Add a new public function of `ares_process_fds()` to work around the
shortcomings of `ares_process_fd()` and `ares_process()`.
This new function allows integrators to specify more than one file
descriptor at a time in an array, with an event mask per file
descriptor. It also adds a flags parameter which can be used to skip
extra expensive processing if they may be calling the function multiple
times before waiting on more events.
This new function is used by the event thread, and all the other
functions internally call this new function simplifying the code
structure.
Authored-By: Brad House (@bradh352)
The main purpose of this PR is to modularize the communication library,
and streamline the code paths for TCP and UDP into a single flow. This
will help us add additional flows such as TLS, and also make sure these
communication functions return a known set of error codes so that
additional error codes can be added for new flows in the future.
It also adds a new optional callback of
`ares_set_notify_pending_write_callback()` that will assist in
aggregating data from multiple queries into a single write operation.
This doesn't apply to UDP, but on TCP and especially TLS in the future
this can be a significant win. This is automatically applied for the
Event Thread.
It also fixes a long standing issue if UDP connection became saturated
and started returning `EWOULDBLOCK` or `EAGAIN` it was treated as a
failure. Since this is more inline with the TCP code now, it will wait
on a write event to retry.
Finally there are additional cleanups due to not needing to be able to
retrieve socket errnos all over the place.
Authored-By: Brad House (@bradh352)
Brad House