wcfs: Shutdown WatchLink on any pin error

If a pin misbehaves or there is IO error or anything else, we want to
stop all communication on the watchlink, cancel on in-flight pin
handlers, and (TODO) kill the client with SIGBUS.

This patch organizes WatchLink shutdown on any pin error.
This functionality is indirectly tested by test_Wcfs_watch_robust and
will be also indirectly tested by faultyprotection tests.

It would be good to have dedicated tests probably, but that is,
hopefully, TODO.

wcfs/wcfs.go

@@ -684,6 +684,9 @@ type WatchLink struct {
 	// serve operates under .serveCtx and can be requested to stop via serveCancel
 	serveCtx    context.Context
 	serveCancel context.CancelFunc
+	down1       sync.Once
+	down        chan struct{}  // ready after shutdown completes
+	pinWG       sync.WaitGroup // all pin handlers are accounted here
 }
 
 // Watch represents watching for changes to 1 BigFile over particular watch link.
@@ -1439,19 +1442,38 @@ func traceIso(format string, argv ...interface{}) {
 // specified context because pinning is critical operation whose failure will lead
 // to client being SIGBUS'ed and so pinning should not be interrupted arbitrarily.
 //
-// pin is invoked by BigFile.readPinWatchers . It is called with atMu rlocked.
+// Corresponding watchlink is shutdown on any error.
+//
+// No error is returned as currently pin handles all errors itself inside, and
+// in the future the only error that pin will not be able to handle itself inside
+// will be considered to be fatal and the filesystem will be switched to EIO mode on that.
 //
-// TODO close watch on any error
-func (w *Watch) pin(blk int64, rev zodb.Tid) (err error) {
-	defer xerr.Contextf(&err, "wlink%d: f<%s>", w.link.id, w.file.zfile.POid())
-	return w._pin(w.link.serveCtx, blk, rev)
+// pin is invoked by BigFile.readPinWatchers . It is called with atMu rlocked.
+func (w *Watch) pin(blk int64, rev zodb.Tid) {
+	w._pin(w.link.serveCtx, blk, rev)
 }
 
 // _pin serves pin and is also invoked directly by WatchLink.setupWatch .
 //
 // It is invoked with ctx being either WatchLink.serveCtx or descendant of it.
 // In all cases it is called with atMu rlocked.
-func (w *Watch) _pin(ctx context.Context, blk int64, rev zodb.Tid) (err error) {
+func (w *Watch) _pin(ctx context.Context, blk int64, rev zodb.Tid) {
+	if ctx.Err() != nil {
+		return // don't enter pinWG if watchlink is down
+	}
+	w.link.pinWG.Add(1)
+	defer w.link.pinWG.Done()
+
+	ctx, cancel := context.WithTimeout(ctx, groot.pinTimeout)
+	defer cancel()
+
+	err := w.__pin(ctx, blk, rev)
+	if err != nil {
+		w.link.shutdown(err)
+	}
+}
+
+func (w *Watch) __pin(ctx context.Context, blk int64, rev zodb.Tid) (err error) {
 	foid := w.file.zfile.POid()
 	revstr := rev.String()
 	if rev == zodb.TidMax {
@@ -1623,9 +1645,8 @@ func (f *BigFile) readPinWatchers(ctx context.Context, blk int64, blkrevMax zodb
 			// NOTE we do not propagate context to pin. Ideally update
 			// watchers should be synchronous, and in practice we just use 30s timeout (TODO).
 			// A READ interrupt should not cause watch update failure.
-			//
-			// TODO close watcher on any error
-			return w.pin(blk, pinrev)
+			w.pin(blk, pinrev) // only fatal error
+			return nil
 		})
 	}
 
@@ -1845,12 +1866,13 @@ func (wlink *WatchLink) setupWatch(ctx context.Context, foid zodb.Oid, at zodb.T
 		blk := blk
 		rev := rev
 		wg.Go(func(ctx context.Context) error {
-			return w._pin(ctx, blk, rev)
+			w._pin(ctx, blk, rev) // only fatal error
+			return nil
 		})
 	}
 	err = wg.Wait()
 	if err != nil {
-		return err
+		return err // should not fail
 	}
 
 	return nil
@@ -1870,6 +1892,7 @@ func (wnode *WatchNode) Open(flags uint32, fctx *fuse.Context) (nodefs.File, fus
 		rxTab:       make(map[uint64]chan string),
 		serveCtx:    serveCtx,
 		serveCancel: serveCancel,
+		down:        make(chan struct{}),
 	}
 
 	head.wlinkMu.Lock()
@@ -1880,6 +1903,36 @@ func (wnode *WatchNode) Open(flags uint32, fctx *fuse.Context) (nodefs.File, fus
 	return wlink.sk.File(), fuse.OK
 }
 
