[release-branch.go1.14] syscall: preserve Windows file permissions for O_CREAT|O_TRUNC

On Windows, calling syscall.Open(file, O_CREAT|O_TRUNC, 0) for a file
that already exists would change the file to be read-only.
That is not how the Unix syscall.Open behaves, so avoid it on
Windows by calling CreateFile twice if necessary.

For #38225
Fixes #39158

Change-Id: I70097fca8863df427cc8a97b9376a9ffc69c6318
Reviewed-on: https://go-review.googlesource.com/c/go/+/234534
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Alex Brainman <alex.brainman@gmail.com>
(cherry picked from commit 567556d78657326c99b8fa84ec2a5ee511a0941b)
Reviewed-on: https://go-review.googlesource.com/c/go/+/234686
Reviewed-by: Emmanuel Odeke <emm.odeke@gmail.com>

2 files changed, 55 insertions, 0 deletions

diff --git a/src/os/os_test.go b/src/os/os_test.go
index 278c19e44b..31b3e67319 100644
--- a/src/os/os_test.go
+++ b/src/os/os_test.go
@@ -2448,3 +2448,38 @@ func TestDirSeek(t *testing.T) {
 		}
 	}
 }
+
+// Test that opening a file does not change its permissions.  Issue 38225.
+func TestOpenFileKeepsPermissions(t *testing.T) {
+	t.Parallel()
+	dir, err := ioutil.TempDir("", "TestOpenFileKeepsPermissions")
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer RemoveAll(dir)
+	name := filepath.Join(dir, "x")
+	f, err := Create(name)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if err := f.Close(); err != nil {
+		t.Error(err)
+	}
+	f, err = OpenFile(name, O_WRONLY|O_CREATE|O_TRUNC, 0)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if fi, err := f.Stat(); err != nil {
+		t.Error(err)
+	} else if fi.Mode()&0222 == 0 {
+		t.Errorf("f.Stat.Mode after OpenFile is %v, should be writable", fi.Mode())
+	}
+	if err := f.Close(); err != nil {
+		t.Error(err)
+	}
+	if fi, err := Stat(name); err != nil {
+		t.Error(err)
+	} else if fi.Mode()&0222 == 0 {
+		t.Errorf("Stat after OpenFile is %v, should be writable", fi.Mode())
+	}
+}
diff --git a/src/syscall/syscall_windows.go b/src/syscall/syscall_windows.go
index 950c281e4d..41bc42a44e 100644
--- a/src/syscall/syscall_windows.go
+++ b/src/syscall/syscall_windows.go
@@ -334,6 +334,26 @@ func Open(path string, mode int, perm uint32) (fd Handle, err error) {
 	var attrs uint32 = FILE_ATTRIBUTE_NORMAL
 	if perm&S_IWRITE == 0 {
 		attrs = FILE_ATTRIBUTE_READONLY
+		if createmode == CREATE_ALWAYS {
+			// We have been asked to create a read-only file.
+			// If the file already exists, the semantics of
+			// the Unix open system call is to preserve the
+			// existing permissions. If we pass CREATE_ALWAYS
+			// and FILE_ATTRIBUTE_READONLY to CreateFile,
+			// and the file already exists, CreateFile will
+			// change the file permissions.
+			// Avoid that to preserve the Unix semantics.
+			h, e := CreateFile(pathp, access, sharemode, sa, TRUNCATE_EXISTING, FILE_ATTRIBUTE_NORMAL, 0)
+			switch e {
+			case ERROR_FILE_NOT_FOUND, _ERROR_BAD_NETPATH, ERROR_PATH_NOT_FOUND:
+				// File does not exist. These are the same
+				// errors as Errno.Is checks for ErrNotExist.
+				// Carry on to create the file.
+			default:
+				// Success or some different error.
+				return h, e
+			}
+		}
 	}
 	h, e := CreateFile(pathp, access, sharemode, sa, createmode, attrs, 0)
 	return h, e