From e4f2b9c0037a757ccb03fe2c8563e33a55e9901e Mon Sep 17 00:00:00 2001
From: Dan Kegel <dkegel@fastly.com>
Date: Sat, 11 Dec 2021 09:55:21 -0800
Subject: [PATCH] os: implement and smoketest os.Unsetenv

---
 src/os/env.go                    |  8 ++++++--
 src/os/env_test.go               | 26 ++++++++++++++++++++++++++
 src/runtime/env_linux.go         | 14 ++++++++++++++
 src/syscall/syscall_libc.go      | 13 +++++++++++++
 src/syscall/syscall_nonhosted.go |  5 +++++
 5 files changed, 64 insertions(+), 2 deletions(-)

diff --git a/src/os/env.go b/src/os/env.go
index d7bae9bb..f232e1b4 100644
--- a/src/os/env.go
+++ b/src/os/env.go
@@ -17,8 +17,12 @@ func Setenv(key, value string) error {
 	return nil
 }
 
-func Unsetenv(_ string) error {
-	return ErrNotImplemented
+func Unsetenv(key string) error {
+	err := syscall.Unsetenv(key)
+	if err != nil {
+		return NewSyscallError("unsetenv", err)
+	}
+	return nil
 }
 
 func LookupEnv(key string) (string, bool) {
diff --git a/src/os/env_test.go b/src/os/env_test.go
index ae7ed718..d2879c34 100644
--- a/src/os/env_test.go
+++ b/src/os/env_test.go
@@ -7,6 +7,7 @@ package os_test
 import (
 	. "os"
 	"reflect"
+	"strings"
 	"testing"
 )
 
@@ -20,6 +21,31 @@ func TestConsistentEnviron(t *testing.T) {
 	}
 }
 
+func TestUnsetenv(t *testing.T) {
+	const testKey = "GO_TEST_UNSETENV"
+	set := func() bool {
+		prefix := testKey + "="
+		for _, key := range Environ() {
+			if strings.HasPrefix(key, prefix) {
+				return true
+			}
+		}
+		return false
+	}
+	if err := Setenv(testKey, "1"); err != nil {
+		t.Fatalf("Setenv: %v", err)
+	}
+	if !set() {
+		t.Error("Setenv didn't set TestUnsetenv")
+	}
+	if err := Unsetenv(testKey); err != nil {
+		t.Fatalf("Unsetenv: %v", err)
+	}
+	if set() {
+		t.Fatal("Unsetenv didn't clear TestUnsetenv")
+	}
+}
+
 func TestLookupEnv(t *testing.T) {
 	const smallpox = "SMALLPOX"      // No one has smallpox.
 	value, ok := LookupEnv(smallpox) // Should not exist.
diff --git a/src/runtime/env_linux.go b/src/runtime/env_linux.go
index 3f7a05b4..5d2164e1 100644
--- a/src/runtime/env_linux.go
+++ b/src/runtime/env_linux.go
@@ -13,6 +13,16 @@ func syscall_setenv_c(key string, val string) {
 	return
 }
 
+// Update the C environment if cgo is loaded.
+// Called from syscall.Unsetenv.
+//go:linkname syscall_unsetenv_c syscall.unsetenv_c
+func syscall_unsetenv_c(key string) {
+	keydata := cstring(key)
+	// ignore any errors
+	libc_unsetenv(&keydata[0])
+	return
+}
+
 // cstring converts a Go string to a C string.
 // borrowed from syscall
 func cstring(s string) []byte {
@@ -25,3 +35,7 @@ func cstring(s string) []byte {
 // int setenv(const char *name, const char *val, int replace);
 //export setenv
 func libc_setenv(name *byte, val *byte, replace int32) int32
+
+// int unsetenv(const char *name);
+//export unsetenv
+func libc_unsetenv(name *byte) int32
diff --git a/src/syscall/syscall_libc.go b/src/syscall/syscall_libc.go
index fa433f34..05df16fd 100644
--- a/src/syscall/syscall_libc.go
+++ b/src/syscall/syscall_libc.go
@@ -142,6 +142,15 @@ func Setenv(key, val string) (err error) {
 	return
 }
 
+func Unsetenv(key string) (err error) {
+	keydata := cstring(key)
+	errCode := libc_unsetenv(&keydata[0])
+	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)) {
@@ -206,6 +215,10 @@ func libc_getenv(name *byte) *byte
 //export setenv
 func libc_setenv(name *byte, val *byte, replace int32) int32
 
+// int unsetenv(const char *name);
+//export unsetenv
+func libc_unsetenv(name *byte) 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 21ad3d63..9e5b70fd 100644
--- a/src/syscall/syscall_nonhosted.go
+++ b/src/syscall/syscall_nonhosted.go
@@ -72,6 +72,11 @@ func Setenv(key, val string) (err error) {
 	return ENOSYS
 }
 
+func Unsetenv(key string) (err error) {
+	// stub for now
+	return ENOSYS
+}
+
 func Environ() []string {
 	env := runtime_envs()
 	envCopy := make([]string, len(env))