Begin the record of merges at the earliest merge kept on any trunk, not per trunk (ADR 0463 section 2, issue 1336)

internal/inventory/batches.go

@@ -75,6 +75,16 @@ func (i *Inventory) KeptOnBranch(ctx context.Context, repository, branch string)
 	return i.chainKeptOnBranch(ctx, c, repository, branch)
 }
 
+// EarliestKept says the controller keeps a merge of some repository's branch, and when it heard the first of all: where its record of
+// merges begins on the whole mesh (novox/hq ADR 0463). A merge made before then is history on every trunk.
+func (i *Inventory) EarliestKept(ctx context.Context) (bool, time.Time, error) {
+	c, err := i.theChain(ctx)
+	if err != nil {
+		return false, time.Time{}, err
+	}
+	return i.chainEarliestKept(ctx, c)
+}
+
 // 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) {
 	c, err := i.theChain(ctx)

internal/inventory/chain_rollouts.go

@@ -437,6 +437,24 @@ func (i *Inventory) chainKeptOnBranch(ctx context.Context, c *chain.Chain, repos
 	return true, first.UTC(), nil
 }
 
+// chainEarliestKept is when the controller heard the first merge it holds a record of, on any repository and branch.
+func (i *Inventory) chainEarliestKept(ctx context.Context, c *chain.Chain) (bool, time.Time, error) {
+	w, err := i.chainWinners(ctx, c)
+	if err != nil {
+		return false, time.Time{}, err
+	}
+	var first time.Time
+	for _, h := range w {
+		if first.IsZero() || h.m.Heard.Before(first) {
+			first = h.m.Heard
+		}
+	}
+	if first.IsZero() {
+		return false, time.Time{}, nil
+	}
+	return true, first.UTC(), nil
+}
+
 func (i *Inventory) chainAloneMerges(ctx context.Context, c *chain.Chain) ([]BatchedMerge, error) {
 	w, err := i.chainWinners(ctx, c)
 	if err != nil {

internal/inventory/chain_rollouts_test.go

@@ -347,3 +347,28 @@ func TestThePlansAreReadFromAViewKeptByOneWatch(t *testing.T) {
 		return len(open) == 3
 	})
 }
+
+// Where the record of merges begins is the earliest merge kept on any repository and branch (novox/hq ADR 0463 §2).
+func TestTheRecordOfMergesBeginsAtTheEarliestMergeKeptOnAnyTrunk(t *testing.T) {
+	inv, c := anInventoryOnTheChain(t)
+	ctx := context.Background()
+	if kept, _, err := inv.chainEarliestKept(ctx, c); err != nil || kept {
+		t.Fatalf("an inventory with no merge kept says it keeps one: %v %v", kept, err)
+	}
+	if err := inv.chainSavePlans(ctx, c, []*Plan{aBatch("plan-1", plansAt), aBatch("plan-2", plansAt)}, nil); err != nil {
+		t.Fatal(err)
+	}
+	later := aMerge("m-late", "plan-1")
+	later.Repository, later.Branch, later.Heard = "novox/mesh-sdk", "main", plansAt.Add(48*time.Hour)
+	earlier := aMerge("m-early", "plan-2")
+	earlier.Repository, earlier.Branch, earlier.Heard = "novox/photos", "nox-mesh", plansAt.Add(time.Hour)
+	for _, m := range []BatchedMerge{later, earlier} {
+		if added, err := inv.chainAddMerge(ctx, c, m); err != nil || !added {
+			t.Fatalf("%s: %v %v", m.Commit, added, err)
+		}
+	}
+	kept, first, err := inv.chainEarliestKept(ctx, c)
+	if err != nil || !kept || !first.Equal(plansAt.Add(time.Hour)) {
+		t.Fatalf("the record begins at %v (%v, %v), want %v", first, kept, err, plansAt.Add(time.Hour))
+	}
+}