1 files changed, 86 insertions, 0 deletions
diff --git a/test/concurrency.test.ts b/test/concurrency.test.ts
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--- /dev/null
+++ b/test/concurrency.test.ts
@@ -0,0 +1,86 @@
+/**
+ * The semaphore, exercised through the only door it has: search().
+ *
+ * These run against a deliberately unreachable CDP endpoint. Every search fails
+ * fast at the connect step, which is exactly what is wanted — the questions here
+ * are about slot bookkeeping, not about Google. Hitting the real browser to test
+ * a counter would be slow, flaky, and rude to whoever is using it.
+ */
+
+import assert from "node:assert/strict";
+import { test } from "node:test";
+
+import { MAX_CONCURRENT_SEARCHES, disconnect, search } from "../src/search.ts";
+
+const UNREACHABLE = "127.0.0.1:1"; // Nothing listens on port 1.
+const quiet = () => {};
+
+const withUnreachableBrowser = async (body: () => Promise<void>): Promise<void> => {
+  const saved = process.env["BU_CDP_HTTP"];
+  process.env["BU_CDP_HTTP"] = UNREACHABLE;
+  disconnect();
+  try {
+    await body();
+  } finally {
+    if (saved === undefined) delete process.env["BU_CDP_HTTP"];
+    else process.env["BU_CDP_HTTP"] = saved;
+    disconnect();
+  }
+};
+
+const attempt = (query: string, signal: AbortSignal) =>
+  search({ query, numResults: 1 }, signal, quiet).then(
+    () => "fulfilled" as const,
+    (e: Error) => e.name,
+  );
+
+test("a failed search returns its slot, so the next one still runs", async () => {
+  await withUnreachableBrowser(async () => {
+    // Three times the cap, run sequentially. If a slot leaked on the failure
+    // path, the run past the cap would hang until the test timed out.
+    for (let i = 0; i < MAX_CONCURRENT_SEARCHES * 3; i++) {
+      const name = await attempt(`q${i}`, AbortSignal.timeout(10_000));
+      assert.equal(name, "BrowserUnavailableError", `attempt ${i}`);
+    }
+  });
+});
+
+test("aborting while queued does not wedge the semaphore", async () => {
+  await withUnreachableBrowser(async () => {
+    // Saturate the cap and queue two more, then cancel the queued ones. The
+    // bug this guards against: a cancelled waiter that is later handed a slot
+    // resolves into nothing, and the slot is never counted back.
+    const cancel = new AbortController();
+    const running = Array.from({ length: MAX_CONCURRENT_SEARCHES }, (_, i) =>
+      attempt(`running${i}`, AbortSignal.timeout(10_000)),
+    );
+    const queued = Array.from({ length: 2 }, (_, i) => attempt(`queued${i}`, cancel.signal));
+    cancel.abort();
+
+    await Promise.all(running);
+    for (const outcome of await Promise.all(queued)) {
+      assert.ok(
+        outcome === "SearchAbortedError" || outcome === "BrowserUnavailableError",
+        `unexpected outcome ${outcome}`,
+      );
+    }
+
+    // The real assertion: the pool still works afterwards. A wedged semaphore
+    // would leave this hanging rather than failing.
+    for (let i = 0; i < MAX_CONCURRENT_SEARCHES + 1; i++) {
+      assert.equal(await attempt(`after${i}`, AbortSignal.timeout(10_000)), "BrowserUnavailableError");
+    }
+  });
+});
+
+test("an unreachable endpoint is reported, never silently swapped for a local browser", async () => {
+  await withUnreachableBrowser(async () => {
+    const error = await search({ query: "q", numResults: 1 }, AbortSignal.timeout(10_000), quiet).then(
+      () => null,
+      (e: Error) => e,
+    );
+    assert.ok(error, "expected a throw");
+    assert.equal(error.name, "BrowserUnavailableError");
+    assert.match(error.message, new RegExp(UNREACHABLE.replace(".", "\\.")));
+  });
+});