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rewatch watch on Linux stops watching a source folder when an overlapping recursive source folder is removed from rescript.json

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A pull request for this has already been merged.

  • #8839 by @cknitt — merged

Assessment

Difficulty
4/5
Estimated time
1-2 days
Newbie friendliness
35/100
Issue type
Bug
Clarity
Clearly specified
Activity status
Stale
Tech stack
ocaml
Domain
devtools

Research direction

Start in rewatch/src/watcher.rs, especially compute_watch_paths and update_watches, and trace how overlapping paths are handled when the config changes. Check notify 5.2.0's Linux INotifyWatcher removal behavior described in the issue, then reproduce with the supplied Linux steps. Done means edits under src/retained are picked up after narrowing the source folders without restarting the watcher.

Written by the indexing model from the issue text.

Description

Found by adversarial experiments on semantic analysts.

What is broken

rescript watch registers each source folder as its own watch, and overlapping folders are allowed. Consider sources [{"dir": "src", "subdirs": true}, "src/retained"], which become ["src/retained"] after an edit to rescript.json. On Linux, edits under src/retained are no longer picked up afterwards.

  • compute_watch_paths gives src (recursive) and src/retained (non-recursive) separate entries.
  • On a config change, update_watches (called at L599) does three things:
    • it keeps src/retained as unchanged;
    • it calls unwatch on the stale src;
    • it registers only the paths that are not kept.
  • On Linux, RecommendedWatcher is notify's INotifyWatcher. In notify 5.2.0 (the version locked in rewatch/Cargo.lock), unwatching a recursive path removes every watched path that starts with it (src/inotify.rs, remove_watch, lines 518–541 of the crate). That includes the separately registered src/retained.
  • update_watches still lists src/retained as registered, so it is never re-registered. The package root watch is non-recursive and does not see src/retained/*.res.

macOS (FSEvents) keeps the child watch, and there the scenario below works. The Linux result follows from the notify code above; I have not run it on Linux.

Reproduction (Linux)

mkdir -p proj/src/retained && cd proj
echo '{"name":"proj","version":"0.0.0","dependencies":{"rescript":"13.0.0-alpha.7"}}' > package.json
cat > rescript.json <<'EOF'
{"name":"proj","sources":[{"dir":"src","subdirs":true},"src/retained"],
 "package-specs":{"module":"esmodule","in-source":true},"suffix":".mjs"}
EOF
echo 'let x = 1' > src/retained/Demo.res
npm install && (npx rescript watch &) && sleep 3
sed -i 's|"sources":.*"src/retained"\],|"sources":["src/retained"],|' rescript.json; sleep 3
echo 'let x = 2' > src/retained/Demo.res; sleep 3
grep 'let x' src/retained/Demo.mjs

Expected: let x = 2;. Per the code above, on Linux Demo.mjs keeps let x = 1; until the watcher restarts.

Introduced by

fb85e7bf8 (#8730). Before it, a full rebuild unregistered every path and registered the complete new list.

Impact

Linux users who narrow an overlapping source configuration while rescript watch runs get silently stale output for the retained folder. One fix is to treat a kept watch as stale when an ancestor recursive watch is removed. Another is to re-register every kept path below a removed recursive path.

Dominant language
ReScript
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Forks
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Merged PRs (30d)
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