Fixed double-writes when write() is called during 'drain' event

[arnaud-lb]

When write() is called in the event handler of a drain event, Buffer can write things twice.

Example:

$buffer = new Buffer();
$buffer->once('drain', function ($buffer) {
    $buffer->write("bar\n");
});

$buffer->write("foo\n");

This write "foo\nfoo\nbar\n" instead of just "foo\nbar\n".

This doesn't happen if $buffer->write("bar\n"); is called on the next tick (which explains why it usually works).

This happens in actual code, e.g. when using Util::pipe() on a Readable which immediately emits a 'data' event when resume() is called.

This PR fixes this by updating the internal buffer before emitting the drain event.

[clue]

Not sure I follow, are these assignments required only in the testing environment to prevent actual writes? Some background info and/or documentation would be much appreciated! 👍

[arnaud-lb]

Yes, I duplicated what testDrain() was doing; it does this to simulate a stream that's not immediately available for writes.

[clue]

Awesome, thanks for spotting and filing this PR! 👍

See my above remark about the test case, otherwise this LGTM 👍

[clue]

Thanks for clearing this up @arnaud-lb! LGTM 👍

[clue]

This happens in actual code, e.g. when using Util::pipe() on a Readable which immediately emits a 'data' event when resume() is called.

IMO this is indeed a major issue.

👍 for getting this in sooner rather than later

Ping @cboden, @WyriHaximus

[WyriHaximus]

LGTM 👍

[cboden]

Thanks @arnaud-lb! Great work!