A re-read of the seat events that fails partway keeps nothing it read (issue 1966)

A created whose dropped lay past the failure was kept again. When the
stream cannot be read whole, only the error is written to bindings-again.

Claude-Session: https://claude.ai/code/session_01NcbgfFSnn24M3GcqttoU7i

modules/mesh-grants/grants_test.go

@@ -3236,7 +3236,15 @@ func TestGrants(t *testing.T) {
 		if got := readAgain("mod-z"); !strings.Contains(got, "holds no created or recreated of mod-z") || !strings.Contains(got, "seat mesh-broker's list") {
 			t.Errorf("a module whose created the stream no longer holds: %s", got)
 		}
-		if len(acts) != 3 || !strings.HasPrefix(acts[0], "grants_bindings_again mod-x: ") {
+		// A re-read that fails partway applies nothing it read: the created whose dropped lay past the failure is not kept.
+		p.History = failingHistory{saidHistory{event("created", "mod-x", "late", "mod-x_late")}}
+		if got := readAgain("mod-x"); !strings.Contains(got, "could not be read, so nothing was kept") || !strings.Contains(got, `"read":0`) {
+			t.Errorf("a failed re-read: %s", got)
+		}
+		if got := bindingsOn("a.mod-x"); len(got) != 1 || got[0].Bucket != "mod-x_windows" {
+			t.Errorf("a failed re-read kept what it read before the failure: %+v", got)
+		}
+		if len(acts) != 4 || !strings.HasPrefix(acts[0], "grants_bindings_again mod-x: ") {
 			t.Errorf("hand acts %v", acts)
 		}
 	})
@@ -4680,3 +4688,10 @@ func oldStateReaders(path string, src []byte) ([]string, error) {
 type saidHistory []grants.SaidEvent
 
 func (h saidHistory) Said() ([]grants.SaidEvent, error) { return h, nil }
+
+// failingHistory is a re-read that fails partway: the events read so far, and the error.
+type failingHistory struct{ read saidHistory }
+
+func (h failingHistory) Said() ([]grants.SaidEvent, error) {
+	return h.read, errors.New("$JS.API.STREAM.MSG.GET.EVENTS at sequence 2: context deadline exceeded")
+}

modules/mesh-grants/internal/grants/bindings.go

@@ -307,7 +307,9 @@ func (p *Part) bindingsAgain(res *Result, module, by string) {
 	out := BindingsAgain{V: 1, Module: module, By: by, At: p.now(), Kept: []string{}, Already: []string{}, Dropped: []string{}}
 	events, err := p.History.Said()
 	if err != nil {
-		out.Error = "the shared events stream could not be read: " + err.Error()
+		// A read that failed partway applies nothing: a created whose dropped lies past the failure would be kept again.
+		out.Error = "the shared events stream could not be read, so nothing was kept: " + err.Error()
+		events = nil
 	}
 	heardCreate := false
 	for _, e := range events {