lib/fs: Remove When watching remove \?\ for drive letters (fixes sync…

…thing#5578)

lib/fs/basicfs_watch.go

@@ -12,6 +12,7 @@ import (
 	"context"
 	"errors"
 	"path/filepath"
+	"runtime"
 
 	"github.com/syncthing/notify"
 )
@@ -22,7 +23,14 @@ import (
 var backendBuffer = 500
 
 func (f *BasicFilesystem) Watch(name string, ignore Matcher, ctx context.Context, ignorePerms bool) (<-chan Event, error) {
-	evalRoot, err := evalSymlinks(f.root)
+	root := f.root
+	// Remove `\\?\` prefix if the path is just a drive letter as a dirty
+	// fix for https://github.com/syncthing/syncthing/issues/5578
+	if runtime.GOOS == "windows" && filepath.Clean(name) == "." && len(root) <= 7 && len(root) > 4 && root[:4] == `\\?\` {
+		root = root[4:]
+	}
+
+	evalRoot, err := evalSymlinks(root)
 	if err != nil {
 		return nil, err
 	}

lib/fs/basicfs_watch_test.go

@@ -149,6 +149,48 @@ func TestWatchRename(t *testing.T) {
 	testScenario(t, name, testCase, expectedEvents, allowedEvents, fakeMatcher{})
 }
 
+// TestWatchWinRoot checks that a watch at a drive letter does not panic due to
+// out of root event on every event.
+// https://github.com/syncthing/syncthing/issues/5695
+func TestWatchWinRoot(t *testing.T) {
+	if runtime.GOOS != "windows" {
+		t.Skip("Windows specific test")
+	}
+
+	outChan := make(chan Event)
+	backendChan := make(chan notify.EventInfo, backendBuffer)
+
+	ctx, cancel := context.WithCancel(context.Background())
+
+	// testFs is Filesystem, but we need BasicFilesystem here
+	root := "D:"
+	fs := newBasicFilesystem(root)
+
+	go func() {
+		defer func() {
+			if r := recover(); r != nil {
+				t.Fatal(r)
+			}
+			cancel()
+		}()
+		fs.watchLoop(".", testDirAbs, backendChan, outChan, fakeMatcher{}, ctx)
+	}()
+
+	path := filepath.Join(root, "foo")
+	backendChan <- fakeEventInfo(path)
+
+	select {
+	case <-time.After(10 * time.Second):
+		cancel()
+		t.Errorf("Timed out before receiving event")
+	case ev := <-outChan:
+		if ev.Name != path {
+			t.Errorf("Unexpected event %v, expected %v", ev.Name, path)
+		}
+	case <-ctx.Done():
+	}
+}
+
 // TestWatchOutside checks that no changes from outside the folder make it in
 func TestWatchOutside(t *testing.T) {
 	outChan := make(chan Event)