Run each part of a repository's own check in the toolchain it declares (hq issue 302)

mesh-lab's merge-check.sh runs in the TypeScript toolchain, which holds no Go
compiler, so its replays register was never compiled before a merge and a
broken one would have merged green. A script now declares a further part with
'# mesh-check-also: <toolchain> <script>'; the build seat runs it in that
toolchain's own container, and mesh/repo-check passes only when every part does.

internal/builder/check_test.go

@@ -480,3 +480,76 @@ func TestARedeliveredCheckRemovesWhatItsEarlierDeliveryLeft(t *testing.T) {
 		t.Errorf("the check left %d container(s) behind", len(left))
 	}
 }
+
+// **A repository in two languages checks both, each in its own toolchain** (novox/hq issue 302): the lab's
+// merge-check.sh runs in the TypeScript toolchain, and its Go replays were said "NOT CHECKED HERE" on every
+// pull request, so a register that did not compile would have merged green. Its script declares the Go
+// part; the check runs it in the Go toolchain's container after the script, and the layer passes only
+// when both do.
+func TestARepositoryInTwoLanguagesIsCheckedInBoth(t *testing.T) {
+	script := "#!/bin/sh\n# mesh-check-toolchain: typescript\n# mesh-check-also: go replays/merge-check.sh\nnpm test\n"
+	parts := ScriptParts([]byte(script))
+	if len(parts) != 2 || parts[0] != (ScriptPart{"typescript", CheckScript}) ||
+		parts[1] != (ScriptPart{"go", "replays/merge-check.sh"}) {
+		t.Fatalf("the parts read as %+v", parts)
+	}
+	if got := ScriptParts([]byte("#!/bin/sh\nset -eu\n")); len(got) != 1 || got[0] != (ScriptPart{"go", CheckScript}) {
+		t.Fatalf("a script declaring nothing reads as %+v", got)
+	}
+
+	// Each part run where its toolchain is: here, the image the part was given is what the command says.
+	held := CheckSpec{Toolchain: "go-image", Toolchains: map[string]string{"typescript": "ts-image", "go": "go-image"}}
+	run := func(t *testing.T, spec CheckSpec, files map[string]string, parts []ScriptPart) (*Layer, []string) {
+		t.Helper()
+		tree := t.TempDir()
+		for name, body := range files {
+			if err := os.MkdirAll(filepath.Dir(filepath.Join(tree, name)), 0o755); err != nil {
+				t.Fatal(err)
+			}
+			if err := os.WriteFile(filepath.Join(tree, name), []byte(body), 0o755); err != nil {
+				t.Fatal(err)
+			}
+		}
+		var images []string
+		var out tail
+		layer := ownCheck(t.Context(), spec, parts, tree, &out, func() bool { return false },
+			func(image, script string) *exec.Cmd {
+				images = append(images, image+" "+script)
+				cmd := exec.CommandContext(t.Context(), "sh", script)
+				cmd.Dir = tree
+				return cmd
+			}, func(string, string, ...any) {})
+		return layer, images
+	}
+	twoParts := []ScriptPart{{"typescript", CheckScript}, {"go", "replays/merge-check.sh"}}
+
+	layer, images := run(t, held, map[string]string{CheckScript: "exit 0\n", "replays/merge-check.sh": "exit 0\n"}, twoParts)
+	if layer.Verdict != "pass" || !strings.Contains(layer.Summary, "replays/merge-check.sh (go)") ||
+		strings.Join(images, ", ") != "ts-image merge-check.sh, go-image replays/merge-check.sh" {
+		t.Errorf("both parts passing answered %+v, ran %v", layer, images)
+	}
+	layer, _ = run(t, held, map[string]string{CheckScript: "exit 0\n",
+		"replays/merge-check.sh": "echo 'not gofmt'\"'\"'d:'; echo register.go; exit 1\n"}, twoParts)
+	if layer.Verdict != "fail" || !strings.Contains(layer.Summary, "replays/merge-check.sh failed") ||
+		!strings.Contains(layer.Summary, "register.go") || !strings.Contains(layer.Summary, "merge-check.sh (typescript) passed") {
+		t.Errorf("a failing Go part answered %+v", layer)
+	}
+	layer, images = run(t, held, map[string]string{CheckScript: "exit 2\n", "replays/merge-check.sh": "exit 0\n"}, twoParts)
+	if layer.Verdict != "fail" || len(images) != 1 {
+		t.Errorf("a failing first part answered %+v and ran %v", layer, images)
+	}
+	layer, _ = run(t, held, map[string]string{CheckScript: "exit 0\n"}, twoParts)
+	if layer.Verdict != "fail" || !strings.Contains(layer.Summary, "does not hold") {
+		t.Errorf("a declared part the repository lacks answered %+v", layer)
+	}
+	layer, _ = run(t, held, map[string]string{CheckScript: "exit 0\n"}, []ScriptPart{{"typescript", CheckScript},
+		{"go", "../elsewhere.sh"}})
+	if layer.Verdict != "fail" || !strings.Contains(layer.Summary, "not a path inside") {
+		t.Errorf("a part outside the repository answered %+v", layer)
+	}
+	layer, _ = run(t, CheckSpec{Toolchains: map[string]string{"typescript": "ts-image"}},
+		map[string]string{CheckScript: "exit 0\n", "replays/merge-check.sh": "exit 0\n"}, twoParts)
+	if layer.Verdict != "error" || !strings.Contains(layer.Summary, "go toolchain") {
+		t.Errorf("a part in a toolchain the mesh does not hold answered %+v — never a pass", layer)
+	}
+}