Keep a merge a cut made history, and build a batch's walk at the branch (hq ADR 0276 review)

internal/inventory/batches.go

@@ -64,6 +64,22 @@ func (i *Inventory) LaterMergesOf(ctx context.Context, repository, branch string
 		order by merged_at desc`, repository, branch, after)
 }
 
+// KeptOnBranch says the controller keeps a merge of a repository's branch, and when it heard its first merge
+// of any: a merge of a branch it keeps merges of, made since then, is the record's to answer, whatever a
+// module's last look says (novox/hq ADR 0276).
+func (i *Inventory) KeptOnBranch(ctx context.Context, repository, branch string) (bool, time.Time, error) {
+	var first *time.Time
+	var kept bool
+	err := i.store.Pool().QueryRow(ctx,
+		`select (select min(heard_at) from batched_merge),
+		        exists (select 1 from batched_merge where repository = lower($1) and branch = $2)`,
+		repository, branch).Scan(&first, &kept)
+	if err != nil || first == nil {
+		return false, time.Time{}, err
+	}
+	return kept, first.UTC(), nil
+}
+
 // AloneMerges is every merge waiting to be walked on its own commit, the newest first (ADR 0276 decision 3).
 func (i *Inventory) AloneMerges(ctx context.Context) ([]BatchedMerge, error) {
 	return i.merges(ctx, `where plan_id is null and alone order by merged_at desc nulls last, heard_at desc`)

internal/inventory/plans.go

@@ -152,6 +152,9 @@ type PlanDelivery struct {
 	TakenOverBy string `json:"taken_over_by,omitempty"`
 	// Batch is the window of a batch; nil once it is cut into a walk.
 	Batch *PlanBatch `json:"batch,omitempty"`
+	// Alone says the walk walks one merge on its own commit, after a failed walk carried it in a later one:
+	// built at that commit, not at the branch.
+	Alone bool `json:"alone,omitempty"`
 }
 
 // Waiting is whether the walk waits for its delivery's word.