Comment 1 for bug 1241371

This seems to be the culprit:

revno: 1049 [merge]
author: Daniel van Vugt <email address hidden>
committer: Tarmac
branch nick: trunk
timestamp: Tue 2013-09-03 10:52:21 +0000
message:
  SwitchingBundle: Simplify and clarify guarantees that compositor_acquire
  always has a buffer to return without blocking or throwing an exception.