unsafe
 	< internal/abi;
 
+	unsafe
+	< internal/godebug;
+
 	# RUNTIME is the core runtime group of packages, all of them very light-weight.
 	internal/abi, internal/cpu, internal/goarch,
-	internal/coverage/rtcov, internal/goexperiment, internal/goos, unsafe
+	internal/coverage/rtcov, internal/goexperiment,
+	internal/goos, internal/godebug, unsafe
 	< internal/bytealg
 	< internal/itoa
 	< internal/unsafeheader
@@ -154,8 +158,6 @@ var depsRules = `
 	< path/filepath
 	< io/ioutil;
 
-	os < internal/godebug;
-
 	path/filepath, internal/godebug < os/exec;
 
 	io/ioutil, os/exec, os/signal

src/internal/godebug/export_test.go

@@ -0,0 +1,7 @@
+// Copyright 2022 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
+
+var Xget = get

src/internal/godebug/godebug.go

@@ -5,11 +5,14 @@
 // Package godebug parses the GODEBUG environment variable.
 package godebug
 
-import "os"
+import _ "unsafe" // go:linkname
+
+//go:linkname getGODEBUG
+func getGODEBUG() string
 
 // Get returns the value for the provided GODEBUG key.
 func Get(key string) string {
-	return get(os.Getenv("GODEBUG"), key)
+	return get(getGODEBUG(), key)
 }
 
 // get returns the value part of key=value in s (a GODEBUG value).

src/internal/godebug/godebug_test.go

@@ -2,9 +2,12 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-package godebug
+package godebug_test
 
-import "testing"
+import (
+	. "internal/godebug"
+	"testing"
+)
 
 func TestGet(t *testing.T) {
 	tests := []struct {
@@ -26,7 +29,7 @@ func TestGet(t *testing.T) {
 		{"foo=bar,baz", "loooooooong", ""},
 	}
 	for _, tt := range tests {
-		got := get(tt.godebug, tt.key)
+		got := Xget(tt.godebug, tt.key)
 		if got != tt.want {
 			t.Errorf("get(%q, %q) = %q; want %q", tt.godebug, tt.key, got, tt.want)
 		}

src/runtime/env_posix.go

@@ -2,8 +2,6 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-//go:build unix || (js && wasm) || windows || plan9
-
 package runtime
 
 import "unsafe"
@@ -48,10 +46,7 @@ var _cgo_setenv unsafe.Pointer   // pointer to C function
 var _cgo_unsetenv unsafe.Pointer // pointer to C function
 
 // Update the C environment if cgo is loaded.
-// Called from syscall.Setenv.
-//
-//go:linkname syscall_setenv_c syscall.setenv_c
-func syscall_setenv_c(k string, v string) {
+func setenv_c(k string, v string) {
 	if _cgo_setenv == nil {
 		return
 	}
@@ -60,10 +55,7 @@ func syscall_setenv_c(k string, v string) {
 }
 
 // Update the C environment if cgo is loaded.
-// Called from syscall.unsetenv.
-//
-//go:linkname syscall_unsetenv_c syscall.unsetenv_c
-func syscall_unsetenv_c(k string) {
+func unsetenv_c(k string) {
 	if _cgo_unsetenv == nil {
 		return
 	}

src/runtime/runtime.go

@@ -65,3 +65,31 @@ func os_runtime_args() []string { return append([]string{}, argslice...) }
 func syscall_Exit(code int) {
 	exit(int32(code))
 }
+
+var godebugenv atomic.Pointer[string] // set by parsedebugvars
+
+//go:linkname godebug_getGODEBUG internal/godebug.getGODEBUG
+func godebug_getGODEBUG() string {
+	if p := godebugenv.Load(); p != nil {
+		return *p
+	}
+	return ""
+}
+
+//go:linkname syscall_runtimeSetenv syscall.runtimeSetenv
+func syscall_runtimeSetenv(key, value string) {
+	setenv_c(key, value)
+	if key == "GODEBUG" {
+		p := new(string)
+		*p = value
+		godebugenv.Store(p)
+	}
+}
+
+//go:linkname syscall_runtimeUnsetenv syscall.runtimeUnsetenv
+func syscall_runtimeUnsetenv(key string) {
+	unsetenv_c(key)
+	if key == "GODEBUG" {
+		godebugenv.Store(nil)
+	}
+}

src/runtime/runtime1.go

@@ -355,6 +355,8 @@ var dbgvars = []dbgVar{
 	{"adaptivestackstart", &debug.adaptivestackstart},
 }
 
+var globalGODEBUG string
+
 func parsedebugvars() {
 	// defaults
 	debug.cgocheck = 1
@@ -372,7 +374,9 @@ func parsedebugvars() {
 		debug.madvdontneed = 1
 	}
 
-	for p := gogetenv("GODEBUG"); p != ""; {
+	globalGODEBUG = gogetenv("GODEBUG")
+	godebugenv.StoreNoWB(&globalGODEBUG)
+	for p := globalGODEBUG; p != ""; {
 		field := ""
 		i := bytealg.IndexByteString(p, ',')
 		if i < 0 {

src/syscall/env_unix.go

@@ -31,11 +31,6 @@ var (
 
 func runtime_envs() []string // in package runtime
 
-// setenv_c and unsetenv_c are provided by the runtime but are no-ops
-// if cgo isn't loaded.
-func setenv_c(k, v string)
-func unsetenv_c(k string)
-
 func copyenv() {
 	env = make(map[string]int)
 	for i, s := range envs {
@@ -67,7 +62,7 @@ func Unsetenv(key string) error {
 		envs[i] = ""
 		delete(env, key)
 	}
-	unsetenv_c(key)
+	runtimeUnsetenv(key)
 	return nil
 }
 
@@ -124,7 +119,7 @@ func Setenv(key, value string) error {
 		envs = append(envs, kv)
 	}
 	env[key] = i
-	setenv_c(key, value)
+	runtimeSetenv(key, value)
 	return nil
 }
 
@@ -135,7 +130,7 @@ func Clearenv() {
 	defer envLock.Unlock()
 
 	for k := range env {
-		unsetenv_c(k)
+		runtimeUnsetenv(k)
 	}
 	env = make(map[string]int)
 	envs = []string{}

src/syscall/env_windows.go

@@ -42,6 +42,7 @@ func Setenv(key, value string) error {
 	if e != nil {
 		return e
 	}
+	runtimeSetenv(key, value)
 	return nil
 }
 
@@ -50,7 +51,12 @@ func Unsetenv(key string) error {
 	if err != nil {
 		return err
 	}
-	return SetEnvironmentVariable(keyp, nil)
+	e := SetEnvironmentVariable(keyp, nil)
+	if e != nil {
+		return e
+	}
+	runtimeUnsetenv(key)
+	return nil
 }
 
 func Clearenv() {

src/syscall/syscall.go

@@ -100,3 +100,7 @@ func (tv *Timeval) Nano() int64 {
 
 func Getpagesize() int
 func Exit(code int)
+
+// runtimeSetenv and runtimeUnsetenv are provided by the runtime.
+func runtimeSetenv(k, v string)
+func runtimeUnsetenv(k string)