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

cmd/mesh-controller/batches_test.go

@@ -2,6 +2,7 @@ package main
 
 import (
 	"bytes"
+	"context"
 	"encoding/json"
 	"io"
 	"os"
@@ -138,8 +139,8 @@ func TestTwoMergesSecondsApartAreOneWalkAtTheLaterCommit(t *testing.T) {
 	if w.Delivery == nil || !slices.Equal(w.Delivery.Merges, want) {
 		t.Fatalf("the walk answers %+v, want %+v", w.Delivery, want)
 	}
-	if len(*asked) != 2 || (*asked)[0][2] != dunst.Commit || (*asked)[1][2] != dunst.Commit {
-		t.Fatalf("the walk did not ask its modules at the commit it carries: %v", *asked)
+	if len(*asked) != 2 || (*asked)[0][2] != "main" || (*asked)[1][2] != "main" {
+		t.Fatalf("the walk did not ask its modules at the branch, which holds the commit it carries: %v", *asked)
 	}
 	// Heard again, as the bus may hand it over twice: never a second merge, nor a second walk.
 	hear(t, open, claude, t0.Add(5*time.Minute))
@@ -625,3 +626,46 @@ func TestARemovedFileIsTheLastMergesWord(t *testing.T) {
 		t.Fatalf("combined as %+v", got)
 	}
 }
+
+// The catch-up hands over what the bus lost after its branch's later merges were cut, and the record keeps it:
+// an earlier merge is named by the walk that carried it; a merge made before a cut and heard after it, which
+// no later merge carries, joins the next batch — neither reads as history and is dropped (review of this change).
+func TestTheCatchUpKeepsWhatACutMadeHistory(t *testing.T) {
+	open := windowed(t)
+	asksWithPaths(t)
+	ctx := t.Context()
+	base := time.Now().UTC().Add(-time.Hour).Truncate(time.Second)
+	hear(t, open, repoMerge("one", "c2", base.Add(10*time.Second)), base.Add(11*time.Second))
+	cutAt(t, open, base.Add(2*time.Minute))
+	ws, _ := walks(t, open)
+	w := ws[0]
+	w.State = inventory.PlanDone
+	if err := open.inventory.SavePlan(ctx, &w); err != nil {
+		t.Fatal(err)
+	}
+	lost := []link.AnnouncedMerge{
+		{SourceMoved: repoMerge("one", "c1", base), At: base.Add(time.Second)},
+		{SourceMoved: repoMerge("one", "c3lost", base.Add(20*time.Second)), At: base.Add(21 * time.Second)},
+	}
+	catalogued := func(ctx context.Context) ([]inventory.Entry, map[string][]inventory.ReadRepository, error) {
+		entries, err := open.inventory.Catalogued(ctx)
+		if err != nil {
+			return nil, nil, err
+		}
+		read, err := readForPlanning(ctx, open.inventory)
+		return entries, read, err
+	}
+	if err := catchUpOnMerges(ctx, time.Now(), unheardMerges{announcedList(lost), open.inventory}, catalogued,
+		(following{open}).SourceMoved, t.Logf); err != nil {
+		t.Fatal(err)
+	}
+	got, _ := open.inventory.PlanByID(ctx, w.ID)
+	if len(got.Delivery.Merges) != 2 || got.Delivery.Merges[0] != (inventory.PlanMerge{Repository: "novox/one",
+		Commit: "c1", Carried: "c2"}) {
+		t.Fatalf("the earlier merge the catch-up handed over is not named by the walk that carried it: %+v",
+			got.Delivery.Merges)
+	}
+	if _, kept, err := open.inventory.MergeOf(ctx, "novox/one", "c3lost"); err != nil || !kept {
+		t.Fatalf("the merge made before the cut and heard after it was dropped: %v %v", kept, err)
+	}
+}