B1g-1: an adopted event whose handler fails is left unacknowledged, the bus asks again

Builds: design 47 G A-222, A-193, A-199

go/views/process_reader.go

@@ -181,7 +181,7 @@ func (r *ProcessReader) Adopt(f *Follower, seat, event string, hear func(context
 
 // AdoptEvent hears a seat's event on the one subscription the followers adopted on that event share, with no picture and no
 // first view (design 47 G A-185, A-199): hear is handed each message of the event beside every adopted follower's handler,
-// and the message is acknowledged. The subscription is made at once when no follower made it, so a part that follows no
+// and the message is acknowledged, unless hear fails: then it is left unacknowledged and the bus asks again (A-222). The subscription is made at once when no follower made it, so a part that follows no
 // view of the seat still hears the event; adopted before Start, it is made at Start.
 func (r *ProcessReader) AdoptEvent(seat, event string, hear func(context.Context, []byte) error) error {
 	if seat == "" || event == "" || hear == nil {
@@ -365,12 +365,17 @@ func (r *ProcessReader) Hear(ctx context.Context) func(jetstream.Msg) {
 		}
 		r.mu.Unlock()
 		if hears != nil {
+			failed := false
 			for _, h := range hears {
 				if err := h.hear(ctx, m.Data()); err != nil {
 					r.logf("views: %s is not taken: %v", m.Subject(), err)
+					// an event adopted with no picture is asked again by the bus (design 47 G A-222)
+					failed = failed || h.bucket == ""
 				}
 			}
-			_ = m.Ack()
+			if !failed {
+				_ = m.Ack()
+			}
 			return
 		}
 		if owned != nil {

go/views/views_test.go

@@ -820,6 +820,28 @@ func subViews(t *testing.T) {
 		if len(hooked) != 2 || !am.acked {
 			t.Errorf("a reader following no view heard %v, acked %v", hooked, am.acked)
 		}
+		// A-222: an adopted event whose handler fails is left unacknowledged, so the bus asks again.
+		failing := &ProcessReader{Module: "reader", Now: now}
+		var tries int
+		if err := failing.AdoptEvent("s", "revision-said", func(context.Context, []byte) error {
+			tries++
+			if tries == 1 {
+				return errors.New("not placed yet")
+			}
+			return nil
+		}); err != nil {
+			t.Fatal(err)
+		}
+		fm := &seatMsg{subject: "mesh.seat.s.event.revision-said.next", data: []byte(`{"revision":10}`)}
+		failing.Hear(context.Background())(fm)
+		if fm.acked {
+			t.Error("a failed adopted event was acknowledged")
+		}
+		fm2 := &seatMsg{subject: fm.subject, data: fm.data}
+		failing.Hear(context.Background())(fm2)
+		if !fm2.acked || tries != 2 {
+			t.Errorf("the event asked again: acked %v after %d tries", fm2.acked, tries)
+		}
 		pf.WaitFor(context.Background(), "4")
 		ended <- true
 		if fd := pfs.next(t); fd.Kind != ViewWaitsKind || fd.ID != "reader.s.revisions" {