buildstep: deepen a merge preview's history from the mirror when it holds the trunk commit, so the preview fetches no history from the forge (issue 1921, try speed S6)

go/buildstep/merge.go

@@ -24,7 +24,8 @@ type MergeResult struct {
 // signature, no hook, nothing of the machine's git configuration that changes a merge. Nothing is pushed.
 //
 // A merge needs both histories, so it makes the fetch whole (the shallow clone FetchCommit made is deepened) and
-// fetches base by its sha. A head that does not merge cleanly is an answer, not an error: the files in conflict, the
+// fetches base by its sha: from the mirror WithMirror names, when it holds base (UpdateMirror to base first), else
+// from remote. A head that does not merge cleanly is an answer, not an error: the files in conflict, the
 // merge undone and the workspace left at base. A head already in base answers a Merge equal to Base and no changed
 // paths. An error is that the merge could not be tried: a base or commit that is not a full sha, no base, a workspace
 // not at commit, or a fetch or git command that failed. remote and the options are those of FetchCommit; the runner's
@@ -67,12 +68,20 @@ func MergeInto(ctx context.Context, run forgekey.Runner, dir, remote, base, comm
 	if err != nil {
 		return res, err
 	}
-	fetch := []string{"fetch", "--no-tags", "origin", base}
-	if strings.TrimSpace(shallow) == "true" {
-		fetch = []string{"fetch", "--no-tags", "--unshallow", "origin", base}
+	fetchFrom := func(source string) error {
+		fetch := []string{"fetch", "--no-tags", source, base}
+		if strings.TrimSpace(shallow) == "true" {
+			fetch = []string{"fetch", "--no-tags", "--unshallow", source, base}
+		}
+		_, err := git(fetch...)
+		return err
 	}
-	if _, err := git(fetch...); err != nil {
-		return res, fmt.Errorf("the trunk commit %s: %w", base, err)
+	// With a mirror brought to the trunk commit, the history is deepened from it, a local fetch (issue 1921, try speed
+	// S6); a mirror that cannot answer leaves it to the forge, as before.
+	if o.mirror == "" || fetchFrom(o.mirror) != nil {
+		if err := fetchFrom("origin"); err != nil {
+			return res, fmt.Errorf("the trunk commit %s: %w", base, err)
+		}
 	}
 	if _, err := git("checkout", "-q", "--detach", base); err != nil {
 		return res, fmt.Errorf("the trunk commit %s cannot be checked out: %w", base, err)

go/buildstep/merge_integration_test.go

@@ -170,3 +170,33 @@ func TestMergeIntoAHeadAlreadyInTheTrunkChangesNothing(t *testing.T) {
 		t.Errorf("result = %+v, %v", res, err)
 	}
 }
+
+// 1921/S6: with a mirror brought to the head and to the trunk commit, the merge asks the forge for nothing: it succeeds
+// with the forge gone.
+func TestMergeIntoWithAMirrorAtTheTrunkNeedsNoForge(t *testing.T) {
+	r := newMergeRig(t)
+	head := r.head(map[string]string{"b.txt": "head\n"})
+	base := r.advance(map[string]string{"a.txt": "one\ntwo\nthree\nfour\n"})
+	mirror := filepath.Join(t.TempDir(), "mirror.git")
+	if err := os.MkdirAll(mirror, 0o755); err != nil {
+		t.Fatal(err)
+	}
+	for _, c := range []string{head, base} {
+		if err := UpdateMirror(context.Background(), forgekey.ExecRunner, mirror, r.remote, c, "", AllowFileRemote()); err != nil {
+			t.Fatal(err)
+		}
+	}
+	if err := FetchCommit(context.Background(), forgekey.ExecRunner, r.ws, r.remote, head, "", AllowFileRemote(), WithMirror(mirror)); err != nil {
+		t.Fatal(err)
+	}
+	if err := os.RemoveAll(r.bare); err != nil { // the forge is gone
+		t.Fatal(err)
+	}
+	res, err := MergeInto(context.Background(), forgekey.ExecRunner, r.ws, r.remote, base, head, AllowFileRemote(), WithMirror(mirror))
+	if err != nil {
+		t.Fatalf("the merge asked the forge: %v", err)
+	}
+	if len(res.Merge) != 40 || !reflect.DeepEqual(res.Changed, []string{"b.txt"}) {
+		t.Errorf("result = %+v", res)
+	}
+}

go/buildstep/merge_test.go

@@ -72,3 +72,39 @@ func TestMergeIntoRunsTheCheckMergesCommands(t *testing.T) {
 		}
 	}
 }
+
+// 1921/S6: with a mirror that holds the trunk commit, the merge deepens the workspace from the mirror, a local fetch,
+// and asks the forge nothing; a mirror that cannot answer falls back to the forge.
+func TestMergeIntoWithAMirrorFetchesTheTrunkFromTheMirror(t *testing.T) {
+	base := strings.Repeat("b", 40)
+	mirror := t.TempDir()
+	answer := func(failMirror bool) *fakeGit {
+		return &fakeGit{rule: func(l string) forgekey.Ran {
+			switch {
+			case strings.HasSuffix(l, "rev-parse HEAD"):
+				return forgekey.Ran{Stdout: sha + "\n"}
+			case strings.Contains(l, "--is-shallow-repository"):
+				return forgekey.Ran{Stdout: "true\n"}
+			case failMirror && strings.Contains(l, " fetch ") && strings.Contains(l, mirror):
+				return forgekey.Ran{Status: 128, Stderr: "fatal: not our ref"}
+			}
+			return forgekey.Ran{}
+		}}
+	}
+	f := answer(false)
+	dir := t.TempDir()
+	if _, err := MergeInto(context.Background(), f.run, dir, "ssh://mesh-forge/o/r.git", base, sha, WithMirror(mirror)); err != nil {
+		t.Fatal(err)
+	}
+	all := strings.Join(f.calls, "\n")
+	if !strings.Contains(all, "fetch --no-tags --unshallow "+mirror+" "+base) || strings.Contains(all, "--unshallow origin") {
+		t.Errorf("the trunk was not fetched from the mirror alone:\n%s", all)
+	}
+	g := answer(true)
+	if _, err := MergeInto(context.Background(), g.run, t.TempDir(), "ssh://mesh-forge/o/r.git", base, sha, WithMirror(mirror)); err != nil {
+		t.Fatal(err)
+	}
+	if all := strings.Join(g.calls, "\n"); !strings.Contains(all, "fetch --no-tags --unshallow origin "+base) {
+		t.Errorf("a mirror that could not answer did not fall back to the forge:\n%s", all)
+	}
+}