How to force closing socket on Linux?

Question

For THIS reason, I want to try something new - close the socket using some system call.

The situation in two words - can't set query timeout of the mysql library (the C API, refer to the link for more info), so I want to try closing the socket to see how the library will react. Probably this is not a good idea, but still wanna try it.

Here's what I've done - there's another started thread - a timer. So, after a specific timeout (let's say 10 second), if there's no response, I want to close the socket. The MYSQL struct has member net, that is also a struct, and holds the fd. But when I try to do this:

shutdown( m_pOwner->m_ptrDBConnection->m_mysql.net.fd, SHUT_RDWR );
close( m_pOwner->m_ptrDBConnection->m_mysql.net.fd );

nothing happens. The returned values from shutdown and close are 0, but the socket is still opened (because after 60sec waiting, there's a returned result from the DB, that means that the mysql client is still waiting for response from the DB.

Any ideas?

Thanks

EDIT - Yes, there's a running transaction, while I'm trying to close the socket. But this is the actual problem - I cannot terminate the query, nor to close the connection, nothing, and I don't wanna wait the whole timeout, which is 20min and 30 sec, or something like this. That's why I'm looking for a brute-force.. :/

Answer 1

Just a shot in the dark, but make sure you cancel/terminate any running transactions. I'm not familiar with the MySQL C API, but I would imagine there is a way to check if there are any active connections/queries. You may not be able to close the socket simply because there are still things running, and they need to be brought to some "resolved" state, be that either committed or rolled back. I would begin there and see what happens. You really don't want to shutdown the socket "brute force" style if you have anything pending anyway because your data would not be in a reliable "state" afterwards - you would not know what transactions succeeded and which ones did not, although I would imagine that MySQL would rollback any pending transactions if the connection failed abruptly.

EDIT: From what I have found via Googling "MySQL stopping runaway query", the consensus seems to be to ask MySQL to terminate the thread of the runaway/long-running query using

KILL thread-id

I would imagine that the thread ID is available to you in the MySQL data structure that contains the socket. You may want to try this, although IIRC to do so requires super user priviledges.

EDIT #2: Apparently MySQL provides a fail-safe mechanism that will restart a closed connection, so forcefully shutting down the socket will not actually terminate the query. Once you close it, MySQL will open another and attempt to complete the query. Turning this off will allow you to close the socket and cause the query to terminate.

The comments below show how the answer was found, and the thought process involved therein.

Answer 2

As far as I know, If shutdown() and close() both return 0 there's no doubt you had successfully closed a socket. The fact is that you could have closed the wrong fd. Or the server could not react properly to a correct shutdown (if so, this could be considered a bug of the server: no reason to still wait for data incoming). I'd keep looking for a supported way to do this.

Answer 3

It looks like you are running into an issue with the TCP wait timer, meaning it will close eventually. [Long story short] it is sort of unavoidable. There was another discussion on this.

close vs shutdown socket?