issue 974: the builders say the checks they run in a bucket that ages, and a builder drops a waiting request for a head and trunk commit another is running

internal/broker/writers.go

@@ -41,6 +41,14 @@ func ownMachine(p Principal, pattern string) bool {
 	return p.Kind == KindNode && len(tokens) > 2 && tokens[2] == p.Node
 }
 
+// holdsTheBuildSeat is a principal holding the build seat, or the controller, which asserts the bucket the holders write.
+func holdsTheBuildSeat(p Principal, _ string) bool {
+	if p.Kind == KindController {
+		return true
+	}
+	return holdsSeatOf(p, "mesh.seat."+TheBuildSeat+".x")
+}
+
 // holdsSeatOf is a principal holding the seat a `mesh.seat.<seat>.…` pattern names: its module's own
 // principal, or the node tools carrying a module that holds it (ADR 0175, the runtime is its modules).
 func holdsSeatOf(p Principal, pattern string) bool {
@@ -153,6 +161,10 @@ var WritersTable = []WriterRow{
 		Others:   "the build seat's holders take it",
 		Subjects: []string{CheckSubjectPrefix(TheBuildSeat, true) + ">", CheckSubjectPrefix(TheBuildSeat, false) + ">"},
 		Writes:   isController},
+	{State: "a pull request's check running", Writer: "the build seat's holders — several by design, each the key of the check it runs",
+		KeptIn: "key-value " + CheckRunningBucket + ", aged out when not renewed", Others: "the build seat's holders read it to drop a duplicate request",
+		Subjects: kvOf(CheckRunningBucket), Writes: holdsTheBuildSeat,
+		Shared: "every machine holding the build seat puts the key of the check it runs and deletes it when the check ends"},
 	{State: "a merge announced", Writer: "one announcer per forge (the hook, or the poll when the hook is absent — never both)",
 		KeptIn: "the bus", Others: "—", Subjects: []string{"mesh.mod.*.event.pull.merged"}, Writes: ownModule},
 	// A pull request's head, before it merges (novox/hq to-be 45 §9): the same one announcer.