Draft: Use TCP KEEPALIVE to force closing socket with offline remotes.

[stephan57160]

Close issue #4729.

[stephan57160]

OK. Let's talk about this lazy implementation.

On Linux, after ::accept(), TCP_KEEPALIVE is not set by default.
Moreover, there is no way for an application to retreive this socket
and then, no way to close() or use setsockopt() on it.
So, the only possibility to avoid socket leakage is TCP_KEEPALIVE.

Regarding the values, the default TCP ones are far too huge (> 2h).
The values below seem reasonnable for a ZYRE application :

120 + 10 * 10 = 220s (3mn 40s)

Note:

A ZYRE client application will reset its side after about 30s, so
no worry about such applications.
What about other applications ?

[bluca]

There are options for this, pretty sure zyre should just set them accordingly, rather than hardcoding?

[stephan57160]

Agreed, and I spent quite some time trying to find something less hard-coded,but I did not find anything.

For instance, when the socket is created internally by a ZYRE node, the application has no possibility to access it.
A least, I did not find any way to do that.

If you have a pointer, I'll see what I can do with it.

[bluca]

ZMQ_TCP_KEEPALIVE
ZMQ_TCP_KEEPALIVE_CNT
ZMQ_TCP_KEEPALIVE_IDLE
ZMQ_TCP_KEEPALIVE_INTVL

[stephan57160]

Not sure to understand what you have in mind.

I you use zmq_set_sockopt(sock, ZMQ_TCP_XXX, ...), you have to handle a bunch of if error cases.
All are handled by tune_tcp_keepalives().

What I can do is to use constants for 10, 10 and 120, right, but I still have to tune TCP KA right after the call to accept(),
as not possible from the application.

[bluca]

I mean that zyre or any other application should just use those options if they want to modify this behaviour

[stephan57160]

Agreed.
Unfortunatelly, ZYRE applications have a limited access to the socket layer.

It's being a long time I worked on this but I'm pretty sure that even the ZACTOR has no view on the accept()'ing socket.

[bluca]

I am pretty sure you can get the zmq socket in all of those cases, and then it's just a matter of setting those options

[stephan57160]

If this way exists, I lost on this Hide & Seek :-)