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, 18 insertions, 5 deletions

diff --git a/src/internal/godebug/export_test.go b/src/internal/godebug/export_test.go
new file mode 100644
index 0000000000..e84d9a9912
--- /dev/null
+++ b/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
diff --git a/src/internal/godebug/godebug.go b/src/internal/godebug/godebug.go
index ac434e5fd8..65a8c4e305 100644
--- a/src/internal/godebug/godebug.go
+++ b/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).
diff --git a/src/internal/godebug/godebug_test.go b/src/internal/godebug/godebug_test.go
index 41b9117b73..d7a2a7a8d8 100644
--- a/src/internal/godebug/godebug_test.go
+++ b/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)
 		}