syscall: Clearenv now unsets env vars on Windows

Previously, `os.Clearenv()` (by way of `syscall.Clearenv`) would simply
set all environment variables' values to `""` rather than actually
unsetting them causing subsequent `os.LookupEnv` calls to return that
they were still set.

Fixes #17902

Change-Id: I54081b4b98665e9a39f55ea7582c8d40bb8a2a22
Reviewed-on: https://go-review.googlesource.com/33168
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Alex Brainman <alex.brainman@gmail.com>

src/os/env_test.go

@@ -95,6 +95,34 @@ func TestUnsetenv(t *testing.T) {
 	}
 }
 
+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 =
+			// http://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.
 	value, ok := LookupEnv(smallpox) // Should not exist.

src/syscall/env_windows.go

@@ -60,7 +60,7 @@ func Clearenv() {
 		// http://blogs.msdn.com/b/oldnewthing/archive/2010/05/06/10008132.aspx
 		for j := 1; j < len(s); j++ {
 			if s[j] == '=' {
-				Setenv(s[0:j], "")
+				Unsetenv(s[0:j])
 				break
 			}
 		}