When `--inspect` is passed, wait until connection before running?

[kitsonk]

There are situations in vscode where when the code is launched with --inspect, Deno runs the program and exits before the vscode debugger can connect, causing problems with being able to debug code where a breakpoint is set. (see: denoland/vscode_deno#557)

Is there a logical way to not run code when --inspect is provided until a client is connected on the port? This is different than --inspect-brk where the code breaks at the first point, it would be to give time for a client to connect and provide break-points.

[devmattrick]

For me, even using --inspect-brk exits immediately in VSCode's debugger.

[kitsonk]

@devmattrick that does not sound like this issue, then.

[bartlomieju]

This is actually what Node.js does too, given:

// foo.js
console.log("foo");

Run in Node.js:

❯ node --inspect foo.js
Debugger listening on ws://127.0.0.1:9229/c1addda4-de08-4c12-a0ed-0e2a1284241a
For help, see: https://nodejs.org/en/docs/inspector
asdf

It runs code and exits immediately.

In comparison, run in Deno:

❯ deno run --inspect foo1.js
Debugger listening on ws://127.0.0.1:9229/ws/a50ae203-ad1d-4e0d-b82c-a4231be1dc35
foo

So I think the behavior is correct, but we need to investigate problems with --inspect-brk.

[kitsonk]

ok, need to check what the behaviour is when debugging NodeJS applications in VSCode and if not using --inspect-brk just "dumps" without starting the debug tools like it does in Deno then.

[bartlomieju]

I did some digging into this issue an here's what I found:

1. I created 3 files (node.js, deno.js, deno.ts), each of the files just prints a console statement:

   console.log("Hello from {name}");

2. For each of the files I opened debugger pane and clicked "Run and Debug", selecting default debugger options (either Node or Deno)
3. The result are as follows:
   • node.js
   • deno.js
   • deno.ts

As can be seen in the screen shots, neither deno.js nor deno.ts print out the message to the console. I'm 90% sure this is caused by https://github.com/denoland/deno/issues/13006

[bartlomieju]

So I fixed the (IMO) underlying issue in https://github.com/denoland/deno/pull/13191 but when I try to run the same code in VSCode there's still no output. It does seem like the process is starting, but nothing shows up in the Debug console... @kitsonk do you have any idea if there's some special handling required for messages to show in that console? Maybe we need to redirect stdio of the spawned process somewhere?

[kitsonk]

I haven't tried the branch, but a recent build off master demonstrates that with --inspect-brk the console output gets sent to the console, just like with Node, so if it isn't behaving exactly like Node.js, something else is amiss.

[bartlomieju]

It does work for me with --inspect-brk too (ie. breakpoint is set and execution pauses); what concerns me is that it doesn't work with just --inspect.

I will add debug logging to Deno's inspector and see what messages are being exchanged between the binary and VSCode.

[kitsonk]

I am sure it is the same if you use about:inspect in Chrome. With --inspect-brk I see Deno in about:inspect and can connect to it. I am going to try with --inspect.

[bartlomieju]

I am sure it is the same if you use about:inspect in Chrome. With --inspect-brk I see Deno in about:inspect and can connect to it. I am going to try with --inspect.

For both Node and Deno I don't see any targets in chrome://inspect - but that's expected IMO, since the code finishes immediately.

[kitsonk]

Ok, I checked out the branch... if I do this:

console.log("hello");

I don't get anything in the output, just like expected, but if I do:

console.log("hello");

await new Promise((res) => {
  setTimeout(() => {
    console.log("deno");
    res();
  }, 5000);
});

Then there is enough time to connect to the inspector port and get the information and I see both lines logged to the console. So I am not sure the waiting is actually working properly in #13191 or at least it isn't working in a way that exhibits the same behaviour as Node, or Deno is just that much faster.

[bartlomieju]

My other suspicion is that VSCode debugger integration requires some special handling, however I haven't found anything yet in the docs about it. Anyway, something to look into after Christmas.

[kitsonk]

Well it doesn't... because if the program takes a bit longer, then everything works fine. For example, on my local machine, using a build of #13191 this works:

console.log("hello");

await new Promise((res) => {
  setTimeout(() => {
    console.log("deno");
    res(undefined);
  }, 750);
});

But this behaves with the "blank" behaviour... the "red bar" never appears:

console.log("hello");

await new Promise((res) => {
  setTimeout(() => {
    console.log("deno");
    res(undefined);
  }, 500);
});

This is my launch.json:

{
  "version": "0.2.0",
  "configurations": [
    {
      "request": "launch",
      "name": "Launch Program",
      "type": "pwa-node",
      "program": "${workspaceFolder}/ins.ts",
      "cwd": "${workspaceFolder}",
      "runtimeExecutable": "deno",
      "runtimeArgs": [
        "run",
        "--inspect",
      ],
      "attachSimplePort": 9229
    }
  ]
}

[bartlomieju]

So, should we just agree that "we're too fast" in the simplest cases and this works as expected?

[kitsonk]

Yeah, because I just tried with Node 15 using:

{
  "version": "0.2.0",
  "configurations": [
    {
      "request": "launch",
      "name": "Launch Program",
      "type": "pwa-node",
      "program": "${workspaceFolder}/ins.mjs",
      "cwd": "${workspaceFolder}",
      "runtimeExecutable": "node",
      "runtimeArgs": [
        "--inspect",
      ],
      "attachSimplePort": 9229
    }
  ]
}

An got the same behaviour... a "blank" debug until I increased the timeout.

[bartlomieju]

Thanks for checking it. I will update the manual page accordingly and close this issue then.

[bartlomieju]

I'm going to close this issue now that denoland/manual#201 is open.