event emitter leak in compression

[usernameisalreadytaken2014]

Here is my keep-alive function.

var ka = setInterval(function() {
    res.write(": keep-alive\n");
    res.flush();
}, 15000);
res.on('close', function() {
    clearInterval(ka);
});

Used on top of a HTTP response that delivers a stream of JSON updates, formatted as text/event-stream (hence the flush).

The keep-alive is useful for SSE streams that only update once in a while, such as transmitting new data points on graphs that have only one data point per minute.

Usually there is a (network) router or two among the hops between client and server, that has a TCP state table with a timeout of 30 seconds, so the above helps to keep the channel open until the next event is ready for the client.

The expressjs/compression module, when enabled, has this side effect:

(node) warning: possible EventEmitter memory leak detected. 11 drain listeners added. Use emitter.setMaxListeners() to increase limit.
Trace
    at Gzip.addListener (events.js:239:17)
    at Gzip.Readable.on (_stream_readable.js:665:33)
    at Gzip.once (events.js:265:8)
    at Gzip.Zlib.flush (zlib.js:448:10)
    at ServerResponse.flush (compression/index.js:70:16)
    at null._repeat (sample.js:2:8)
    at wrapper [as _onTimeout] (timers.js:264:19)
    at Timer.listOnTimeout (timers.js:92:15)

I'm not adding any event listeners, however. Just flushing the output stream after writing.

Hmm, thinking about it, the keep-alive function is irrelevant. When using text/event-stream, the first big chunk of data will usually be pushed with just a single flush at the end, but once the stream gets "up to date" or whatever, every complete JSON object written will be followed by a flush().

Probably just random chance that this happened to me inside the keep-alive function...

Anyway, the error disappears if the compression module is disabled.

[dougwilson]

Calling res.flush() will result in a listener leak in certain versions of Node.js due to a Node.js bug we cannot work-around. Which version of Node.js are you running?

[dougwilson]

Looks like Node.js has never fixed this issue yet. Here is a PR to Node.js core to fix: https://github.com/nodejs/node-v0.x-archive/pull/25679

[usernameisalreadytaken2014]

Hmm, I am running v.4.2.1 on armv6l

[dougwilson]

@usernameisalreadytaken2014 , correct, it is not fixed. There is a PR to fix this in Node.js core: https://github.com/nodejs/node-v0.x-archive/pull/25679 if you want to get them to accept the patch :)

[usernameisalreadytaken2014]

Hmm okay then. I left a note over in the nodejs tracker. Fingers crossed