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Feature request: callback to know when the sub-process is started, of why it failed to exec

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#265 15 comments 1 reaction 1 assignee View on GitHub

@deepak1556 is already working on this.

Since Dec 19, 2025.

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Description

enhancement help wanted

Environment details

  • OS: GNU/Linux
  • OS version: 16.04
  • node-pty version: master

Issue description

When using the spawn function, I didn't find any way to know when the process has been successfully started, or if the exec failed for some reason. My concrete use case is that I need to start a process on a pty. On success, I need to report to the caller that the process has been started*. On failure (e.g. if the executable doesn't exist), I should return the error code. I haven't found a way to do this using the current API.

I have implemented something in our fork of node-pty here, but unfortunately my employer doesn't want me to sign the Microsoft CLA, and thus I can't contribute it. But I thought I would share the idea, to get some feedback. And if somebody wants to implement something similar in the upstream repo, I guess you can't copy the code but you can get some good inspiration from it.

In my patch, I add an exec event to the IPty/ITerminal interface, with an optional error parameter. If the exec goes well, the exec callback is invoked without parameter. If there is an error, it is invoked with an errno string (e.g. ENOENT).

On Linux/macOS, since we use fork + exec to spawn a new process, it's a bit difficult to get some feedback on the exec (since it happens in a new process). I looked at how libuv does (what node's child_process.spawn relies on) and did something similar, which is to use a pipe between the child process and our process to report success or failure.

For Windows, it looks like we just need to catch an exception on failure, here's what I did.

Any feedback is appreciated, and if anybody is willing to work on an upstream version of this, I would be happy to help.

  • My criterion for "started" just means that the exec system call returned "success". Many things can go wrong after that, like dynamic libraries not found, but that's out of scope here.
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