.

wcfs/wcfs.go

@@ -1353,25 +1353,32 @@ func (wlink *WatchLink) setupWatch(ctx context.Context, foid zodb.Oid, at zodb.T
 		return fmt.Errorf("file not yet known or is not a ZBigFile")
 	}
 
+	headAt := head.zconn.At()
+
 	// XXX wait head.zconn.At() ≥ at
 	// XXX <~> f.δtail.Head() ≥ at (?)
 
 	if at < bfdir.δFtail.Tail() {
-		// XXX err += head.at?
-		return fmt.Errorf("at is too far away back from head/at")
+		return fmt.Errorf("at is too far away back from head/at (%s)", headAt)
 	}
 
 	// TODO register w to f here early, so that READs going in parallel to us
 	// preparing and processing initial pins, also sends pin for read
 	// blocks. If we don't we can miss to send pin for a freshly read which
 	// could have revision > w.at .
+	w = &Watch{
+		link:   wlink,
+		file:   f,
+		at:     at,
+		pinned: make(SetI64),
+	}
 
 	toPin := map[int64]zodb.Tid{} // blk -> @rev
 
 	// XXX locking
 
 	// pin all tracked file blocks that were changed (at, head] range
-	fhead := tidmin(f.δtail.Head(), head.zconn.At())
+	fhead := tidmin(f.δtail.Head(), headAt)
 	for _, δfile := range f.δtail.SliceByRev(at, fhead) {
 		for _, blk := range δfile.Changev {
 			_, already := toPin[blk]