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test: fix flaky test-fs-watchfile on macOS #13252

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test: fix flaky test-fs-watchfile on macOS #13252

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test: fix flaky test-fs-watchfile on macOS 
On macOS, a watcher created with fs.watch() does not necessarily
start receiving events immediately. So it can miss a change by
fs.writefile() if it comes very soon after the watcher is created. Fix
test flakiness caused by this by using `setInterval()` to repeat the
write action.

Fixes: #13248
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@Trott
Trott committed May 27, 2017
commit ada66cc07d137c43153db154c3a997a60d37946e
9 changes: 6 additions & 3 deletions test/parallel/test-fs-watchfile.js
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Original file line number Diff line number Diff line change
Expand Up @@ -72,12 +72,15 @@ if (common.isLinux || common.isOSX || common.isWindows || common.isAix) {
if (err) assert.fail(err);

fs.watch(dir, common.mustCall(function(eventType, filename) {
clearInterval(interval);
this._handle.close();
assert.strictEqual(filename, 'foo.txt');
}));

fs.writeFile(`${dir}/foo.txt`, 'foo', common.mustCall(function(err) {
if (err) assert.fail(err);
}));
const interval = setInterval(() => {
fs.writeFile(`${dir}/foo.txt`, 'foo', common.mustCall(function(err) {
if (err) assert.fail(err);
}));
}, 1);

@refack refack May 27, 2017

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[question] is 1 safe? does it not cause a busy-wait-ish situation?

@refack refack May 27, 2017

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maybe change to try {fs.writeFileSych ...?

@Trott Trott May 27, 2017

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is 1 safe?

It won't block the event loop or anything like that, if that's what you mean. The way intervals work, the next one only gets added to the queue after the first one finishes, and gets added to execute in the next tick of the event loop at the earliest.

@refack refack May 27, 2017

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I figured it will not block the loop, but maybe stress the I/O. Although obviously the process shouldn't be doing anything else at this point.
I agree it's a least arbitrary value you could put here, so guess it could be read a sort of nextTickInterval idiom?

}));
}