syscall: keep write access when O_TRUNC is used on Windows

CL 618836 introduces a regression where O_APPEND and O_TRUNC could
not be used together on Windows.

This CL fixes the issue by keeping the write access when O_TRUNC is used
, which is required when overwriting data (as per the file
access rights docs: https://learn.microsoft.com/en-us/windows/win32/fileio/file-access-rights-constants).

Fixes #69902.

Change-Id: I77ec60ca6929124dd4490bdad6c3280c4db3efcb
Reviewed-on: https://go-review.googlesource.com/c/go/+/620575
Reviewed-by: Ian Lance Taylor <iant@google.com>
Reviewed-by: Alex Brainman <alex.brainman@gmail.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Damien Neil <dneil@google.com>

src/syscall/syscall_windows.go

@@ -362,8 +362,12 @@ func Open(name string, flag int, perm uint32) (fd Handle, err error) {
 		access |= GENERIC_WRITE
 	}
 	if flag&O_APPEND != 0 {
-		access &^= GENERIC_WRITE
 		access |= FILE_APPEND_DATA
+		// Remove GENERIC_WRITE access unless O_TRUNC is set,
+		// in which case we need it to truncate the file.
+		if flag&O_TRUNC == 0 {
+			access &^= GENERIC_WRITE
+		}
 	}
 	sharemode := uint32(FILE_SHARE_READ | FILE_SHARE_WRITE)
 	var sa *SecurityAttributes

src/syscall/syscall_windows_test.go

@@ -15,29 +15,39 @@ import (
 	"testing"
 )
 
-func TestOpen_Dir(t *testing.T) {
+func TestOpen(t *testing.T) {
 	t.Parallel()
 
 	dir := t.TempDir()
+	file := filepath.Join(dir, "a")
+	f, err := os.Create(file)
+	if err != nil {
+		t.Fatal(err)
+	}
+	f.Close()
+
 	tests := []struct {
+		path string
 		flag int
 		err  error
 	}{
-		{syscall.O_RDONLY, nil},
-		{syscall.O_CREAT, nil},
-		{syscall.O_RDONLY | syscall.O_CREAT, nil},
-		{syscall.O_RDONLY | syscall.O_TRUNC, syscall.ERROR_ACCESS_DENIED},
-		{syscall.O_WRONLY | syscall.O_RDWR, syscall.EISDIR},
-		{syscall.O_WRONLY, syscall.EISDIR},
-		{syscall.O_RDWR, syscall.EISDIR},
+		{dir, syscall.O_RDONLY, nil},
+		{dir, syscall.O_CREAT, nil},
+		{dir, syscall.O_RDONLY | syscall.O_CREAT, nil},
+		{file, syscall.O_APPEND | syscall.O_WRONLY | os.O_CREATE, nil},
+		{file, syscall.O_APPEND | syscall.O_WRONLY | os.O_CREATE | os.O_TRUNC, nil},
+		{dir, syscall.O_RDONLY | syscall.O_TRUNC, syscall.ERROR_ACCESS_DENIED},
+		{dir, syscall.O_WRONLY | syscall.O_RDWR, syscall.EISDIR},
+		{dir, syscall.O_WRONLY, syscall.EISDIR},
+		{dir, syscall.O_RDWR, syscall.EISDIR},
 	}
 	for i, tt := range tests {
-		h, err := syscall.Open(dir, tt.flag, 0)
+		h, err := syscall.Open(tt.path, tt.flag, 0o660)
 		if err == nil {
 			syscall.CloseHandle(h)
 		}
 		if err != tt.err {
-			t.Errorf("%d: Open got %v, want %v", i, err, tt.err)
+			t.Errorf("%d: Open got %q, want %q", i, err, tt.err)
 		}
 	}
 }