"drain" event listener leak when unpiping to response

This is sort of a weird scenario, but this test case fails:

  it('should clean up event listeners when res is unpiped to', function (done) {
    var listenerCount
    var server = createServer({ threshold: 0 }, function (req, res) {
      var times = 0
      var int = setInterval(function () {
        var rs = fs.createReadStream('does not exist')
        rs.on('error', function (e) {
          listenerCount = res.listenerCount('drain')
          rs.unpipe(res)
        })
        rs.pipe(res)
        if (times++ > 12) {
          clearInterval(int)
          res.end('hello, world')
        }
      })
    })

    request(server)
      .get('/')
      .set('Accept-Encoding', 'gzip')
      .expect(function () {
        assert.ok(listenerCount < 2)
      })
      .expect(200, done)
  })

I hit this in some code that retries creating a read stream until the source exists. We clean up from our side: rs.on("error", e => { rs.unpipe("res"); }). Seems like compression needs to be cleaning up its listeners when "unpipe" happens.

there is also a leak caused by mapping ServerResponse.once to ServerResponse.on so there is no easy way to wait for 'drain' events to happen by just using the response.once('drain', ...)

support res.removeListener('drain'), res.once('drain')

support res.removeLister("drain"), res.once("drain")

If anyone came here looking for a workaround for the ability to call res.once('drain', ...) without have node warn about MaxListenersExceededWarning: Possible EventEmitter memory leak detected. 11 drain listeners added to [Gzip]. Use emitter.setMaxListeners() to increase limit..

This is a bit of a mess, but you can do something like this:

let onDrain;

// add a single drain listener early on
res.on('drain', () => {
    if (onDrain) {
        // call the onDrain callback at most once
        onDrain();
        onDrain = undefined;
    }
});

// later, when you would normally call res.once('drain', ...)
onDrain = () => {
    // resume your input stream or whatever
};

I hope that helps someone feeling stuck on this old issue.

support res.removeLister("drain"), res.once("drain")

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"drain" event listener leak when unpiping to response #135

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"drain" event listener leak when unpiping to response #135

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matthiasg commented on Apr 17, 2018

jesseskinner commented on Mar 7, 2022

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