wcfs: Cleanup wlinkTab entry when client drops opened head/watch handle

The code was already behaving like that but there was XXX to do it. Add
test to verify it is actually done.

Opened WatchLink handle is released after RELEASE because
read in WatchLink.serve, after RELEASE, returns EOF and then the code
inside WCFS does all necessary WatchLink-related cleanup:

https://lab.nexedi.com/nexedi/wendelin.core/-/blob/wendelin.core-2.0.alpha3-26-g79e6f7b9/wcfs/wcfs.go#L1828-1872

/reviewed-by @levin.zimmermann
/reviewed-on !18

wcfs/wcfs.go

@@ -1831,7 +1831,6 @@ func (wnode *WatchNode) Open(flags uint32, fctx *fuse.Context) (nodefs.File, fus
 	}
 
 	head.wlinkMu.Lock()
-	// XXX del wlinkTab[w] on w.sk.File.Release
 	head.wlinkTab[wlink] = struct{}{}
 	head.wlinkMu.Unlock()
 

wcfs/wcfs_test.py

@@ -541,7 +541,7 @@ class tDB(tWCFS):
         assert len(t._wlinks)  == 0
         t._wc_zheadfh.close()
 
-        t.assertStats({'PinnedBlk': 0, 'ZHeadLink': 0})  # FIXME + WatchLink, Watch
+        t.assertStats({'WatchLink': 0, 'Watch': 0, 'PinnedBlk': 0, 'ZHeadLink': 0})
 
     # open opens wcfs file corresponding to zf@at and starts to track it.
     # see returned tFile for details.
@@ -955,8 +955,11 @@ class tWatchLink(wcfs.WatchLink):
         # this tWatchLink currently watches the following files at particular state.
         t._watching = {}    # {} foid -> tWatch
 
+        tdb.assertStats({'WatchLink': len(tdb._wlinks)})
+
     def close(t):
-        t.tdb._wlinks.remove(t)
+        tdb = t.tdb
+        tdb._wlinks.remove(t)
         super(tWatchLink, t).close()
 
         # disable all established watches
@@ -965,6 +968,8 @@ class tWatchLink(wcfs.WatchLink):
             w.pinned = {}
         t._watching = {}
 
+        tdb.assertStats({'WatchLink': len(tdb._wlinks)})
+
 
 # ---- infrastructure: watch setup/adjust ----