RR-2: the reader reads an accepted kind's stream

check or delivery; the same kind name may ride both.

Builds: design 47 G §409, §415

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

go/seatdef/seatdef.go

@@ -223,6 +223,12 @@ type Verb struct {
 	NeedsApproval bool `json:"needs-approval,omitempty"`
 }
 
+// The streams an accepted kind may ride (design 47 G §415).
+const (
+	StreamCheck    = "check"
+	StreamDelivery = "delivery"
+)
+
 // Message is one accepted message or one event of the seat, its body a JSON Schema.
 type Message struct {
 	// Name is the message's name; an event's may be lower-case words joined by dots (pull.updated), as a module's
@@ -245,6 +251,9 @@ type Message struct {
 	Keep *Keep `json:"keep,omitempty"`
 	// Callers says who may send an accepted kind, as a verb's (design 47 G §338).
 	Callers *Callers `json:"callers,omitempty"`
+	// Stream is the stream an accepted kind rides, StreamCheck or StreamDelivery; its subject is `<stream>.<kind>`, and
+	// the same kind name may ride more than one stream (design 47 G §415).
+	Stream string `json:"stream,omitempty"`
 	// MaxAgeDays and MaxMiB bound an event kept KeepStream: how long a message is kept and how much the stream holds
 	// (design 47 *What the gate owes the forge*: the stream `owed`, 7 days and at most 64 MiB). Given with KeepStream
 	// alone.
@@ -814,10 +823,16 @@ func checkMessages(field string, ms []Message, whole bool) Problems {
 		}
 		if !named.MatchString(m.Name) {
 			ps = append(ps, Problem{where + ".name", "missing, or not a lower-case name"})
-		} else if seen[m.Name] {
+		} else if key := m.Stream + "/" + m.Name; seen[key] {
 			ps = append(ps, Problem{where + ".name", fmt.Sprintf("%q twice", m.Name)})
 		}
-		seen[m.Name] = true
+		seen[m.Stream+"/"+m.Name] = true
+		switch {
+		case m.Stream != "" && field != "accepts":
+			ps = append(ps, Problem{where + ".stream", "only an accepted kind rides a stream"})
+		case m.Stream != "" && m.Stream != StreamCheck && m.Stream != StreamDelivery:
+			ps = append(ps, Problem{where + ".stream", fmt.Sprintf("%q is not %s or %s", m.Stream, StreamCheck, StreamDelivery)})
+		}
 		mode := m.legacyMode()
 		switch {
 		case m.Keep != nil && field != "events":

go/seatdef/seatdef_test.go

@@ -143,6 +143,13 @@ func TestADefinitionIsReadStrictlyNamingWhatIsWrong(t *testing.T) {
 		{"a verb marked for approval", strings.Replace(whole, `"name":"list",`, `"name":"list","needs-approval":true,`, 1), ""},
 		{"a verb marked with a word that is no boolean", strings.Replace(whole, `"name":"list",`, `"name":"list","needs-approval":"yes",`, 1), "not a seat definition's JSON"},
 		{"a bodies section", strings.Replace(whole, `"family":`, `"bodies":{"Finding":{"type":"object"}},"family":`, 1), ""},
+		{"one kind on two streams", strings.Replace(whole, `"events":`,
+			`"accepts":[{"name":"step","body":{},"stream":"check"},{"name":"step","body":{},"stream":"delivery"}],"events":`, 1), ""},
+		{"one kind twice on a stream", strings.Replace(whole, `"events":`,
+			`"accepts":[{"name":"step","body":{},"stream":"check"},{"name":"step","body":{},"stream":"check"}],"events":`, 1), `"step" twice`},
+		{"a stream that is none", strings.Replace(whole, `"events":`, `"accepts":[{"name":"step","body":{},"stream":"mail"}],"events":`, 1),
+			`accepts[0].stream: "mail" is not check or delivery`},
+		{"an event on a stream", strings.Replace(whole, `"name":"created"`, `"name":"created","stream":"check"`, 1), "only an accepted kind rides a stream"},
 		{"over 256 KiB", listGrain[:len(listGrain)-1] + `,"description":"` + strings.Repeat("x", MaxSize) + `"}`,
 			ErrTooLarge.Error()},
 	}