src/os: make Environ() return a copy of the environment

src/os/env_test.go

@@ -122,7 +122,33 @@ func TestUnsetenv(t *testing.T) {
 	}
 }
 
-// TODO: add back TestClearenv() and fix the errors it finds
+func TestClearenv(t *testing.T) {
+	const testKey = "GO_TEST_CLEARENV"
+	const testValue = "1"
+
+	// reset env
+	defer func(origEnv []string) {
+		for _, pair := range origEnv {
+			// Environment variables on Windows can begin with =
+			// https://blogs.msdn.com/b/oldnewthing/archive/2010/05/06/10008132.aspx
+			i := strings.Index(pair[1:], "=") + 1
+			if err := Setenv(pair[:i], pair[i+1:]); err != nil {
+				t.Errorf("Setenv(%q, %q) failed during reset: %v", pair[:i], pair[i+1:], err)
+			}
+		}
+	}(Environ())
+
+	if err := Setenv(testKey, testValue); err != nil {
+		t.Fatalf("Setenv(%q, %q) failed: %v", testKey, testValue, err)
+	}
+	if _, ok := LookupEnv(testKey); !ok {
+		t.Errorf("Setenv(%q, %q) didn't set $%s", testKey, testValue, testKey)
+	}
+	Clearenv()
+	if val, ok := LookupEnv(testKey); ok {
+		t.Errorf("Clearenv() didn't clear $%s, remained with value %q", testKey, val)
+	}
+}
 
 func TestLookupEnv(t *testing.T) {
 	const smallpox = "SMALLPOX"      // No one has smallpox.

src/syscall/syscall_libc.go

@@ -235,20 +235,41 @@ func Mprotect(b []byte, prot int) (err error) {
 }
 
 func Environ() []string {
-	environ := libc_environ
-	var envs []string
-	for *environ != nil {
-		// Convert the C string to a Go string.
-		length := libc_strlen(*environ)
+	// calculate total memory required
+	var length uintptr
+	var vars int
+	for environ := libc_environ; *environ != nil; {
+		length += libc_strlen(*environ)
+		vars++
+		environ = (*unsafe.Pointer)(unsafe.Pointer(uintptr(unsafe.Pointer(environ)) + unsafe.Sizeof(environ)))
+	}
+
+	// allocate our backing slice for the strings
+	b := make([]byte, length)
+	// and the slice we're going to return
+	envs := make([]string, 0, vars)
+
+	// loop over the environment again, this time copying over the data to the backing slice
+	for environ := libc_environ; *environ != nil; {
+		length = libc_strlen(*environ)
+		// construct a Go string pointing at the libc-allocated environment variable data
 		var envVar string
 		rawEnvVar := (*struct {
 			ptr    unsafe.Pointer
 			length uintptr
 		})(unsafe.Pointer(&envVar))
 		rawEnvVar.ptr = *environ
 		rawEnvVar.length = length
-		envs = append(envs, envVar)
-		// This is the Go equivalent of "environ++" in C.
+		// pull off the number of bytes we need for this environment variable
+		var bs []byte
+		bs, b = b[:length], b[length:]
+		// copy over the bytes to the Go heap
+		copy(bs, envVar)
+		// convert trimmed slice to string
+		s := *(*string)(unsafe.Pointer(&bs))
+		// add s to our list of environment variables
+		envs = append(envs, s)
+		// environ++
 		environ = (*unsafe.Pointer)(unsafe.Pointer(uintptr(unsafe.Pointer(environ)) + unsafe.Sizeof(environ)))
 	}
 	return envs