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Debug worker_threads #26609
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Fwiw, I recently wrote @eugeneo an email about this. We implemented something similar to the
Targetdomain that Chrome uses, but under a different name. I don’t know if the plan is for Devtools to use the Node.js-specific domain, or for us to eventually use theTargetdomain as well./cc @nodejs/v8-inspector
/cc @pavelfeldman
@aslushnikov , @a1ph, do you have cycles to look at it?
@ak239 , anyone? :) Now that we have the flattened session support settled in chrome land, we can probably name this domain Target and introduce the flattened sessionId dance in Node.
NDB has added support for worker_threads. Do we expect something similar in future?
GoogleChromeLabs/ndb#281Reacted by EdenGottlieb and coderaiserReacted by Emily Marigold Klassen@gauravmahto Unfortunately, DevTools does not support this yet, https://bugs.chromium.org/p/chromium/issues/detail?id=968472 is their open issue for this.
Reacted by Gaurav M and Hongcai DengThere are two separate things:
- DevTools folks can adopt ndb code to support workers,
- additional instrumentation should be added in node to support stepping between main and worker threads.
@ak239 Maybe this helps solve the second part?
@addaleax there are three methods available on V8Inspector: storeCurrentStackTrace,externalAsyncTaskStarted,externalAsyncTaskFinished.
To support step into between threads we need to call storeCurrentStackTrace before postMessage, pass V8StackTraceId to another thread and call externalAsyncTaskStarted / finished on another thread when we are processing this message. V8StackTraceId is actually pair of ints. As soon as it is done, when in
ndbyou press step into on post message, debugger will automatically go to the worker and pause there when worker is about to process message.Second idea that is implemented in Chrome, it is doing the same for Worker constructor: store at constructor call, and call started/finished for main worker script. When this one is done, user can click step into worker constructor and go to worker script.
In case any debugger implementors come across there -- Node has an extension to the debugger protocol which allows debugging worker threads -- with minimal work given your debugger is already multi-target aware, like VS Code's. However this is not documented anywhere so I have no idea how stable it may be, YMMV.
Reacted by Arka Pratim ChaudhuriDebugging workers with
--inspect-brkis possible with some hacky patching. Passing it asexecArgvwon't work but usingnode:inspectorinside the worker works.This does not work:
import { isMainThread, Worker } from "node:worker_threads"; import { fileURLToPath } from "node:url"; const filename = fileURLToPath(import.meta.url); if (isMainThread) { new Worker(filename, { execArgv: ["--inspect-brk"] }); } else { debugger; // No stop here }
The
node:inspectorworks. Chrome DevTools stop at thedebugger.import { isMainThread, Worker } from "node:worker_threads"; import { fileURLToPath } from "node:url"; +import inspector from "node:inspector"; const filename = fileURLToPath(import.meta.url); if (isMainThread) { new Worker(filename, { execArgv: ["--inspect-brk"] }); } else { + if (process.execArgv.includes("--inspect-brk")) { + inspector.open(); + inspector.waitForDebugger(); + } debugger; // Stops in worker thread! }Adding the
process.execArgvchecks in userland seems like unnecessary work - maybe Node could do this for us?Reacted by peelz, Dean Xu, Nathaniel, Mattias Buelens, Tamir Duberstein, bobOnGitHub and Max CoplanReacted by Max CoplanDebugging workers with --inspect-brk is possible with some hacky patching. Passing it as execArgv won't work but using node:inspector inside the worker works.
Could you please elaborate on this? Does this enable debugging a worker with chrome devtools? Because I'm not having any success - the worker doesn't show up in chrome after executing
inspector.open();inspector.waitForDebugger();in it, it seems like it just freezesDoes this enable debugging a worker with chrome devtools?
Yes, just save the code above into file and run with
node. Here's picture from Chrome dev tools:
Reacted by DanielHmm, @AriPerkkio do you know if it works with require too? Just adding
const inspector = require("node:inspector");at the top of the worker I'm trying to debug, https://github.com/nodejs/node/blob/55d2eb53d78040472371eedd0e9e7922748dfb0e/lib/internal/modules/esm/worker.js makes the worker freeze for me (become unresponsive at 0% CPU usage) 😢And nothing shows up in chrome
I'm trying to get a debugger to where this error is thrown, see reproduction steps there: swc-project/swc-node#736 (comment) and I'm following these steps to build node 20.9.0 https://www.devdungeon.com/content/build-nodejs-source
Fyi, VS Code's JavaScript debugger automatically debugs node_workers using the NodeWorker domain (#26609 (comment)). So if you just need to debug a worker, easiest way is to run "Create JavaScript Debug Terminal" in VS Code; everything run in that terminal will be debugged.
Reacted by Romain LebesleReacted by Daniel and NReacted by Daniel@connor4312 TYSM, that got me a debugger inside of the worker!
Unfortunately it doesn't seem like the vscode debugger can go to function definitions on all functions (
[[FunctionLocation]]) and also the "break on caught exceptions" doesn't seem to work within a worker but I got further, thanks :)But what about the CLI debugger?
I'd like to be able to pass
inspectas a positional argument to the worker causing it to start the CLI debugger just like we would do withnode inspect script.js.As things are, I am unable to start the internal (CLI) debugger for a worker thread on Node v22.9.0 (Linux) with :
node inspect-cli.jsconst { Worker, isMainThread } = require('node:worker_threads'); if(isMainThread) { const worker = new Worker(__filename, { execArgv: ["--inspect-brk"] }) worker.on('error', console.error) worker.on('exit', (code) => console.log(`Worker stopped with exit code ${code}`)) } else { debugger }
What happens is, the process hangs, and here are the NODE_DEBUG and strace logs:
Here's what the debugger yields when running the
scriptscommand after attaching a debugger client usingnode inspect -p PID:Spoiler
[user@pc node-playground]$ node inspect -p 123 connecting to 127.0.0.1:9229 ... ok debug> scripts 22: node:events 24: node:buffer 31: node:async_hooks 32: node:timers 34: node:path 37: node:querystring 42: node:fs 47: node:util 62: node:url 66: node:diagnostics_channel 75: inspect-cli.js 76: node:worker_threads 80: node:stream 92: node:string_decoder 95: node:stream/promises 101: node:inspector 102: node:tty 103: node:net
Finally, I think it would only make sense that nodejs start the CLI debugger:
- As soon as it comes across a
debugger - When an exception occurs if a given ENV is set accordingly
- As soon as it comes across a
Can I use the Session from the
node:inspectormodule and connect to a worker?- Yes. Check the NodeTarget domain.…On Mon, Jan 13, 2025 at 10:36 PM Cyan ***@***.***> wrote: Can I use the Session from the node:inspector module and connect to a worker? — Reply to this email directly, view it on GitHub <#26609 (comment)>, or unsubscribe <https://github.com/notifications/unsubscribe-auth/AACGJLMHWIPWWUAC5XC2K6L2KSV7HAVCNFSM6AAAAABQU7O4DGVHI2DSMVQWIX3LMV43OSLTON2WKQ3PNVWWK3TUHMZDKOBZGE2DEOJQGI> . You are receiving this because you were mentioned.Message ID: ***@***.***>
- linked a pull request that will close this issueinspector: support for worker inspection in chrome devtools #56759
on Jan 29, 2025
There is no possible to debug worker threads, only through event messages to main thread
You can't step into this worker in the chrome dev tools
debuggerstatements also don't work inside thread