lab_check: work in a fresh directory each time, and name what an earlier check left (hq issue 624)

lab_check cloned into one fixed path, <work>/check, after an rm -rf whose result it ignored; a tree an
earlier check left that the lab's account could not remove made every check fail on "destination path
already exists". Each check now works in a fresh check-* directory it removes when done, and clears
what earlier checks left, naming any path it could not remove.

modules/lab/test/runs.test.ts

@@ -194,3 +194,35 @@ test("the lab asks the package registry for read only", () => {
   assert.ok(m.requires.includes("npm-package-registry"));
   assert.equal(m.contributes?.["npm-package-registry"]?.access, "read");
 });
+
+// lab_check failed for ever on "destination path '<work>/check' already exists": it cloned into one fixed
+// path and ignored whether removing the last one worked (novox/hq issue 624). Each check now works in a
+// fresh directory, and what earlier checks left is cleared or named.
+import { checkDir, clearChecks } from "../tools/runs.ts";
+
+test("each check gets a fresh directory of its own", () => {
+  const work = mkdtempSync(join(tmpdir(), "lab-work-"));
+  const a = checkDir(work);
+  const b = checkDir(work);
+  assert.notEqual(a, b);
+  assert.ok(a.startsWith(join(work, "check-")) && existsSync(a) && existsSync(b));
+  rmSync(work, { recursive: true, force: true });
+});
+
+test("what earlier checks left is cleared, and what cannot be is named rather than tripped over", () => {
+  const work = mkdtempSync(join(tmpdir(), "lab-work-"));
+  for (const d of ["check/src", "check-setup/mesh-lab", "check-abc", "run-x", "keep"]) mkdirSync(join(work, d), { recursive: true });
+  // A tree this account cannot empty: a directory it may not write, with something in it.
+  mkdirSync(join(work, "check-stuck", "locked"), { recursive: true });
+  writeFileSync(join(work, "check-stuck", "locked", "file"), "x");
+  chmodSync(join(work, "check-stuck", "locked"), 0o555);
+  const said = clearChecks(work);
+  chmodSync(join(work, "check-stuck", "locked"), 0o755);
+  for (const gone of ["check", "check-setup", "check-abc"]) assert.equal(existsSync(join(work, gone)), false, gone);
+  assert.ok(existsSync(join(work, "run-x")) && existsSync(join(work, "keep")));
+  if (process.getuid?.() !== 0) {
+    assert.equal(said.length, 1);
+    assert.match(said[0] as string, /check-stuck, left by an earlier check, could not be removed/);
+  }
+  rmSync(work, { recursive: true, force: true });
+});