File descriptor handlers leak on fork + exec

[feliperuiz]

When forking and execing, file descriptors get leaked to the child process, which is not always ideal. One can always call fcntl and set O_CLOEXEC after establishing a connection, but this can lead to race conditions if a different thread is trying to fork.

Apart from OS compatibility, are there any cases where one might want this sort of behaviour, meaning the sockets couldn't be created with SOCK_CLOEXEC? If so, how could we implement some sort of flag or another Connect* in order to accommodate for that?

I've been bitten by this at work and I went with calling fcntl, but I'd like to have a more atomic fix, and I'm willing to contribute a patch to this, I'd just like to have some input on what sort of API/behaviour is best. :)