.

wcfs/wcfs.go

@@ -1389,6 +1389,7 @@ func (wlink *WatchLink) setupWatch(ctx context.Context, foid zodb.Oid, at zodb.T
 	bfdir := head.bfdir
 
 	// XXX locking
+	// XXX head.zconnMu.RLock() + defer unlock (see vvv for unpin vs pin and locked head)
 
 	// XXX if watch was already established - we need to update it
 	w := wlink.byfile[foid]
@@ -1442,7 +1443,7 @@ func (wlink *WatchLink) setupWatch(ctx context.Context, foid zodb.Oid, at zodb.T
 	// block which could have revision > w.at:	XXX test
 	//
 	//                1   3    2   4
-	//	-----.----x---o----x---x------.----------
+	//	-----.----x---o----x---x------]----------
 	//           ↑                        ↑
 	//          w.at                     head
 	//
@@ -1454,6 +1455,22 @@ func (wlink *WatchLink) setupWatch(ctx context.Context, foid zodb.Oid, at zodb.T
 	// f.watches; if w ∉ f.watches at that moment, w will miss to receive
 	// pin for #3.
 	//
+	// NOTE for `unpin blk` head we can be sure there won't be simultaneous
+	// `pin blk` request, because:
+	//
+	// - unpin means blk was previously pinned,
+	// - blk was pinned means it is tracked by δFtail,
+	// - if blk is tracked and δFtail says there is no δblk ∈ (at, head],
+	//   there is indeed no blk change in that region,
+	// - which means that δblk with rev > w.at might be only > head,
+	// - but such δblk are processed with zhead wlocked and we keep zhead
+	//   rlocked during pin setup. XXX rlock zhead
+	//
+	//          δ                      δ
+	//      ----x----.------------]----x----
+	//               ↑            ↑
+	//              w.at         head
+	//
 	// XXX locking
 	// XXX register only if watch was created anew, not updated?
 	f.watches[w] = struct{}{}