syscall: only remove write data access when O_APPEND is set on Windows

There is no need to remove all write accesses when O_APPEND is set,
only the FILE_WRITE_DATA access. This will allow files opened with
O_APPEND and O_WRONLY to be have their attributes and ACLs modified.

Change-Id: I6fe3b25e87b141a9eb30805f395fec31242fd35d
Reviewed-on: https://go-review.googlesource.com/c/go/+/620615
LUCI-TryBot-Result: Go LUCI <[email protected]>
Reviewed-by: Michael Pratt <[email protected]>
Reviewed-by: Damien Neil <[email protected]>

Author: qmuntal

src/os/os_test.go

@@ -3593,3 +3593,29 @@ func TestCopyFSWithSymlinks(t *testing.T) {
 		t.Fatal("comparing two directories:", err)
 	}
 }
+
+func TestAppendDoesntOverwrite(t *testing.T) {
+	name := filepath.Join(t.TempDir(), "file")
+	if err := WriteFile(name, []byte("hello"), 0666); err != nil {
+		t.Fatal(err)
+	}
+	f, err := OpenFile(name, O_APPEND|O_WRONLY, 0)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if _, err := f.Write([]byte(" world")); err != nil {
+		f.Close()
+		t.Fatal(err)
+	}
+	if err := f.Close(); err != nil {
+		t.Fatal(err)
+	}
+	got, err := ReadFile(name)
+	if err != nil {
+		t.Fatal(err)
+	}
+	want := "hello world"
+	if string(got) != want {
+		t.Fatalf("got %q, want %q", got, want)
+	}
+}

src/syscall/syscall_windows.go

@@ -362,12 +362,14 @@ func Open(name string, flag int, perm uint32) (fd Handle, err error) {
 		access |= GENERIC_WRITE
 	}
 	if flag&O_APPEND != 0 {
-		access |= FILE_APPEND_DATA
-		// Remove GENERIC_WRITE access unless O_TRUNC is set,
-		// in which case we need it to truncate the file.
+		// Remove GENERIC_WRITE unless O_TRUNC is set, in which case we need it to truncate the file.
+		// We can't just remove FILE_WRITE_DATA because GENERIC_WRITE without FILE_WRITE_DATA
+		// starts appending at the beginning of the file rather than at the end.
 		if flag&O_TRUNC == 0 {
 			access &^= GENERIC_WRITE
 		}
+		// Set all access rights granted by GENERIC_WRITE except for FILE_WRITE_DATA.
+		access |= FILE_APPEND_DATA | FILE_WRITE_ATTRIBUTES | _FILE_WRITE_EA | STANDARD_RIGHTS_WRITE | SYNCHRONIZE
 	}
 	sharemode := uint32(FILE_SHARE_READ | FILE_SHARE_WRITE)
 	var sa *SecurityAttributes

src/syscall/types_windows.go

@@ -93,6 +93,7 @@ const (
 
 	FILE_LIST_DIRECTORY   = 0x00000001
 	FILE_APPEND_DATA      = 0x00000004
+	_FILE_WRITE_EA        = 0x00000010
 	FILE_WRITE_ATTRIBUTES = 0x00000100
 
 	FILE_SHARE_READ              = 0x00000001