Descriptors equal or higher than FD_SETSIZE can't be used with select()

[vnsavage]

As per select(2) manual page: Executing FD_CLR() or FD_SET() with a value of fd that is negative or is equal to or larger than FD_SETSIZE will result in undefined behavior.

If we don't check whether the file descriptor is less than FD_SETSIZE it's easy to trigger segfaults in the extension. This is easy to reproduce with the following script:

<?php

for ( $i = 0; $i < 1024; $i++ ) {
    socket_create_pair(AF_UNIX, SOCK_STREAM, 0, $socketpair);
    $sockets[] = $socketpair;
}

memcache_get(memcache_connect('memcache_host, 11211), 'some_key');

Long term the extension should probably be switched to a different polling API, but in the short term this patch will trigger Fatal Errors when we run out of allowed file descriptors so that segfaults are not generated.

[pmurzakov]

I don't know how it is possible as this bug has been present at least since 16 years ago and never been reported and now we both found it at the same time. I also encountered it in our production system a few days ago and implemented alternative API based on poll() which doesn't have this limitation. (leaving select() as default option for compatibility)

here the PR #101

[vnsavage]

Quite the coincidence indeed, switching to poll() sounds good 👍