os: optimise cgo clearenv

src/runtime/cgo/clearenv.go

@@ -0,0 +1,15 @@
+// Copyright 2024 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.
+
+//go:build linux
+
+package cgo
+
+import _ "unsafe" // for go:linkname
+
+//go:cgo_import_static x_cgo_clearenv
+//go:linkname x_cgo_clearenv x_cgo_clearenv
+//go:linkname _cgo_clearenv runtime._cgo_clearenv
+var x_cgo_clearenv byte
+var _cgo_clearenv = &x_cgo_clearenv

src/runtime/cgo/gcc_clearenv.c

@@ -0,0 +1,19 @@
+// Copyright 2024 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.
+
+//go:build linux
+
+#include "libcgo.h"
+
+#include <stdlib.h>
+
+/* Stub for calling clearenv */
+void
+x_cgo_clearenv(void **_unused)
+{
+	_cgo_tsan_acquire();
+	clearenv();
+	_cgo_tsan_release();
+}
+

src/runtime/runtime_clearenv.go

@@ -0,0 +1,29 @@
+// Copyright 2024 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.
+
+//go:build linux
+
+package runtime
+
+import "unsafe"
+
+var _cgo_clearenv unsafe.Pointer // pointer to C function
+
+// Clear the C environment if cgo is loaded.
+func clearenv_c() {
+	if _cgo_clearenv == nil {
+		return
+	}
+	asmcgocall(_cgo_clearenv, nil)
+}
+
+//go:linkname syscall_runtimeClearenv syscall.runtimeClearenv
+func syscall_runtimeClearenv(env map[string]int) {
+	clearenv_c()
+	// Did we just unset GODEBUG?
+	if _, ok := env["GODEBUG"]; ok {
+		godebugEnv.Store(nil)
+		godebugNotify(true)
+	}
+}

src/runtime/runtime_noclearenv.go

@@ -0,0 +1,18 @@
+// Copyright 2024 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.
+
+//go:build !linux
+
+package runtime
+
+import _ "unsafe" // for go:linkname
+
+//go:linkname syscall_runtimeClearenv syscall.runtimeClearenv
+func syscall_runtimeClearenv(env map[string]int) {
+	// The system doesn't have clearenv(3) so emulate it by unsetting all of
+	// the variables manually.
+	for k := range env {
+		syscall_runtimeUnsetenv(k)
+	}
+}

src/syscall/env_unix.go

@@ -129,9 +129,8 @@ func Clearenv() {
 	envLock.Lock()
 	defer envLock.Unlock()
 
-	for k := range env {
-		runtimeUnsetenv(k)
-	}
+	runtimeClearenv(env)
+
 	env = make(map[string]int)
 	envs = []string{}
 }

src/syscall/syscall.go

@@ -104,3 +104,7 @@ func Exit(code int)
 // runtimeSetenv and runtimeUnsetenv are provided by the runtime.
 func runtimeSetenv(k, v string)
 func runtimeUnsetenv(k string)
+
+// runtimeClearenv is provided by the runtime (on platforms without
+// clearenv(3), it is just a wrapper around runtimeUnsetenv).
+func runtimeClearenv(env map[string]int)