go/zodb: Fix IStorage implementation not to deadlock driver if watcher fails

Before the patch if storage.watcher fails, storage.driver.Close is not
called, and so the driver will continue to send to .drvWatchq, but noone
is receiving from it.

a5dbb92b (go/zodb: Require drivers to close watchq on Close), provides
the guarantee that the driver will stop sending on drvWatchq right after
drv.Close call.

go/zodb/storage.go

@@ -171,6 +171,7 @@ type storage struct {
 	down        chan struct{} // ready when no longer operational
 	downOnce    sync.Once     // shutdown may be due to both Close and IO error in watcher|Sync
 	downErr     error         // reason for shutdown
+	drvCloseErr error         // err from .driver.Close()
 
 	// watcher
 
@@ -193,6 +194,11 @@ func (s *storage) shutdown(reason error) {
 	s.downOnce.Do(func() {
 		close(s.down)
 		s.downErr = fmt.Errorf("not operational due: %s", reason)
+		// - if called by Close or failed Sync: driver.Close will close
+		//   drvWatchq and cause watcher to stop.
+		// - if called by failed watcher: closing driver will prevent
+		//   drvWatchq<- deadlock in driver because we no longer read from it.
+		s.drvCloseErr = s.driver.Close()
 	})
 }
 
@@ -206,7 +212,7 @@ func (s *storage) Iterate(ctx context.Context, tidMin, tidMax Tid) ITxnIterator
 
 func (s *storage) Close() error {
 	s.shutdown(fmt.Errorf("closed"))
-	return s.driver.Close() // this will close drvWatchq and cause watcher stop
+	return s.drvCloseErr
 }
 
 // loading goes through cache - this way prefetching can work