os: optimise cgo clearenv

src/runtime/cgo/clearenv.go

@@ -0,0 +1,15 @@
+// Copyright 2025 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 +1,18 @@
+// Copyright 2025 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 2025 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 2025 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

@@ -126,9 +126,8 @@ func Clearenv() {
 	envLock.Lock()
 	defer envLock.Unlock()
 
-	for k := range env {
-		runtimeUnsetenv(k)
-	}
+	runtimeClearenv(env)
+
 	env = make(map[string]int)
 	envs = []string{}
 }

src/syscall/env_unix_test.go

@@ -0,0 +1,160 @@
+// Copyright 2025 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 unix || (js && wasm) || plan9 || wasip1
+
+package syscall_test
+
+import (
+	"fmt"
+	"strconv"
+	"strings"
+	"syscall"
+	"testing"
+)
+
+type env struct {
+	name, val string
+}
+
+func genDummyEnv(tb testing.TB, size int) []env {
+	tb.Helper()
+	envList := make([]env, size)
+	for idx := range size {
+		envList[idx] = env{
+			name: fmt.Sprintf("DUMMY_VAR_%d", idx),
+			val:  fmt.Sprintf("val-%d", idx*100),
+		}
+	}
+	return envList
+}
+
+func setDummyEnv(tb testing.TB, envList []env) {
+	tb.Helper()
+	for _, env := range envList {
+		if err := syscall.Setenv(env.name, env.val); err != nil {
+			tb.Fatalf("setenv %s=%q failed: %v", env.name, env.val, err)
+		}
+	}
+}
+
+func setupEnvCleanup(tb testing.TB) {
+	tb.Helper()
+	originalEnv := map[string]string{}
+	for _, env := range syscall.Environ() {
+		fields := strings.SplitN(env, "=", 2)
+		name, val := fields[0], fields[1]
+		originalEnv[name] = val
+	}
+	tb.Cleanup(func() {
+		syscall.Clearenv()
+		for name, val := range originalEnv {
+			if err := syscall.Setenv(name, val); err != nil {
+				tb.Fatalf("could not reset env %s=%q: %v", name, val, err)
+			}
+		}
+	})
+}
+
+func TestClearenv(t *testing.T) {
+	setupEnvCleanup(t)
+
+	t.Run("DummyVars-4096", func(t *testing.T) {
+		envList := genDummyEnv(t, 4096)
+		setDummyEnv(t, envList)
+
+		if env := syscall.Environ(); len(env) < 4096 {
+			t.Fatalf("env is missing dummy variables: %v", env)
+		}
+		for idx := range 4096 {
+			name := fmt.Sprintf("DUMMY_VAR_%d", idx)
+			if _, ok := syscall.Getenv(name); !ok {
+				t.Fatalf("env is missing dummy variable %s", name)
+			}
+		}
+
+		syscall.Clearenv()
+
+		if env := syscall.Environ(); len(env) != 0 {
+			t.Fatalf("clearenv should've cleared all variables: %v still set", env)
+		}
+		for idx := range 4096 {
+			name := fmt.Sprintf("DUMMY_VAR_%d", idx)
+			if val, ok := syscall.Getenv(name); ok {
+				t.Fatalf("clearenv should've cleared all variables: %s=%q still set", name, val)
+			}
+		}
+	})
+
+	// Test that GODEBUG getting cleared by Clearenv also resets the behaviour.
+	t.Run("GODEBUG", func(t *testing.T) {
+		envList := genDummyEnv(t, 100)
+		setDummyEnv(t, envList)
+
+		doNilPanic := func() (ret any) {
+			defer func() {
+				ret = recover()
+			}()
+			panic(nil)
+			return "should not return"
+		}
+
+		// Allow panic(nil).
+		if err := syscall.Setenv("GODEBUG", "panicnil=1"); err != nil {
+			t.Fatalf("setenv GODEBUG=panicnil=1 failed: %v", err)
+		}
+
+		got := doNilPanic()
+		if got != nil {
+			t.Fatalf("GODEBUG=panicnil=1 did not allow for nil panic: got %#v", got)
+		}
+
+		// Disallow panic(nil).
+		syscall.Clearenv()
+
+		if env := syscall.Environ(); len(env) != 0 {
+			t.Fatalf("clearenv should've cleared all variables: %v still set", env)
+		}
+
+		got = doNilPanic()
+		if got == nil {
+			t.Fatalf("GODEBUG=panicnil=1 being unset didn't reset panicnil behaviour")
+		}
+		if godebug, ok := syscall.Getenv("GODEBUG"); ok {
+			t.Fatalf("GODEBUG still exists in environment despite being unset: GODEBUG=%q", godebug)
+		}
+	})
+}
+
+func BenchmarkClearenv(b *testing.B) {
+	setupEnvCleanup(b)
+	b.ResetTimer()
+	for _, size := range []int{100, 1000, 10000} {
+		b.Run(strconv.Itoa(size), func(b *testing.B) {
+			envList := genDummyEnv(b, size)
+			b.ResetTimer()
+			for b.Loop() {
+				// Ideally we would use StopTimer to hide the environment
+				// variable setup cost but that causes go test to loop
+				// seemingly infinitely....
+				setDummyEnv(b, envList)
+				syscall.Clearenv()
+			}
+		})
+	}
+}
+
+func BenchmarkSetenv(b *testing.B) {
+	setupEnvCleanup(b)
+	b.ResetTimer()
+	for _, size := range []int{100, 1000, 10000} {
+		b.Run(strconv.Itoa(size), func(b *testing.B) {
+			envList := genDummyEnv(b, size)
+			b.ResetTimer()
+			for b.Loop() {
+				setDummyEnv(b, envList)
+			}
+		})
+	}
+}

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)