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>

1 files changed, 28 insertions, 0 deletions

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)
+	}
+}