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fs: add AbortSignal support to watch #37190

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fs: add AbortSignal support to watch #37190

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7 changes: 7 additions & 0 deletions doc/api/fs.md
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Original file line number Diff line number Diff line change
Expand Up @@ -4133,6 +4133,9 @@ this API: [`fs.utimes()`][].
<!-- YAML
added: v0.5.10
changes:
- version: REPLACEME
pr-url: https://github.com/nodejs/node/pull/37190
description: Added support for closing the watcher with an AbortSignal.
- version: v7.6.0
pr-url: https://github.com/nodejs/node/pull/10739
description: The `filename` parameter can be a WHATWG `URL` object using
Expand All @@ -4152,6 +4155,7 @@ changes:
`false`.
* `encoding` {string} Specifies the character encoding to be used for the
filename passed to the listener. **Default:** `'utf8'`.
* `signal` {AbortSignal} allows closing the watcher with an AbortSignal.
* `listener` {Function|undefined} **Default:** `undefined`
* `eventType` {string}
* `filename` {string|Buffer}
Expand All @@ -4174,6 +4178,9 @@ The listener callback is attached to the `'change'` event fired by
[`fs.FSWatcher`][], but it is not the same thing as the `'change'` value of
`eventType`.

If a `signal` is passed, aborting the corresponding AbortController will close
the returned [`fs.FSWatcher`][].

### Caveats

<!--type=misc-->
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11 changes: 11 additions & 0 deletions lib/fs.js
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Original file line number Diff line number Diff line change
Expand Up @@ -1579,6 +1579,17 @@ function watch(filename, options, listener) {
if (listener) {
watcher.addListener('change', listener);
}
if (options.signal) {
if (options.signal.aborted) {
process.nextTick(() => watcher.close());
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Why on next tick?

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So that users have a chance to subscribe to the "close" event.

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@benjamingr benjamingr Feb 3, 2021 •
edited
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(Just because watch returns an EventEmitter watcher and not a promise or a callback)

} else {
const listener = () => watcher.close();
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nit: you could move the listener declaration above and pass it in process.nextTick to avoid code duplication.

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I actually find this way more readable though I don't have very strong feelings about it.

options.signal.addEventListener('abort', listener);
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@aduh95 aduh95 Feb 4, 2021 •
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Not sure if that matters / is more efficient, but at least it makes it clearer that this is just a one timer:

Suggested change
options.signal.addEventListener('abort', listener);
options.signal.addEventListener('abort', listener, { once: true });

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This is a common issue that gets brought up from time to time. The reason I like doing { once: true } less is that it creates the false impression we are relying on the listener being a one-time listener for cleanup.

That assumption has lead to memory leaks in our code before - this way the cleanup is explicit (when the watcher is closed) and I think it makes bugs slightly less likely.

watcher.once('close', () => {
options.signal.removeEventListener('abort', listener);
});
}
}

return watcher;
}
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32 changes: 32 additions & 0 deletions test/parallel/test-fs-watch-abort-signal.js
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@@ -0,0 +1,32 @@
// Flags: --expose-internals
'use strict';

// Verify that AbortSignal integration works for fs.watch

const common = require('../common');

if (common.isIBMi)
common.skip('IBMi does not support `fs.watch()`');

const fs = require('fs');
const fixtures = require('../common/fixtures');


{
// Signal aborted after creating the watcher
const file = fixtures.path('empty.js');
const ac = new AbortController();
const { signal } = ac;
const watcher = fs.watch(file, { signal });
watcher.once('close', common.mustCall());
setImmediate(() => ac.abort());
}
{
// Signal aborted before creating the watcher
const file = fixtures.path('empty.js');
const ac = new AbortController();
const { signal } = ac;
ac.abort();
const watcher = fs.watch(file, { signal });
watcher.once('close', common.mustCall());
}