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.
The three-way hand-off has a problem: there's no way to arrange for the
state of the migration to be unambiguous in case of failure. If the
final "disconnect" message is lost (as in, the destination never
receives it whether it is sent by the sender or not), the destination
has no option but to quit with an error status and let a human sort it
out. However, at that point we can either arrange to have a .INCOMPLETE
file still on disc or not - and it doesn't matter which we choose, we
can still end up with dataloss by picking a specific calamity to have
befallen the sender.
Given this, it makes sense to fall back to a simpler protocol: just send
all the data, then send a "disconnect" message. This has the same
downside that we need a human to sort out specific failure cases, but
combined with --unlink before sending "disconnect" (see next patch) it
will always be possible for a human to disambiguate, whether the
destination quit with an error status or not.
Changing behaviour so that instead of rebinding after a successful
migration and continuing as an ordinary server, we simply quit with a
0 exit code and let our caller restart us as a server if they want to.
This means that everything in listen.c, listen.h, and anything making
reference to a rebind address is unneeded.
This is important because if we try to rebind after a migration and
someone else is in the way, any clients trying to reconnect to us will
instead be connecting to the squatter.
If a second mirror command is run while the first is still going,
flexnbd needs to prevent the second because we only have one dirty map.
Also, the shutdown becomes Complicated if we allow more than one mirror
at a time.
If the client makes a write that's out of range, by the time we get to
validate the message at the server end the client has already stuffed
the socket with data we can't use, so we have to flush it.
This patch also fixes a potential problem in the acceptance tests where
the error field was being returned as an array rather than a value.
The mirror_super signals the commit state to the control thread via an
mbox, and this mbox is moved to control. It was owned by mirror_super,
but the problem with that is that mirror_super can free the mbox before
the control client has been scheduled to receive the message. If it's
owned by the control object, that can't happen.
If the mirror attempt failed and we were able to report an error to the
user, it makes no sense to attempt a retry. We don't have a way to
abort a mirror attempt yet, so if the user got a setting wrong and it's
failing for that reason, the only recourse they'd have would be to
restart the server.
