turbo-tasks-fs: Don't print NotFound watcher errors (#99794)
### What?
Stop the Turbopack disk watcher from printing `watch error … NotFound`
for paths that vanish while notify is scanning them, and send the
remaining watcher errors to stderr instead of stdout.
### Why?
In polling mode, notify's `PollWatcher` walks the tree with walkdir.
When a directory is deleted between being listed and being opened or
stat'ed, notify reports an `Io` error with `NotFound`. Deleting a large
tree (`rm -rf`, a package install) produces a burst of these. A
filesystem fuzzer saw about 4000 in two minutes. Each one was printed
with `println!`, so a library was spamming the user's stdout.
These errors don't signal a correctness problem. The watcher already
invalidates the reported path recursively, and the next scan reports the
removal anyway. The fuzzer runs had no missed invalidations.
### How?
- A small private classifier recognizes notify errors of kind `Io`
wrapping `std::io::ErrorKind::NotFound`. It matches on the structured
error kinds, not on message text.
- The watcher thread skips the print for those errors. Batch
invalidation and batch-delay scheduling still run for every error, so
invalidation behavior doesn't change.
- Other notify error kinds, including `PathNotFound`, are still
reported, now on stderr.
### Verification
- `cargo test -p turbo-tasks-fs --lib watcher::tests`: 19 passed,
covering native and polling watchers in both recursive and non-recursive
modes
- `cargo clippy -p turbo-tasks-fs --tests -- -D warnings`
- Not run: a deletion stress test reproducing the race (the fuzzer
finding above).
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Co-authored-by: vercel-fleet-prod[bot] <318278635+vercel-fleet-prod[bot]@users.noreply.github.com>
Co-authored-by: Tobias Koppers <1365881+sokra@users.noreply.github.com>
