.

wcfs/wcfs.go

@@ -1420,9 +1420,10 @@ func (wlink *WatchLink) setupWatch(ctx context.Context, foid zodb.Oid, at zodb.T
 	}
 
 
-	// XXX check at >= w.at		-> reject?
+	// check at >= w.at
+	// XXX we might want to allow going back in history if we need it.
 	if !(at >= w.at) {
-		panic("TODO")
+		return fmt.Errorf("going back ZZ history is forbidden")
 	}
 
 

wcfs/wcfs_test.py

@@ -1379,6 +1379,26 @@ def test_wcfs_watch_before_create():
     wl.close()
 
 
+# verify that watch @at_i -> @at_j ↓ is rejected
+# XXX we might want to allow going back in history.
+@func
+def test_wcfs_watch_going_back():
+    t = tDB(); zf = t.zfile
+    defer(t.close)
+
+    at1 = t.commit(zf, {2:'c1'})
+    at2 = t.commit(zf, {2:'c2'})
+    f = t.open(zf)
+    f.assertData(['','','c2'])
+
+    wl = t.openwatch()
+    wl.watch(zf, at2, {})
+    wl.sendReq(timeout(), b"watch %s @%s" % (h(zf._p_oid), h(at1))) == \
+        "error setup watch f<%s> @%s: " % (h(zf._p_oid), h(at1)) + \
+        "going back in history is forbidden"
+    wl.close()
+
+
 # verify that wcfs kills slow/faulty client who does not reply to pin in time.
 @xfail  # protection against faulty/slow clients
 @func
@@ -1710,8 +1730,6 @@ def test_wcfs_watch_2files():
     # XXX watch @at when file did not existed -> error
 
 
-    # XXX going not only up, but also down at1 <- at2 <- at3    ?   -> forbid?
-
     # XXX new watch request while previous watch request is in progress (over the same /head/watch handle)