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child_process: fix data loss with readable event #5036

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child_process: fix data loss with readable event #5036

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child_process: fix data loss with readable event 
This commit prevents child process stdio streams from being
automatically flushed on child process exit/close if a 'readable'
event handler has been attached at the time of exit.

Without this, child process stdio data can be lost if the process
exits quickly and a `read()` (e.g. from a 'readable' handler)
hasn't had the chance to get called yet.

Fixes: #5034
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@mscdex
mscdex committed Feb 4, 2016
commit e528bdc8d9515bc1f3929ef2187ea09aff0eaf6d
2 changes: 1 addition & 1 deletion lib/internal/child_process.js
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Original file line number Diff line number Diff line change
Expand Up @@ -217,7 +217,7 @@ util.inherits(ChildProcess, EventEmitter);
function flushStdio(subprocess) {
if (subprocess.stdio == null) return;
subprocess.stdio.forEach(function(stream, fd, stdio) {
if (!stream || !stream.readable)
if (!stream || !stream.readable || stream._readableState.readableListening)
return;
stream.resume();
});
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18 changes: 15 additions & 3 deletions test/parallel/test-child-process-flush-stdio.js
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Original file line number Diff line number Diff line change
Expand Up @@ -8,10 +8,22 @@ const p = cp.spawn('echo');
p.on('close', common.mustCall(function(code, signal) {
assert.strictEqual(code, 0);
assert.strictEqual(signal, null);
spawnWithReadable();
}));

p.stdout.read();

setTimeout(function() {
p.kill();
}, 100);
function spawnWithReadable() {
const buffer = [];
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I removed the timeout here because I'm quite wary of such things as I've seen tests with timeouts like this become flaky.

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you could choose to use common.platformTimeout

setTimeout(function() {
  p.kill();
}, common.platformTimeout(100);

That should solve the falkyness

const p = cp.spawn('echo', ['123']);
p.on('close', common.mustCall(function(code, signal) {
assert.strictEqual(code, 0);
assert.strictEqual(signal, null);
assert.strictEqual(Buffer.concat(buffer).toString().trim(), '123');
}));
p.stdout.on('readable', function() {
let buf;
while (buf = this.read())
buffer.push(buf);
});
}