+// shutdown shuts down communication over watchlink due to specified reason and
+// marks the watchlink as no longer active.
+//
+// Only the first shutdown call has the effect, but all calls wait for the
+// actual shutdown to complete.
+//
+// NOTE shutdown can be invoked under atMu.R from pin.
+func (wlink *WatchLink) shutdown(reason error) {
+	wlink.down1.Do(func() {
+		// mark wlink as down; this signals serve loop to exit and cancels all in-progress pins
+		wlink.serveCancel()
+
+		// give client a chance to be notified if shutdown was due to some logical error
+		if reason != nil {
+			ctx, cancel := context.WithTimeout(context.Background(), 1*time.Second)
+			defer cancel()
+			_ = wlink.send(ctx, 0, fmt.Sprintf("error: %s", reason))
+		}
+
+		// NOTE unregistering watches and wlink itself is done on serve exit, not
+		//      here, to avoid AB-BA deadlock on atMu and e.g. WatchLink.byfileMu .
+		//	It is ok to leave some watches still present in BigFile.watchTab
+		//	until final cleanup because pin becomes noop on down watchlink.
+
+		close(wlink.down)
+	})
+
+	<-wlink.down
+}
+
 // serve serves client initiated watch requests and routes client replies to
 // wcfs initiated pin requests.
 func (wlink *WatchLink) serve(ctx context.Context) {
@@ -1910,13 +1963,6 @@ func (wlink *WatchLink) _serve(ctx context.Context) (err error) {
 		delete(head.wlinkTab, wlink)
 		head.wlinkMu.Unlock()
 
-		// give client a chance to be notified if it was due to some logical error
-		if err != nil {
-			ctx, cancel := context.WithTimeout(context.Background(), 1*time.Second)
-			defer cancel()
-			_ = wlink.send(ctx, 0, fmt.Sprintf("error: %s", err))
-		}
-
 		// close .sk
 		// closing .sk.tx wakes up rx on client side.
 		err2 := wlink.sk.Close()
@@ -1928,15 +1974,28 @@ func (wlink *WatchLink) _serve(ctx context.Context) (err error) {
 	// watch handlers are spawned in dedicated workgroup
 	//
 	// Pin handlers are run either inside - for pins run from setupWatch, or,
-	// for pins run from readPinWatchers, outside.
-	// Upon serve exit we cancel watch and pin handlers ran inside and wait for their completion.
+	// for pins run from readPinWatchers, under wlink.pinWG.
+	// Upon serve exit we cancel them all and wait for their completion.
 	wg := xsync.NewWorkGroup(ctx)
 	defer func() {
-		// cancel all handlers on both error and ok return.
-		// ( ok return is e.g. when we received "bye", so if client
-		//   sends "bye" and some pin handlers are in progress - they
-		//   anyway don't need to wait for client replies anymore )
-		wlink.serveCancel()
+		// cancel all watch and pin handlers on both error and ok return.
+		//
+		// For ok return, when we received "bye", we want to cancel
+		// in-progress pin handlers without killing clients. That's why
+		// we call shutdown ourselves.
+		//
+		// For error return, we want any in-progress, and so will
+		// become failed, pin handler to result in corresponding client
+		// to become killed (TODO). That's why we trigger only cancel
+		// ourselves and let failed pin handlers to invoke shutdown
+		// with their specific reason.
+		//
+		// NOTE this affects pin handlers invoked by both setupWatch and readPinWatchers.
+		if err != nil {
+			wlink.serveCancel()
+		} else {
+			wlink.shutdown(nil)
+		}
 
 		// wait for setupWatch and pin handlers spawned from it to complete
 		err2 := wg.Wait()
@@ -1944,6 +2003,12 @@ func (wlink *WatchLink) _serve(ctx context.Context) (err error) {
 			err = err2
 		}
 
+		// wait for all other pin handlers to complete
+		wlink.pinWG.Wait()
+
+		// make sure that shutdown is actually invoked if it was an
+		// error and there were no in-progress pin handlers
+		wlink.shutdown(err)
 	}()
 
 	// cancel main thread on any watch handler error
@@ -1951,7 +2016,7 @@ func (wlink *WatchLink) _serve(ctx context.Context) (err error) {
 	defer mainCancel()
 	wg.Go(func(ctx context.Context) error {
 		// monitor is always canceled - either due to parent ctx cancel, error in workgroup,
-		// or return from serve and running "cancel all handlers ..." above
+		// or return from serve and running "cancel all watch handlers ..." above.
 		<-ctx.Done()
 		mainCancel()
 		return nil