migration: Fix race on qemu_file_shutdown()

In qemu_file_shutdown(), there's a possible race if with current order of
operation.  There're two major things to do:

  (1) Do real shutdown() (e.g. shutdown() syscall on socket)
  (2) Update qemufile's last_error

We must do (2) before (1) otherwise there can be a race condition like:

      page receiver                     other thread
      -------------                     ------------
      qemu_get_buffer()
                                        do shutdown()
        returns 0 (buffer all zero)
        (meanwhile we didn't check this retcode)
      try to detect IO error
        last_error==NULL, IO okay
      install ALL-ZERO page
                                        set last_error
      --> guest crash!

To fix this, we can also check retval of qemu_get_buffer(), but not all
APIs can be properly checked and ultimately we still need to go back to
qemu_file_get_error().  E.g. qemu_get_byte() doesn't return error.

Maybe some day a rework of qemufile API is really needed, but for now keep
using qemu_file_get_error() and fix it by not allowing that race condition
to happen.  Here shutdown() is indeed special because the last_error was
emulated.  For real -EIO errors it'll always be set when e.g. sendmsg()
error triggers so we won't miss those ones, only shutdown() is a bit tricky
here.

Cc: Daniel P. Berrange <berrange@redhat.com>
Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Signed-off-by: Peter Xu <peterx@redhat.com>
Reviewed-by: Juan Quintela <quintela@redhat.com>
Signed-off-by: Juan Quintela <quintela@redhat.com>

migration/qemu-file.c

@@ -79,6 +79,30 @@ int qemu_file_shutdown(QEMUFile *f)
     int ret = 0;
 
     f->shutdown = true;
+
+    /*
+     * We must set qemufile error before the real shutdown(), otherwise
+     * there can be a race window where we thought IO all went though
+     * (because last_error==NULL) but actually IO has already stopped.
+     *
+     * If without correct ordering, the race can happen like this:
+     *
+     *      page receiver                     other thread
+     *      -------------                     ------------
+     *      qemu_get_buffer()
+     *                                        do shutdown()
+     *        returns 0 (buffer all zero)
+     *        (we didn't check this retcode)
+     *      try to detect IO error
+     *        last_error==NULL, IO okay
+     *      install ALL-ZERO page
+     *                                        set last_error
+     *      --> guest crash!
+     */
+    if (!f->last_error) {
+        qemu_file_set_error(f, -EIO);
+    }
+
     if (!qio_channel_has_feature(f->ioc,
                                  QIO_CHANNEL_FEATURE_SHUTDOWN)) {
         return -ENOSYS;
@@ -88,9 +112,6 @@ int qemu_file_shutdown(QEMUFile *f)
         ret = -EIO;
     }
 
-    if (!f->last_error) {
-        qemu_file_set_error(f, -EIO);
-    }
     return ret;
 }