lib/versioner: Clean the versions dir of symlinks, not the full folder

GitHub-Pull-Request: https://github.com/syncthing/syncthing/pull/4289

lib/model/model.go

@@ -178,8 +178,13 @@ func (m *Model) StartDeadlockDetector(timeout time.Duration) {
 func (m *Model) StartFolder(folder string) {
 	m.fmut.Lock()
 	m.pmut.Lock()
-	folderType := m.startFolderLocked(folder)
 	folderCfg := m.folderCfgs[folder]
+
+	if folderCfg.Versioning.Type != "" && m.cfg.RawCopy().OriginalVersion < 21 {
+		m.attemptSymlinkRecovery(folderCfg)
+	}
+
+	folderType := m.startFolderLocked(folder)
 	m.pmut.Unlock()
 	m.fmut.Unlock()
 
@@ -2721,3 +2726,76 @@ func rootedJoinedPath(root, rel string) (string, error) {
 
 	return joined, nil
 }
+
+func (m *Model) attemptSymlinkRecovery(fcfg config.FolderConfiguration) {
+	fs, ok := m.folderFiles[fcfg.ID]
+	if !ok {
+		return
+	}
+
+	// The window during which we had a broken release out, roughly.
+	startDate := time.Date(2017, 8, 8, 6, 0, 0, 0, time.UTC)
+	endDate := time.Date(2017, 8, 8, 12, 0, 0, 0, time.UTC)
+
+	// Look through all our files looking for deleted symlinks.
+	fs.WithHave(protocol.LocalDeviceID, func(intf db.FileIntf) bool {
+		if !intf.IsSymlink() {
+			return true
+		}
+
+		symlinkPath, err := rootedJoinedPath(fcfg.Path(), intf.FileName())
+		if err != nil {
+			// odd
+			return true
+		}
+
+		if _, err := os.Lstat(symlinkPath); err == nil {
+			// The symlink exists. Our work here is done.
+			return true
+		}
+
+		fi := intf.(protocol.FileInfo)
+		if !fi.Deleted && fi.SymlinkTarget != "" {
+			// We haven't noticed the delete and put it into the
+			// index yet. Great! We can restore the symlink.
+			l.Infoln("Restoring incorrectly deleted symlink", symlinkPath)
+			os.Symlink(fi.SymlinkTarget, symlinkPath)
+			return true
+		}
+
+		// It's deleted. Check if it was deleted in the bad window.
+		if fi.ModTime().Before(startDate) || !fi.ModTime().Before(endDate) {
+			return true
+		}
+
+		// Try to find an older index entry.
+		for deviceID := range m.cfg.Devices() {
+			olderFI, ok := fs.Get(deviceID, fi.Name)
+			if !ok {
+				// This device doesn't have it.
+				continue
+			}
+			if olderFI.Deleted || !olderFI.IsSymlink() {
+				// The device has something deleted or not a
+				// symlink, doesn't help us.
+				continue
+			}
+			if olderFI.Version.GreaterEqual(fi.Version) {
+				// The device has something newer. We should
+				// chill and let the puller handle it. No
+				// need to look further for this specific
+				// symlink.
+				return true
+			}
+
+			if olderFI.SymlinkTarget != "" {
+				// It has symlink data. Restore the symlink.
+				l.Infoln("Restoring incorrectly deleted symlink", symlinkPath)
+				os.Symlink(olderFI.SymlinkTarget, symlinkPath)
+				return true
+			}
+		}
+
+		return true
+	})
+}