.

wcfs/wcfs.go

@@ -1436,13 +1436,20 @@ func (wlink *WatchLink) setupWatch(ctx context.Context, foid zodb.Oid, at zodb.T
 	// register w to f here early, so that READs going in parallel to us
 	// preparing and processing initial pins, also send pins for read
 	// blocks. If we don't, we can miss to send pin for a freshly read
-	// block which could have revision > w.at .	XXX test
+	// block which could have revision > w.at:	XXX test
 	//
 	//                1   3    2   4
-	//	-----.----x---o----x---x-----------------
-	//           ↑
-	//          w.at
+	//	-----.----x---o----x---x------.----------
+	//           ↑                        ↑
+	//          w.at                     head
 	//
+	// Here blocks #1, #2 and #4 were previously accessed, are thus tracked
+	// by δFtail and are changed after w.at - they will be returned by vvv
+	// δFtail query and pin-sent to w. Block #3 was not yet accessed but
+	// was also changed after w.at . As head/file[#3] might be accessed
+	// simultaneously to watch setup, and f.readBlk will be checking
+	// f.watches; if w ∉ f.watches at that moment, w will miss to receive
+	// pin for #3.
 	//
 	// XXX locking
 	// XXX register only if watch was created anew, not updated?