This is similar to the pusher thread, but a separate thread because git
pushes can be done in parallel with exports, and updating a big export
should not prevent other git pushes going out in the meantime.
The exportThread only runs at most every 30 seconds, since updating an
export is more expensive than pushing. This may need to be tuned.
Added a separate channel for export commits; the committer records a
commit in that channel.
Also, reconnectRemotes records a dummy commit, to make the exporter
thread wake up and make sure all exports are up-to-date. So,
connecting a drive with a directory special remote export will
immediately update it, and getting online will automatically
update S3 and WebDAV exports.
The transfer queue is not involved in exports. Instead, failed
exports are retried much like failed pushes.
This commit was sponsored by Ewen McNeill.
Lacking error handling, reconnection, credentials configuration,
and doesn't actually do anything when it receives an incoming notification.
Other than that, it might work! :)
Hooked up everything that needs to notify on pushes. Note that
syncNewRemote does not notify. This is probably ok, and I'd need to thread
more state through to make it do so.
This is only set up to support a single push notification method; I didn't
use a NotificationBroadcaster. Partly because I don't yet know what info
about pushes needs to be communicated, so my data types are only
preliminary.
SampleMVar won't work; between getting the current value and changing
it, another thread could made a change, which would get lost.
TMVar works well; this update situation is handled by atomic transactions.
