[Qemu-devel] failed migration makes monitor stuck

[Michael Tokarev]

After some debugging I found a programming error in
error handling in migration, but I'm not sure how to
fix it.

When migration starts, monitor gets suspended, calling
monitor_suspend() routine which increments assotiated
suspend_cnt counter.

At the end of migration, in migrate_fd_cleanup(),
monitor_resume() gets called, which decrements the
counter.

But monitor_resume() gets also called from another
place, in migrate_fd_put_buffer(), in case we
encountered a write error.

So, suppose a tcp endpoint has disconnected, or the
exec: program terminated due to error or whatnot --
in all these cases write will fail, and we'll call
monitor_resume() twice as a result: once in this
place in migrate_fd_put_buffer(), and once more at
the end in migrate_fd_cleanup().

This results in suspend_cnt being decremented twice,
with the resultant value being -1.

So monitor_can_read() will return 0 from now on, since
it compares suspend_cnt with 0.  And hence, monitor will
stop working.

To me it looks like monitor_resume() call should be
removed from migrate_fd_put_buffer(), but I'm not sure
_why_ it were here in the first place.

There's more: monitor_suspend() gets called from within
protocol handlers (using migrate_fd_monitor_suspend()
routine), -- are we sure that all current and future
protocol handlers will call this function?

Thanks!

/mjt