Memory leak when awaiting in inspector `Debugger.pause` callback

[timfish]

Version

20.10.0

Platform

All

Subsystem

inspector

What steps will reproduce the bug?

The following code leaks memory at about 10MB per second on my machine:

import { Session } from "node:inspector/promises";

const session = new Session();
session.connect();

session.on("Debugger.paused", async (event) => {
  for (const frame of event.params.callFrames) {
    const localScope = frame.scopeChain.find((scope) => scope.type === "local");

    if (localScope?.object?.objectId) {
      const props = await session.post("Runtime.getProperties", {
        objectId: localScope.object.objectId,
        ownProperties: true,
      });
    }
  }
});

await session.post("Debugger.enable");
await session.post("Debugger.setPauseOnExceptions", { state: "all" });

setInterval(() => {
  console.log(`${Math.floor(process.memoryUsage().rss / 1024 / 1024)} MB`);
}, 1000);

setInterval(() => {
  try {
    throw new Error("Hello");
  } catch {}
}, 1);

How often does it reproduce? Is there a required condition?

Every time

What is the expected behavior? Why is that the expected behavior?

No memory leaked

What do you see instead?

I see ~10MB leaked every second which is around 10kB per debugger pause event.

Additional information

I found that that the following code:

import { Session } from "node:inspector/promises";

const session = new Session();
session.connect();

session.on("Debugger.resumed", () => {
  console.log("Debugger resumed");
});

session.on("Debugger.paused", async (event) => {
  console.log("Debugger paused start");

  for (const frame of event.params.callFrames) {
    const localScope = frame.scopeChain.find((scope) => scope.type === "local");

    if (localScope?.object?.objectId) {
      console.log("Runtime.getProperties");
      const props = await session.post("Runtime.getProperties", {
        objectId: localScope.object.objectId,
      });
      console.log("Runtime.getProperties returned");
    }
  }

  console.log("Debugger.paused end");
});

await session.post("Debugger.enable");
await session.post("Debugger.setPauseOnExceptions", { state: "all" });

setTimeout(() => {
  try {
    throw new Error("Hello");
  } catch {}
}, 1000);

Outputs:

Debugger paused start
Runtime.getProperties
Debugger resumed
Runtime.getProperties returned
Runtime.getProperties
Runtime.getProperties returned
Runtime.getProperties
Runtime.getProperties returned
Debugger.paused end

The debugger does not wait for Debugger.resume. Instead it resumes automatically as soon as we call await session.post and I guess calling Runtime.getProperties after the resume causes something to be leaked?

[anonrig]

cc @legendecas @joyeecheung any suggestions?

[legendecas]

The retained scope information with 'Debugger.paused' must be released with a 'Debugger.resume' call when the debugger is still paused.

For instance:

session.on("Debugger.paused", (event) => {
  // Call 'Debugger.resume' synchronously to release the remote object in `event`.
  session.post("Debugger.resume");
});

Notably, using breakpoints with a same-thread inspector session is not a good practice because the inspected script can not be distinguished from the inspector session codes and the program is literally been resumed when the 'Debugger.paused' fires.

[timfish]

The issue is that the debugger resumes on it's own when I call await session.post("Runtime.getProperties").

For example, see this full code example I posted to a discussion:
https://github.com/orgs/nodejs/discussions/51393

I had to add isPaused to the above example to keep track of the paused status because If you call 'Debugger.paused' when it's not paused it throws an error.

session.on("Debugger.paused", (event) => {
  // Call 'Debugger.resume' synchronously to release the remote object in `event`.
  session.post("Debugger.resume");
});

So it not possible to call anything asynchronous from within the Debugger.paused callback? In that case the newer promises API can't be used?

[theanarkh]

I think you can use the example as follows.

import { Session } from "node:inspector";

const session = new Session();
session.connect();

session.on("Debugger.paused", async (event) => {
  for (const frame of event.params.callFrames) {
    const localScope = frame.scopeChain.find((scope) => scope.type === "local");
    if (localScope?.object?.objectId) {
       session.post("Runtime.getProperties", {
        objectId: localScope.object.objectId,
        ownProperties: true,
      }, (err, result) => {
        // console.log("Runtime.getProperties");
      });
    }
  }
  session.post("Debugger.resume");
});

session.post("Debugger.enable", (err) => {
  !err && session.post("Debugger.setPauseOnExceptions", { state: "all" });
});

setInterval(() => {
  console.log(`${Math.floor(process.memoryUsage().rss / 1024 / 1024)} MB`);
}, 1000);

setInterval(() => {
  try {
    throw new Error("Hello");
  } catch {}
}, 100);

because VM will call resume after Debugger.pause event, so you can not use await in Debugger.pause event.

[legendecas]

As mentioned above, it is not recommended to use breakpoints with same thread inspector sessions. Instead, try to connect to the inspector with websocket connections or with a worker thread:

import { Session } from "node:inspector/promises";
import { Worker, isMainThread } from 'node:worker_threads';

if (!isMainThread) {
  inspectMainThread();
} else {
  mainThreadWork();

  new Worker(new URL(import.meta.url));
    .on('exit', (code) => console.log('worker exited', code));
}

function mainThreadWork() {
  setInterval(() => {
    try {
      throw new Error("Hello");
    } catch {}
  }, 1);
}

async function inspectMainThread() {
  setInterval(() => {
    console.log(`${Math.floor(process.memoryUsage().rss / 1024 / 1024)} MB`);
  }, 1000);

  const session = new Session();
  session.connectToMainThread();

  session.on("Debugger.paused", async (event) => {
    // `await` can be used here.

    // After all asynchronous work is done, resume the main thread.
    session.post('Debugger.resume');
  });

  await session.post("Debugger.enable");
  await session.post("Debugger.setPauseOnExceptions", { state: "all" });
}