Dune doesn't respond to RPC methods when run in watch mode in the dune repository

[gridbugs]

Expected Behavior

It should be possible to run _build/default/bin/main.exe build --watch from the dune repository, and then in a second terminal run _build/default/bin/main.exe rpc ping and have the second command return immediately.

Actual Behavior

The ping command hangs forever. I've only been able to reproduce this in the dune repository. Running a freshly-built dune binary in other projects seems to work as expected.

Reproduction

1. Compile dune
2. Run _build/default/bin/main.exe build --watch from the root of the dune directory
3. In a second terminal, run _build/default/bin/main.exe rpc ping also from the root of the dune directory.

Specifications

• Version of dune (output of dune --version): 82c670e
• Version of ocaml (output of ocamlc --version): 4.14.1
• Operating system (distribution and version): NixOS

Additional information

The first commit with this behaviour is 8bea7ea

[rgrinberg]

I wasn't able to reproduce this. Anything else you could provide us?

This PR removes threads for RPC altogether. Does it fix the issue for you?

@Alizter could you try reproducing?

[gridbugs]

This #7418 removes threads for RPC altogether. Does it fix the issue for you?

I don't see the problem on that PR branch.

I tried reproducing on a different machine in my home and I couldn't. It could be related to the fact that my main machine (where I see the problem) has a relatively large number of cores (32).

[rgrinberg]

(where I see the problem) has a relatively large number of cores (32).

Hmm, I'm not sure what difference would this make. You could try changing -j to pretend that you're a low core machine to see if it fixes something.

[gridbugs]

It still hangs with -j1 (on the server).