ocaml/dune

Formatting dune files on save while the build is running

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#13,525 opened on Feb 2, 2026

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good first issuerpc

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Description

Desired Behavior

Imagine you have an editor extension enabled that can format your dune files (dune, dune-project, etc.) on save, using dune format-dune-file under the hood. I think now if you have a build running in watch mode in the project, this no longer works. The format command conflicts with the build locks.

I think this use to work before dune.3.21 but I suspect something changed.

Repro?

I'd appreciate if someone else can confirm as I am not sure exactly sure from where this workflow comes from (dune.el?). I have noticed it via some other layer with the reformatter emacs package (using this setup). Not sure if this applies to other users. Thanks!

Contributor guide

Research direction
Investigate how dune acquires locks during 'dune format dune file' and how it conflicts with watch mode build locks. Check git history for changes around version 3.21 that may have introduced this behavior.
Tech stack
None
Domain
build system
Issue type
Bug
DifficultyEstimated implementation difficulty for a new contributor, from 1 for very small changes to 5 for expert-level work.
3
Estimated timeA rough time range for an experienced contributor to investigate, implement, test, and prepare a pull request.
Half day
Activity statusHow available the issue appears right now: fresh, active, stale, blocked, or waiting on maintainer input.
Active
ClarityHow clearly the issue explains the expected change, acceptance criteria, and next step.
Mostly clear
Prerequisites
OCamldune
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
40