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>

3 files changed, 35 insertions, 11 deletions

diff --git a/src/runtime/env_posix.go b/src/runtime/env_posix.go
index 94a19d80d8..0eb4f0d7a3 100644
--- a/src/runtime/env_posix.go
+++ b/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
 	}
diff --git a/src/runtime/runtime.go b/src/runtime/runtime.go
index 50f68a327c..25b714de4e 100644
--- a/src/runtime/runtime.go
+++ b/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)
+	}
+}
diff --git a/src/runtime/runtime1.go b/src/runtime/runtime1.go
index b0a458d187..a29608329c 100644
--- a/src/runtime/runtime1.go
+++ b/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 {