TVars were not updated atomically, which was ok when each thread got its
own External that was the only thing using these TVars. But, with the
async extension, several External instances can share the same var, so
it needs to be a TMVar to avoid read/write conflicts.
In particular, this makes PREPARE only be sent once.
Moving jobid generation to the git-annex side lets it be simplified a
lot.
Note that it will also be possible to generate one jobid per connection,
rather than a new job per request. That will make overflow not an issue,
and will avoid some work, and will simplify some of the code.
Receive loop looks right. Still need the send loop.
And, a complication is that some messages git-annex
sends need to be wrapped in REPLY_ASYNC, while others
do not. So will probably need to split externalSend
into two.
Now that I've started implementation, I see it's really necessary that
every message the special remote sends use the protocol, otherwise
nasty edge cases abound.
Since there's a race here, and since Kyle saw an exception leak out,
which I have not been able to reproduce that. See my comment for what
I think might be going on.
Note that, I used tryNonAsync, because it seems a later tryNonAsync
caught the exception. I don't actually understand how it did, as I
understand exception classification, it's the data type, not the way it
was thrown. One possibility is that the async exception may have been wrapped
in some other, non-async exception, and Show displayed it the same way.
