Find a rechecked pull request's group by its repository and number (issue 867, ADR 0382 §1)

A recheck rebuilds the pull request without its head branch name, so the sibling lookup found no group and said nothing: mesh-host#114's recheck was judged without its sibling. With no branch name, the group is the one whose member_details hold the pull request; an older mesh-delivery says the siblings could not be learned. TestReplay867Recheck replays it.

Co-Authored-By: Claude <noreply@anthropic.com>

cmd/mesh-controller/check_siblings.go

@@ -57,19 +57,39 @@ func nameOf(repository string) string {
 // learned, empty when they could: the group's members carry no head or base — a mesh-delivery from before the rule.
 func siblingsFor(p link.PullUpdated, entries []inventory.Entry, beside map[string]link.CheckedOut,
 	lines []groupsLine) ([]link.CheckSibling, string) {
-	if p.Head == "" {
-		return nil, ""
-	}
+	own := strings.ToLower(p.Owner + "/" + p.Repo)
 	var group *groupsLine
-	for i := range lines {
-		if lines[i].ID == p.Head {
-			group = &lines[i]
+	if p.Head == "" {
+		// A recheck (mesh-delivery's `recheck` through `delivery check`) carries no head branch name: the group is the one
+		// whose members hold this very pull request, by repository and number.
+		learnable := false
+		for i := range lines {
+			for _, m := range lines[i].Details {
+				learnable = true
+				if p.Number != 0 && m.Number == p.Number && strings.ToLower(m.Repository) == own {
+					group = &lines[i]
+				}
+			}
+		}
+		if group == nil {
+			for _, l := range lines {
+				if len(l.Details) == 0 && len(l.Members) > 0 && !learnable {
+					return nil, "mesh-delivery named the groups' members without their pull requests (a mesh-delivery from before novox/hq ADR 0382): " +
+						"the group of a pull request announced without its head branch cannot be found"
+				}
+			}
+			return nil, "" // a lone pull request
+		}
+	} else {
+		for i := range lines {
+			if lines[i].ID == p.Head {
+				group = &lines[i]
+			}
+		}
+		if group == nil {
+			return nil, ""
 		}
 	}
-	if group == nil {
-		return nil, ""
-	}
-	own := strings.ToLower(p.Owner + "/" + p.Repo)
 	if len(group.Details) == 0 && len(group.Members) > 0 {
 		return nil, "mesh-delivery named the group's members without their heads and bases (a mesh-delivery from before novox/hq ADR 0382)"
 	}