It's safe to terminate the proxy at any point in its lifecycle, so
there's no point using signalfd() (and the associated select() +
non-blocking I/O gubbins) in it. We might want to use non-blocking
I/O in the future for other reasons, of course, at which point it
might become sensible to use signalfd() again. For now, this makes
us reliably responsive to TERM,INT and QUIT in a way that we weren't
previously.
In the event of a fiemap ioctl failing (when the file is on a tmpfs,
for instance), we would free() serve->allocation_map, but it would
remain not NULL, leading to segfaults in client.c when responding to
write requests.
Keeping the free() behaviour is more hassle than it's worth, as there
are synchronization problems with setting serve->allocation_map to
NULL, so we just omit the free() instead to avoid the segfault. This
is safe because we never consult the map until allocation_map_built is
set to true, and we never do that when the builder thread fails.
This makes it easier for the tests (and supervisor) to guarantee to be
able to connect to the server socket.
Also this patch moves freeing the mirror supervisor into the server
thread.
server startup (sparse write avoidance doesn't happen until it is finished).
Added mutex to bitset functions, which were already being called from
multiple threads. Rewrote allocation map builder to request file
information in multiple chunks, to avoid uninterruptible wait and dynamic
memory allocation.
This prevents the supervisor from thinking that the migration completed
successfully.
In order to do this, I've introduced a new lock around the start (and
finish) of the migration so that we avoid a race between the signal
handler in the server_accept loop and the control thread mirror startup.
Without that, we'd risk successfully starting a migration after the
SIGTERM handler fired, which would be Bad.
Building the allocation map takes time, which scales with the size of the disc
being presented. By building that map in the space between bind() and accept(),
we leave the process in a useless state after the only good signal we have for
"we are ready" and the state where it is actually ready. This was breaking
migrations of large files.
