@redis/core-team adding a trivial API, please approve.

Two more requests:

1. Let's add an optional timeout argument (using pthread_mutex_timedlock) so that we don't have to add another API in the future.
2. Let's store the return value into errno so that users can distinguish between the different types of errors (we already have other APIs doing that)

@oranagra Why manipulate errno and not have a proper API to return errors?

IMHO it feels more natural and matches the rest of the API. We don't have any module API that returns errno (yet).
Come to think of it most of the POSIX api returns 0 or non-0 and the error comes from errno. So why is the mutex API different?

After discussing this with @oranagra, our conclusions are:

1. Using errno is somewhat of a code smell but it's already the practice in other Module API functions so we'll stick with that for the sake of consistency.
2. We need an optional timeout argument as described above.
   @swilly22 can you incorporate these changes into the PR?

@yossigo sure thing.

Turns out pthread_mutex_timedlock isn’t supported on the following platforms:

Mac OS X 10.13, FreeBSD 5.2.1, NetBSD 7.1, OpenBSD 3.8, Minix 3.1.8, AIX 5.1, HP-UX 11, IRIX 6.5, Solaris 9, Cygwin 1.7.x, mingw, MSVC 14, Android 4.4.

This is a suggested workaround:

#if defined(_POSIX_TIMEOUTS)  && _POSIX_TIMEOUTS >= 200112L
   r = pthread_mutex_timedlock(&mutList, &to);
#else
   r = pthread_mutex_trylock(&mutList); // must check this
#endif

In this case I suggest we revert.

yes, let's revert..
the only other option is to add a separate module API for timedlock, which may or may not exist, and have modules check for it's existence at runtime before using.

@oranagra I am not in favor of a Module API that is OS specific and may or may not be available regardless of Redis version. We can just not support timeouts.

@redis/core-team please approve.