From cff4493ca0ff7435a533be27b89ea0fe7daf9877 Mon Sep 17 00:00:00 2001
From: Dan Kegel <dkegel@fastly.com>
Date: Sun, 5 Dec 2021 13:24:00 -0800
Subject: [PATCH] os: implement and smoketest os.Setenv

---
 src/os/env.go                    | 12 ++++++++++
 src/os/env_test.go               | 38 ++++++++++++++++++++++++++++++++
 src/runtime/env_linux.go         | 27 +++++++++++++++++++++++
 src/syscall/syscall_libc.go      | 14 ++++++++++++
 src/syscall/syscall_nonhosted.go |  5 +++++
 5 files changed, 96 insertions(+)
 create mode 100644 src/os/env_test.go
 create mode 100644 src/runtime/env_linux.go

diff --git a/src/os/env.go b/src/os/env.go
index 585650e7..d7bae9bb 100644
--- a/src/os/env.go
+++ b/src/os/env.go
@@ -9,6 +9,18 @@ func Getenv(key string) string {
 	return v
 }
 
+func Setenv(key, value string) error {
+	err := syscall.Setenv(key, value)
+	if err != nil {
+		return NewSyscallError("setenv", err)
+	}
+	return nil
+}
+
+func Unsetenv(_ string) error {
+	return ErrNotImplemented
+}
+
 func LookupEnv(key string) (string, bool) {
 	return syscall.Getenv(key)
 }
diff --git a/src/os/env_test.go b/src/os/env_test.go
new file mode 100644
index 00000000..ae7ed718
--- /dev/null
+++ b/src/os/env_test.go
@@ -0,0 +1,38 @@
+// Copyright 2010 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 os_test
+
+import (
+	. "os"
+	"reflect"
+	"testing"
+)
+
+func TestConsistentEnviron(t *testing.T) {
+	e0 := Environ()
+	for i := 0; i < 10; i++ {
+		e1 := Environ()
+		if !reflect.DeepEqual(e0, e1) {
+			t.Fatalf("environment changed")
+		}
+	}
+}
+
+func TestLookupEnv(t *testing.T) {
+	const smallpox = "SMALLPOX"      // No one has smallpox.
+	value, ok := LookupEnv(smallpox) // Should not exist.
+	if ok || value != "" {
+		t.Fatalf("%s=%q", smallpox, value)
+	}
+	defer Unsetenv(smallpox)
+	err := Setenv(smallpox, "virus")
+	if err != nil {
+		t.Fatalf("failed to release smallpox virus")
+	}
+	_, ok = LookupEnv(smallpox)
+	if !ok {
+		t.Errorf("smallpox release failed; world remains safe but LookupEnv is broken")
+	}
+}
diff --git a/src/runtime/env_linux.go b/src/runtime/env_linux.go
new file mode 100644
index 00000000..3f7a05b4
--- /dev/null
+++ b/src/runtime/env_linux.go
@@ -0,0 +1,27 @@
+// +build linux
+
+package runtime
+
+// Update the C environment if cgo is loaded.
+// Called from syscall.Setenv.
+//go:linkname syscall_setenv_c syscall.setenv_c
+func syscall_setenv_c(key string, val string) {
+	keydata := cstring(key)
+	valdata := cstring(val)
+	// ignore any errors
+	libc_setenv(&keydata[0], &valdata[0], 1)
+	return
+}
+
+// cstring converts a Go string to a C string.
+// borrowed from syscall
+func cstring(s string) []byte {
+	data := make([]byte, len(s)+1)
+	copy(data, s)
+	// final byte should be zero from the initial allocation
+	return data
+}
+
+// int setenv(const char *name, const char *val, int replace);
+//export setenv
+func libc_setenv(name *byte, val *byte, replace int32) int32
diff --git a/src/syscall/syscall_libc.go b/src/syscall/syscall_libc.go
index 4451cf76..fa433f34 100644
--- a/src/syscall/syscall_libc.go
+++ b/src/syscall/syscall_libc.go
@@ -132,6 +132,16 @@ func Getenv(key string) (value string, found bool) {
 	}
 }
 
+func Setenv(key, val string) (err error) {
+	keydata := cstring(key)
+	valdata := cstring(val)
+	errCode := libc_setenv(&keydata[0], &valdata[0], 1)
+	if errCode != 0 {
+		err = getErrno()
+	}
+	return
+}
+
 func Mmap(fd int, offset int64, length int, prot int, flags int) (data []byte, err error) {
 	addr := libc_mmap(nil, uintptr(length), int32(prot), int32(flags), int32(fd), uintptr(offset))
 	if addr == unsafe.Pointer(^uintptr(0)) {
@@ -192,6 +202,10 @@ func libc_write(fd int32, buf *byte, count uint) int
 //export getenv
 func libc_getenv(name *byte) *byte
 
+// int setenv(const char *name, const char *val, int replace);
+//export setenv
+func libc_setenv(name *byte, val *byte, replace int32) int32
+
 // ssize_t read(int fd, void *buf, size_t count);
 //export read
 func libc_read(fd int32, buf *byte, count uint) int
diff --git a/src/syscall/syscall_nonhosted.go b/src/syscall/syscall_nonhosted.go
index 0fd7ffc5..21ad3d63 100644
--- a/src/syscall/syscall_nonhosted.go
+++ b/src/syscall/syscall_nonhosted.go
@@ -67,6 +67,11 @@ func Getenv(key string) (value string, found bool) {
 	return "", false
 }
 
+func Setenv(key, val string) (err error) {
+	// stub for now
+	return ENOSYS
+}
+
 func Environ() []string {
 	env := runtime_envs()
 	envCopy := make([]string, len(env))