Comment 15 for bug 1514094

Thanks Blake for the test in the real MAAS environment.

As mentioned yesterday I was able to reproduce those and was working on them this morning.
- I found an issues which didn't show up before as part of it is based on a race with older levels of the userspace toolchain.
- There was an issue - again only with older userspace toolchains - which only triggered if the backing device was set up when the caching device was not yet fully initialized
- I also found that there was a potential issue of a yaml changing cache mode on a cache device (those are set per backing device) which would have caused a non insightful "IOError". This now fails more gracefully with a proper message.

All those issues were fixed in my branch (lp:~paelzer/curtin/fix-1514094-v1) as of revision 328 and along all that I further extended the debug output of these code paths.

Since it has shown races I ran my tests 18 times over Lunch and all worked.
It would be great if you could put it to its trial in your environment once more to see if there is more to do.