all: Support syncing ownership (fixes #1329) (#8434)

This adds support for syncing ownership on Unixes and on Windows. The
scanner always picks up ownership information, but it is not applied
unless the new folder option "Sync Ownership" is set.

Ownership data is stored in a new FileInfo field called "platform data". This
is intended to hold further platform-specific data in the future
(specifically, extended attributes), which is why the whole design is a
bit overkill for just ownership.

lib/scanner/walk_test.go

@@ -543,6 +543,79 @@ func TestWalkReceiveOnly(t *testing.T) {
 	}
 }
 
+func TestScanOwnershipPOSIX(t *testing.T) {
+	// This test works on all operating systems because the FakeFS is always POSIXy.
+
+	fakeFS := fs.NewFilesystem(fs.FilesystemTypeFake, "TestScanOwnership")
+	current := make(fakeCurrentFiler)
+
+	fakeFS.Create("root-owned")
+	fakeFS.Create("user-owned")
+	fakeFS.Lchown("user-owned", "1234", "5678")
+	fakeFS.Mkdir("user-owned-dir", 0755)
+	fakeFS.Lchown("user-owned-dir", "2345", "6789")
+
+	expected := []struct {
+		name     string
+		uid, gid int
+	}{
+		{"root-owned", 0, 0},
+		{"user-owned", 1234, 5678},
+		{"user-owned-dir", 2345, 6789},
+	}
+
+	files := walkDir(fakeFS, ".", current, nil, 0)
+	if len(files) != len(expected) {
+		t.Fatalf("expected %d items, not %d", len(expected), len(files))
+	}
+	for i := range expected {
+		if files[i].Name != expected[i].name {
+			t.Errorf("expected %s, got %s", expected[i].name, files[i].Name)
+			continue
+		}
+
+		if files[i].Platform.Unix == nil {
+			t.Error("failed to load POSIX data on", files[i].Name)
+			continue
+		}
+		if files[i].Platform.Unix.UID != expected[i].uid {
+			t.Errorf("expected %d, got %d", expected[i].uid, files[i].Platform.Unix.UID)
+		}
+		if files[i].Platform.Unix.GID != expected[i].gid {
+			t.Errorf("expected %d, got %d", expected[i].gid, files[i].Platform.Unix.GID)
+		}
+	}
+}
+
+func TestScanOwnershipWindows(t *testing.T) {
+	if runtime.GOOS != "windows" {
+		t.Skip("This test only works on Windows")
+	}
+
+	testFS := fs.NewFilesystem(fs.FilesystemTypeBasic, t.TempDir())
+	current := make(fakeCurrentFiler)
+
+	fd, err := testFS.Create("user-owned")
+	if err != nil {
+		t.Fatal(err)
+	}
+	fd.Close()
+
+	files := walkDir(testFS, ".", current, nil, 0)
+	if len(files) != 1 {
+		t.Fatalf("expected %d items, not %d", 1, len(files))
+	}
+	t.Log(files[0])
+
+	// The file should have an owner name set.
+	if files[0].Platform.Windows == nil {
+		t.Fatal("failed to load Windows data")
+	}
+	if files[0].Platform.Windows.OwnerName == "" {
+		t.Errorf("expected owner name to be set")
+	}
+}
+
 func walkDir(fs fs.Filesystem, dir string, cfiler CurrentFiler, matcher *ignore.Matcher, localFlags uint32) []protocol.FileInfo {
 	cfg, cancel := testConfig()
 	defer cancel()