gate: find an approved merge by its ask, not by which member's run was seen first, so a group's turns are written before its first member is handed at every look (ADR 0548 §3; issue 568, review round 1)

modules/mesh-delivery-gate/cmd/mesh-delivery-gate/turns.go

@@ -102,7 +102,13 @@ type approvedMerge struct {
 func (h *Holder) approvedMerges() []approvedMerge {
 	var out []approvedMerge
 	seen := map[string]bool{}
-	for _, d := range h.deliveries {
+	ids := make([]string, 0, len(h.deliveries))
+	for id := range h.deliveries {
+		ids = append(ids, id)
+	}
+	sort.Strings(ids) // the same owner of an ask at every look, whichever member's run holds it
+	for _, id := range ids {
+		d := h.deliveries[id]
 		r := d.Ready
 		if r == nil || r.Approved == nil || r.Declined != nil || r.AskID == "" || len(r.Heads) == 0 || seen[r.AskID] ||
 			d.State.Final() || d.State == Rejected || d.Base != MergeBranch || r.RefusedSince != nil || h.allMerged(r.Heads) {
@@ -146,7 +152,8 @@ func (h *Holder) mergeOfDelivery(d *Delivery) (approvedMerge, bool) {
 		return approvedMerge{}, false
 	}
 	for _, am := range h.approvedMerges() {
-		if am.run == c {
+		// By the ask, not the run: a group's members may each hold a run of the same ask, and the merge is the ask's.
+		if am.run.AskID == c.AskID {
 			return am, true
 		}
 	}