.

wcfs/wcfs.go

@@ -1250,6 +1250,7 @@ func (f *BigFile) updateWatchers(ctx context.Context, blk int64, treepath []btre
 			blkrev, _ = f.LastBlkRev(ctx, blk, f.head.zconn.At())
 			blkrevRough = false
 
+			// XXX w.at could be only ↑ ? if not - locking is more complex
 			if blkrev <= w.at {
 				continue
 			}
@@ -1370,6 +1371,7 @@ func (w *Watch) _pin(ctx context.Context, blk int64, rev zodb.Tid) (err error) {
 	// XXX locking?
 	// XXX simultaneous calls?
 
+	// XXX ignore vvv ? (w.at could be ↑ after precheck in read (XXX or setupBlk)
 	if !(rev == zodb.TidMax || rev <= w.at) {
 		panicf("f<%s>: wlink%d: pin #%d @%s: watch.at (%s) < rev",
 			foid, w.link.id, blk, rev, w.at)
@@ -1426,6 +1428,7 @@ func (wlink *WatchLink) setupWatch(ctx context.Context, foid zodb.Oid, at zodb.T
 	defer head.zheadMu.RUnlock()
 	headAt := head.zconn.At()
 
+	// XXX δFtail locking? (or ForgetPast is called only with zheadMu.W ?)
 	if at != zodb.InvalidTid && at < bfdir.δFtail.Tail() {
 		return fmt.Errorf("too far away back from head/at (@%s); δt = %s",
 			headAt, headAt.Time().Sub(at.Time().Time))
@@ -1501,6 +1504,7 @@ func (wlink *WatchLink) setupWatch(ctx context.Context, foid zodb.Oid, at zodb.T
 	//
 	// XXX locking
 	// XXX register only if watch was created anew, not updated?
+	w.at = at
 	f.watchTab[w] = struct{}{}
 	wlink.byfile[foid] = w
 
@@ -1536,10 +1540,6 @@ func (wlink *WatchLink) setupWatch(ctx context.Context, foid zodb.Oid, at zodb.T
 		}
 	}
 
-	// XXX locking
-	// XXX do at the same time when registering ^^^, so that simultaneous READs see correct w.at
-	w.at = at
-
 	wg, ctx := errgroup.WithContext(ctx)
 	for blk, rev := range toPin {
 		blk := blk