R-5: a kept call is asked again only while its key is open and not replaced

busretry.Open and DoKept, process.Conn.CallKeptWhileOpen.

Builds: design 47 G §351, §425, §460

Claude-Session: https://claude.ai/code/session_01JG73KEq4zoDfM9WtYpcQgZ

go/busretry/busretry.go

@@ -84,6 +84,45 @@ func DoRepeatable[T any](what string, try func() (T, error), blackout func() boo
 	})
 }
 
+// Open is what an asker passes a kept call beside the ask's key (design 47 G §351, §425, §460): Open reads the asker's own
+// state and says whether the key is still open; Replaced, when the asker has one, says an event that replaces the ask was
+// heard. The ask is made again only while Open answers yes and Replaced says no. An error of Open is written to Logf
+// (standard error when nil), naming the key, and the ask is made again: a kept call is one the callee takes once by a
+// key, so asking again is safe and not asking would be silent. Open is always given.
+type Open struct {
+	Key      string
+	Open     func(key string) (bool, error)
+	Replaced func() bool
+	Logf     func(format string, args ...any)
+}
+
+func (o Open) stillOpen() bool {
+	if o.Replaced != nil && o.Replaced() {
+		return false
+	}
+	open, err := o.Open(o.Key)
+	if err != nil {
+		logf := o.Logf
+		if logf == nil {
+			logf = func(f string, a ...any) { fmt.Fprintf(os.Stderr, "[mesh-sdk] "+f+"\n", a...) }
+		}
+		logf("busretry: whether the ask %q is still open could not be read, so it is asked again: %v", o.Key, err)
+		return true
+	}
+	return open
+}
+
+// DoKept is DoRepeatable for a kept call that is asked again only while its key is open and not replaced (Open).
+func DoKept[T any](what string, o Open, try func() (T, error), blackout func() bool) (T, error) {
+	if o.Open == nil {
+		var zero T
+		return zero, fmt.Errorf("%s: a kept call names its key's open function", what)
+	}
+	return do(what, try, func(err error) bool {
+		return (TimedOut(err) || (Undelivered(err) && (blackout == nil || blackout()))) && o.stillOpen()
+	})
+}
+
 // TimedOut says err is a request's deadline passing without an answer: the request may have been delivered, so only a
 // call marked safe to make again (DoRepeatable) asks it again. A callee's own answer (Answered) never counts.
 func TimedOut(err error) bool {

go/busretry/busretry_test.go

@@ -68,3 +68,44 @@ func TestABusThatAnswersNothingIsWaitedOut(t *testing.T) {
 		}
 	}
 }
+
+// Design 47 G §351, §460: a kept call is asked again only while its key is open and not replaced; an error of `open` is
+// logged naming the key and the ask is made again.
+func TestAKeptCallIsAskedAgainOnlyWhileItsKeyIsOpen(t *testing.T) {
+	for _, row := range []struct {
+		name     string
+		open     func(n int) (bool, error) // by the number of asks made so far
+		replaced bool
+		tries    int
+		logged   bool
+	}{
+		{"open throughout, then answered", func(int) (bool, error) { return true, nil }, false, 3, false},
+		{"closed after the first ask: not asked again", func(int) (bool, error) { return false, nil }, false, 1, false},
+		{"replaced: not asked again", func(int) (bool, error) { return true, nil }, true, 1, false},
+		{"open errs: logged and asked again", func(int) (bool, error) { return false, errors.New("state unreadable") }, false, 3, true},
+	} {
+		t.Cleanup(Waits(10*time.Second, time.Millisecond, 4*time.Millisecond))
+		tries, logs := 0, []string{}
+		o := Open{Key: "ask-7", Open: func(k string) (bool, error) {
+			if k != "ask-7" {
+				t.Errorf("%s: open asked about %q", row.name, k)
+			}
+			return row.open(tries)
+		}, Replaced: func() bool { return row.replaced },
+			Logf: func(f string, a ...any) { logs = append(logs, fmt.Sprintf(f, a...)) }}
+		_, err := DoKept("the test", o, func() (int, error) {
+			tries++
+			if tries < 3 {
+				return 0, nats.ErrNoResponders
+			}
+			return tries, nil
+		}, nil)
+		if tries != row.tries || (row.tries == 3) != (err == nil) || (len(logs) > 0) != row.logged ||
+			(row.logged && !strings.Contains(logs[0], "ask-7")) {
+			t.Errorf("%s: %d asks, err %v, logs %v", row.name, tries, err, logs)
+		}
+	}
+	if _, err := DoKept("x", Open{Key: "k"}, func() (int, error) { return 0, nil }, nil); err == nil {
+		t.Errorf("a kept call with no open function was made")
+	}
+}

go/process/call.go

@@ -76,6 +76,20 @@ func (c *Conn) CallKeptOn(node, key string, body any) (json.RawMessage, error) {
 	return c.callKeptOn(called, fallback, body)
 }
 
+// CallKeptWhileOpen is CallKept for an ask the asker can close or replace: it is asked again only while open says its
+// key is still open (design 47 G §351, §460).
+func (c *Conn) CallKeptWhileOpen(open busretry.Open, key string, body any) (json.RawMessage, error) {
+	called, fallback, err := callSubjectsOn(key, "", c.callerToken())
+	if err != nil {
+		return nil, err
+	}
+	data, err := json.Marshal(body)
+	if err != nil {
+		return nil, err
+	}
+	return busretry.DoKept("the call on "+fallback, open, askBoth(c.request, called, fallback, data), nil)
+}
+
 func (c *Conn) callKeptOn(called, fallback string, body any) (json.RawMessage, error) {
 	data, err := json.Marshal(body)
 	if err != nil {