go/zodb: Rework watchq to also receive errors

This continues 4d2c8b1d (go/zodb: Require drivers to provide
notifications for database change events) and a6580062 (go/zodb: Require
drivers to provide at₀ on open) and reworks watchq to receive not only
commit events, but also errors from watcher in storage driver.

go/zodb/open.go

@@ -49,9 +49,7 @@ type DriverOptions struct {
 	//
 	// The storage driver will send only and all events in (at₀, +∞] range,
 	// where at₀ is at returned by driver open.
-	//
-	// TODO extend watchq to also receive errors from watcher.
-	Watchq chan<- CommitEvent
+	Watchq chan<- Event
 }
 
 // DriverOpener is a function to open a storage driver.

go/zodb/storage/fs1/filestorage.go

@@ -98,8 +98,8 @@ type FileStorage struct {
 	txnhMax TxnHeader // (both with .Len=0 & .Tid=0 if database is empty)
 	downErr error     // !nil when the storage is no longer operational
 
-	// driver client <- watcher: database commits.
-	watchq chan<- zodb.CommitEvent
+	// driver client <- watcher: database commits | errors.
+	watchq chan<- zodb.Event
 
 	down     chan struct{}  // ready when storage is no longer operational
 	downOnce sync.Once      // shutdown may be due to both Close and IO error in watcher
@@ -483,6 +483,9 @@ func (fs *FileStorage) watcher(w *fsnotify.Watcher, errFirstRead chan<- error) {
 	fs.shutdown(err)
 
 	if fs.watchq != nil {
+		if err != nil {
+			fs.watchq <- &zodb.EventError{err}
+		}
 		close(fs.watchq)
 	}
 }
@@ -679,7 +682,7 @@ mainloop:
 				case <-fs.down:
 					return nil
 
-				case fs.watchq <- zodb.CommitEvent{it.Txnh.Tid, δoid}:
+				case fs.watchq <- &zodb.EventCommit{it.Txnh.Tid, δoid}:
 					// ok
 				}
 			}

go/zodb/storage/fs1/filestorage_test.go

@@ -377,7 +377,7 @@ func TestWatch(t *testing.T) {
 	// force tfs creation & open tfs at go side
 	at := xcommit(0, xtesting.ZRawObject{0, b("data0")})
 
-	watchq := make(chan zodb.CommitEvent)
+	watchq := make(chan zodb.Event)
 	fs, at0 := xfsopenopt(t, tfs, &zodb.DriverOptions{ReadOnly: true, Watchq: watchq})
 	if at0 != at {
 		t.Fatalf("opened @ %s  ; want %s", at0, at)
@@ -423,10 +423,22 @@ func TestWatch(t *testing.T) {
 			xtesting.ZRawObject{i, b(datai)})
 
 		// TODO also test for watcher errors
-		e := <-watchq
+		event := <-watchq
 
-		if objvWant := []zodb.Oid{0, i}; !(e.Tid == at && reflect.DeepEqual(e.Changev, objvWant)) {
-			t.Fatalf("watch:\nhave: %s %s\nwant: %s %s", e.Tid, e.Changev, at, objvWant)
+		var δ *zodb.EventCommit
+		switch event := event.(type) {
+		default:
+			panic(fmt.Sprintf("unexpected event: %T", event))
+
+		case *zodb.EventError:
+			t.Fatal(event.Err)
+
+		case *zodb.EventCommit:
+			δ = event
+		}
+
+		if objvWant := []zodb.Oid{0, i}; !(δ.Tid == at && reflect.DeepEqual(δ.Changev, objvWant)) {
+			t.Fatalf("watch:\nhave: %s %s\nwant: %s %s", δ.Tid, δ.Changev, at, objvWant)
 		}
 
 		checkLastTid(at)

go/zodb/storage/zeo/zeo.go

@@ -45,8 +45,8 @@ type zeo struct {
 	mu      sync.Mutex
 	lastTid zodb.Tid
 
-	// driver client <- watcher: database commits.
-	watchq chan<- zodb.CommitEvent // FIXME stub
+	// driver client <- watcher: database commits | errors.
+	watchq chan<- zodb.Event // FIXME stub
 
 	url string // we were opened via this
 }

go/zodb/zodb.go

@@ -426,9 +426,26 @@ type Committer interface {
 	// TpcAbort(txn)
 }
 
+// Event represents one database event.
+//
+// Possible events are:
+//
+//	- EventError	an error happened
+//	- EventCommit	a transaction was committed
+type Event interface {
+	event()
+}
+
+func (_ *EventError)  event() {}
+func (_ *EventCommit) event() {}
+
+// EventError is event descrbing an error observed by watcher.
+type EventError struct {
+	Err error
+}
 
-// CommitEvent is event describing one observed database commit.
-type CommitEvent struct {
+// EventCommit is event describing one observed database commit.
+type EventCommit struct {
 	Tid     Tid   // ID of committed transaction
 	Changev []Oid // ID of objects changed by committed transaction
 }