Reversion introduced in version 8.20201007, one release after the 1st
release with the extension.
Surprisingly, hClose can hang if another thread is reading from the
handle. This is because it uses takeMVar.
The use of cancel here does mean that, if receiveMessageAddonProcess
or Remote.External.AsyncExtension.receiveloop allocated some resource in
a non-async-exception safe way, they might not get a chance to clean it up.
They do not appear to, and anyway, this only happens when git-annex is
shutting down, so any recource that did leak would not be a problem.
This commit was sponsored by Boyd Stephen Smith Jr. on Patreon.
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.
