.

wcfs/wcfs.go

@@ -1235,7 +1235,7 @@ func (f *BigFile) readBlk(ctx context.Context, blk int64, dest []byte) (err erro
 
 		case <-loading.ready:
 			if loading.err == nil {
-				copy(dest, loading.blkdata)	// XXX copy
+				copy(dest, loading.blkdata) // TODO avoid copy
 			}
 			return loading.err
 		}
@@ -1255,9 +1255,16 @@ func (f *BigFile) readBlk(ctx context.Context, blk int64, dest []byte) (err erro
 		return err
 	}
 
+	if f.head.rev == 0 {
+		// update δFtail index
+		// see "3) for */head/data the following invariant is maintained..."
+		δFtail := f.head.bfdir.δFtail
+		δFtail.Track(f.zfile, blk, treepath, blkcov, zblk)	// XXX pass in zblk.rev here?
+
 		// we have the data - it can be used after watchers are updated
 		// XXX should we use ctx here? (see readPinWatchers comments)
 		f.readPinWatchers(ctx, blk, treepath, blkcov, zblk, blkrevMax)
+	}
 
 	// data can be used now
 	close(loading.ready)
@@ -1470,7 +1477,8 @@ func (w *Watch) _pin(ctx context.Context, blk int64, rev zodb.Tid) (err error) {
 //
 // See "7.2) for all registered client@at watchers ..."
 //
-// Called with f.head.zheadMu rlocked.
+// Must be called only for f under head/
+// Must be called with f.head.zheadMu rlocked.
 //
 // XXX do we really need to use/propagate caller context here? ideally update
 // watchers should be synchronous, and in practice we just use 30s timeout.
@@ -1478,23 +1486,18 @@ func (w *Watch) _pin(ctx context.Context, blk int64, rev zodb.Tid) (err error) {
 func (f *BigFile) readPinWatchers(ctx context.Context, blk int64, treepath []btree.LONode, blkcov btree.LKeyRange, zblk ZBlk, blkrevMax zodb.Tid) {
 	// only head/ is being watched for
 	if f.head.rev != 0 {
-		return
+		panic("BUG: readPinWatchers: called for file under @revX/")
 	}
 
 //	fmt.Printf("S: read #%d -> pin watchers (#%d)\n", blk, len(f.watchTab))
 
-	// update δFtail index	XXX -> move upper into readBlk ?
-	// see "3) for */head/data the following invariant is maintained..."
-	bfdir := f.head.bfdir
-	δFtail := bfdir.δFtail
-	δFtail.Track(f.zfile, blk, treepath, blkcov, zblk)	// XXX pass in zblk.rev here?
-
 	// make sure that file[blk] on clients side stays as of @w.at state.
 
 	// try to use blkrevMax only as the first cheap criteria to skip updating watchers.
 	// This is likely to be the case, since most watchers should be usually close to head.
 	// If using blkrevMax only turns out to be not sufficient, we'll
 	// consult δFtail, which might involve recomputing it.
+	δFtail := f.head.bfdir.δFtail
 	blkrev := blkrevMax
 	blkrevRough := true