Normally the postcopy recover phase should only exist for a super short
period, that's the duration when QEMU is trying to recover from an
interrupted postcopy migration, during which handshake will be carried out
for continuing the procedure with state changes from PAUSED -> RECOVER ->
POSTCOPY_ACTIVE again.

Here RECOVER phase should be super small, that happens right after the
admin specified a new but working network link for QEMU to reconnect to
dest QEMU.

However there can still be case where the channel is broken in this small
RECOVER window.

If it happens, with current code there's no way the src QEMU can got kicked
out of RECOVER stage. No way either to retry the recover in another channel
when established.

This patch allows the RECOVER phase to fail itself too - we're mostly
ready, just some small things missing, e.g. properly kick the main
migration thread out when sleeping on rp_sem when we found that we're at
RECOVER stage.  When this happens, it fails the RECOVER itself, and
rollback to PAUSED stage.  Then the user can retry another round of
recovery.

To make it even stronger, teach QMP command migrate-pause to explicitly
kick src/dst QEMU out when needed, so even if for some reason the migration
thread didn't got kicked out already by a failing rethrn-path thread, the
admin can also kick it out.

This will be an super, super corner case, but still try to cover that.

One can try to test this with two proxy channels for migration:

  (a) socat unix-listen:/tmp/src.sock,reuseaddr,fork tcp:localhost:10000
  (b) socat tcp-listen:10000,reuseaddr,fork unix:/tmp/dst.sock

So the migration channel will be:

                      (a)          (b)
  src -> /tmp/src.sock -> tcp:10000 -> /tmp/dst.sock -> dst

Then to make QEMU hang at RECOVER stage, one can do below:

  (1) stop the postcopy using QMP command postcopy-pause
  (2) kill the 2nd proxy (b)
  (3) try to recover the postcopy using /tmp/src.sock on src
  (4) src QEMU will go into RECOVER stage but won't be able to continue
      from there, because the channel is actually broken at (b)

Before this patch, step (4) will make src QEMU stuck in RECOVER stage,
without a way to kick the QEMU out or continue the postcopy again.  After
this patch, (4) will quickly fail qemu and bounce back to PAUSED stage.

Admin can also kick QEMU from (4) into PAUSED when needed using
migrate-pause when needed.

After bouncing back to PAUSED stage, one can recover again.

Reported-by: Xiaohui Li <xiaohli@redhat.com>
Reviewed-by: Fabiano Rosas <farosas@suse.de>
Bugzilla: https://bugzilla.redhat.com/show_bug.cgi?id=2111332


Reviewed-by: Juan Quintela <quintela@redhat.com>
Signed-off-by: Peter Xu <peterx@redhat.com>
Signed-off-by: Juan Quintela <quintela@redhat.com>
Message-ID: <20231017202633.296756-3-peterx@redhat.com>