dune build -w exclusion does not work in Mac OS

[camlspotter]

Expected Behavior

The file event monitoring of dune build -w should ignore the file creation of an Emacs temp file like .#foobar.ml

Actual Behavior

Generation of .#foobar.ml triggers the rebuild, which should not.

Reproduction

1. Use Mac OS
2. Execute dune build -w in your project
3. See the message waiting for filesystem changes...
4. Open a terminal and execute touch .#foobar.ml
5. dune build -w restarts building, which is not an expected behavior.

Specifications

• dune.3.11.1
• Mac OS

Additional information

The issue does not reproduce in Linux.

[emillon]

Hi,
The core of the issue is that the logic for excluded files is different for each backend, and that for fsevents (the backend used on macos), the logic is partially implemented.

There are 2 ways to do this kind of filtering:

• either when initializing the backend, we tell it to ignore certain files or pattern
• or whenever an event fires, we check the path against the list of exclusions

In the case of fsevents, there's an exclusion list of the first type defined here:

dune/src/dune_file_watcher/dune_file_watcher.ml

Line 588 in 013ae8a

:: ([ "_esy"; "_opam"; ".git"; ".hg" ]

But the function that sets exclusions cannot be called with more that 8 entries:

dune/src/fsevents/fsevents.ml

Line 318 in 013ae8a

"Fsevents.set_exclusion_paths: 8 directories should be enough for anybody"

But these are just directories, not patterns, so I think that the length check is there to prevent a solution like evaluating a glob and passing that to the macos APIs.

Regarding the other kind of filtering, we have a list of "standard exclusions" defined here:

dune/src/dune_config_file/dune_config_file.ml

Line 256 in 013ae8a

let standard_watch_exclusions =

It's the default of --watch-exclusions, which is passed when the watcher backend is initialized here:

dune/src/dune_file_watcher/dune_file_watcher.ml

Line 677 in 013ae8a

let create_default ?fsevents_debounce ~watch_exclusions ~scheduler () =

Then, each backend is able to postprocess events using that list.
As you can see, in the Fsevents case, neither ~should_exclude nor ~watch_exclusions is passed.

So I think that one way to fix that is to add an extra argument to create_fsevents and do the filtering in that function when an event arrives.

Do you want to have a go at this? Otherwise I can guide someone with this.

[emillon]

I'm going to advertise this issue a bit more since I think it's a good thing work on for new contributors. Intermediate familiarity with ocaml is required but I don't think it's necessary to know how dune work in detail.

If you do want to work on this issue, please post here before opening a PR, to avoid duplicate work.

[PoorlyDefinedBehaviour]

I'll try to do it today (in like 8 hours from now).

[emillon]

Thanks @PoorlyDefinedBehaviour !