.

wcfs/wcfs.go

@@ -1485,7 +1485,7 @@ func (wlink *WatchLink) setupWatch(ctx context.Context, foid zodb.Oid, at zodb.T
 	}
 
 	// make sure zhead.At stays unchanged while we are preparing the watch
-	// (see vvv e.g. about unpin to @head for why it is neeed)
+	// (see vvv e.g. about unpin to @head for why it is needed)
 	head.zheadMu.RLock()
 	defer head.zheadMu.RUnlock()
 	headAt := head.zconn.At()
@@ -1521,10 +1521,16 @@ func (wlink *WatchLink) setupWatch(ctx context.Context, foid zodb.Oid, at zodb.T
 		return nil
 	}
 
+	// request exclusive access to the watch to change .at and compute pins.
+	// The lock will be downgraded from W to R after pins computation is done.
+	// Pins will be executed with atMu.R only - with the idea not to block
+	// other client that read-access the file simultaneously to setupWatch.
+	w.atMu.Lock()
 
 	// check at >= w.at
 	// XXX we might want to allow going back in history if we need it.
-	if !(at >= w.at) {	// XXX locking
+	if !(at >= w.at) {
+		w.atMu.Unlock()
 		return fmt.Errorf("going back in history is forbidden")
 	}
 
@@ -1568,7 +1574,7 @@ func (wlink *WatchLink) setupWatch(ctx context.Context, foid zodb.Oid, at zodb.T
 	//   unpinned, because we update w.at to requested at early.
 	//
 	// XXX register only if watch was created anew, not updated?
-	w.at = at			// XXX locking
+	w.at = at
 	// NOTE registering f.watchTab[w] and wlink.byfile[foid] = w must come together.
 	f.watchTab[w] = struct{}{}	// XXX locking
 	wlink.byfile[foid] = w		// XXX locking
@@ -1576,12 +1582,9 @@ func (wlink *WatchLink) setupWatch(ctx context.Context, foid zodb.Oid, at zodb.T
 	// XXX defer -> unregister watch if error?
 
 	// pin all tracked file blocks that were changed in (at, head] range.
-
-	// XXX locking
-
 	toPin := map[int64]zodb.Tid{} // blk -> @rev
 
-	for _, δfile := range bfdir.δFtail.SliceByFileRev(f, at, headAt) {
+	for _, δfile := range bfdir.δFtail.SliceByFileRev(f, at, headAt) {	// XXX locking δFtail
 		for blk := range δfile.Blocks {
 			_, already := toPin[blk]
 			if already {
@@ -1593,9 +1596,10 @@ func (wlink *WatchLink) setupWatch(ctx context.Context, foid zodb.Oid, at zodb.T
 	}
 
 	// if a block was previously pinned, but ∉ δ(at, head] -> unpin it to head.
-	for blk, pinPrev := range w.pinned {	// XXX locking
+	for blk, pinPrev := range w.pinned {
 		// only 1 setupWatch can be run simultaneously for one file
 		// XXX assert pinPrev.rev != zodb.TidMax
+
 		pinNew, pinning := toPin[blk]
 		if !pinning {
 			toPin[blk] = zodb.TidMax // @head
@@ -1609,6 +1613,14 @@ func (wlink *WatchLink) setupWatch(ctx context.Context, foid zodb.Oid, at zodb.T
 		_ = pinNew
 	}
 
+	// downgrade atMu.W -> atMu.R
+	// XXX there is no primitive to do Wlock->Rlock atomically, but we are
+	// ok with that since we prepared eveyrhing to handle simultaneous pins
+	// from other reads.
+	w.atMu.Unlock()
+	w.atMu.RLock()
+	defer w.atMu.RUnlock()
+
 	wg, ctx := errgroup.WithContext(ctx)
 	for blk, rev := range toPin {
 		blk := blk