[nfc] Add working regression test for #2564

The previous attempt at a test didn't fail because it didn't actually exercise the inspector protocol. If it had, it would have encounterd a WebSocket error and failed.

This commit removes the previous test, uncomments out the profiling test, factors it out into a separate function, then uses that function to implement two separate test cases, one to actually test profiling, and one to test repeated inspector connections.

src/workerd/server/tests/inspector/driver.mjs

@@ -54,68 +54,71 @@ async function connectInspector(port) {
   });
 }
 
-// TODO(soon): This test reproduces a null pointer dereference in workerd (possibly the same issue
-//   as https://github.com/cloudflare/workerd/issues/2564), but the test doesn't fail. :(
-test('Can repeatedly connect and disconnect to the inspector port', async () => {
-  for (let i = 0; i < 5; ++i) {
-    let inspectorClient = await connectInspector(
-      await workerd.getListenInspectorPort()
+async function profileAndExpectDeriveBitsFrames(inspectorClient) {
+  // Enable and start profiling.
+  await inspectorClient.Profiler.enable();
+  await inspectorClient.Profiler.start();
+
+  // Drive the worker with a test request. A single one is sufficient.
+  let httpPort = await workerd.getListenPort('http');
+  const response = await fetch(`http://localhost:${httpPort}`);
+  await response.arrayBuffer();
+
+  // Stop and disable profiling.
+  const profile = await inspectorClient.Profiler.stop();
+  await inspectorClient.Profiler.disable();
+
+  // Figure out which function name was most frequently sampled.
+  let hitCountMap = new Map();
+
+  for (let node of profile.profile.nodes) {
+    if (hitCountMap.get(node.callFrame.functionName) === undefined) {
+      hitCountMap.set(node.callFrame.functionName, 0);
+    }
+    hitCountMap.set(
+      node.callFrame.functionName,
+      hitCountMap.get(node.callFrame.functionName) + node.hitCount
     );
+  }
 
-    // Drive the worker with a test request.
-    let httpPort = await workerd.getListenPort('http');
-    const response = await fetch(`http://localhost:${httpPort}`);
-    let body = await response.arrayBuffer();
-    console.log(body);
+  let max = {
+    name: null,
+    count: 0,
+  };
 
-    await inspectorClient.close();
+  for (let [name, count] of hitCountMap) {
+    if (count > max.count) {
+      max.name = name;
+      max.count = count;
+    }
   }
+
+  // The most CPU-intensive function our test script runs is `deriveBits()`, so we expect that to be
+  // the most frequently sampled function.
+  assert.equal(max.name, 'deriveBits');
+  assert.notEqual(max.count, 0);
+}
+
+// Regression test for https://github.com/cloudflare/workerd/issues/1754.
+//
+// At one time, workerd produced only "(program)" frames.
+test('Profiler mostly sees deriveBits() frames', async () => {
+  let inspectorClient = await connectInspector(
+    await workerd.getListenInspectorPort()
+  );
+  await profileAndExpectDeriveBitsFrames(inspectorClient);
+  await inspectorClient.close();
 });
 
-// TODO(soon): Re-enable once https://github.com/cloudflare/workerd/issues/2564 is solved.
-// test("Profiler mostly sees deriveBits() frames", async () => {
-//   let inspectorClient = await connectInspector(await workerd.getListenInspectorPort());
-
-//   // Enable and start profiling.
-//   await inspectorClient.Profiler.enable();
-//   await inspectorClient.Profiler.start();
-
-//   // Drive the worker with a test request. A single one is sufficient.
-//   let httpPort = await workerd.getListenPort("http");
-//   const response = await fetch(`http://localhost:${httpPort}`);
-//   await response.arrayBuffer();
-
-//   // Stop and disable profiling.
-//   const profile = await inspectorClient.Profiler.stop();
-//   await inspectorClient.Profiler.disable();
-
-//   // Figure out which function name was most frequently sampled.
-//   let hitCountMap = new Map();
-
-//   for (let node of profile.profile.nodes) {
-//     if (hitCountMap.get(node.callFrame.functionName) === undefined) {
-//       hitCountMap.set(node.callFrame.functionName, 0);
-//     }
-//     hitCountMap.set(node.callFrame.functionName,
-//         hitCountMap.get(node.callFrame.functionName) + node.hitCount);
-//   }
-
-//   let max = {
-//     name: null,
-//     count: 0,
-//   };
-
-//   for (let [name, count] of hitCountMap) {
-//     if (count > max.count) {
-//       max.name = name;
-//       max.count = count;
-//     }
-//   }
-
-//   // The most CPU-intensive function our test script runs is `deriveBits()`, so we expect that to be
-//   // the most frequently sampled function.
-//   assert.equal(max.name, "deriveBits");
-//   assert.notEqual(max.count, 0);
-
-//   await inspectorClient.close();
-// });
+// Regression test for https://github.com/cloudflare/workerd/issues/2564.
+//
+// At one time, workerd segfaulted on the second inspector connection.
+test('Can repeatedly reconnect the inspector and profiling still works', async () => {
+  for (let i = 0; i < 4; ++i) {
+    let inspectorClient = await connectInspector(
+      await workerd.getListenInspectorPort()
+    );
+    await profileAndExpectDeriveBitsFrames(inspectorClient);
+    await inspectorClient.close();
+  }
+});