Avoid trying to git push/pull to special remotes, but still do transfer
scans of them, after git pull from any other remotes, so we know about
any values that have been placed on them.
I think this makes sense.. Unless the assistant is running on the server,
the repo won't be updated, so it might as well be bare.
Non-bare repos will be handled by the pairing configurator, later.
The code to maintain that TChan in parallel with the list was buggy,
the two were not always the same. And all that TChan was needed for was
blocking on the next transfer, which can be accomplished just as well by
checking the size and retrying, thanks to STM.
Also, this is faster, and uses less memory. Total win.
I had an intuition that throwTo might be blocking because an exception was
caught and the exception handler was running. This seems to be the case,
and is avoided by using try. However, I can't really find anywhere in
throwTo's documentation that justifies this behavior.
When multiple downloads of a key are queued, it starts the first, but leaves the
other downloads in the queue. This ensures that we don't lose a queued
download if the one that got started failed.
Run code that pops off the next queued transfer and adds it to the active
transfer map within an allocated transfer slot, rather than before
allocating a slot. Fixes the transfers display, which had been displaying
the next transfer as a running transfer, while the previous transfer was
still running.
Currently only the web special remote is readonly, but it'd be possible to
also have readonly drives, or other remotes. These are handled in the
assistant by only downloading from them, and never trying to upload to
them.
The expensive transfer scan now scans a whole set of remotes in one pass.
So at startup, or when network comes up, it will run only once.
Note that this can result in transfers from/to higher cost remotes being
queued before other transfers of other content from/to lower cost remotes.
Before, low cost remotes were scanned first and all their transfers came
first. When multiple transfers are queued for a key, the lower cost ones
are still queued first. However, this could result in transfers from slow
remotes running for a long time while transfers of other data from faster
remotes waits.
I expect to make the transfer queue smarter about ordering
and/or make it allow multiple transfers at a time, which should eliminate
this annoyance. (Also, it was already possible to get into that situation,
for example if the network was up, lots of transfers from slow remotes
might be queued, and then a disk is mounted and its faster transfers have
to wait.)
Also note that this means I don't need to improve the code in
Assistant.Sync that currently checks if any of the reconnected remotes
have diverged, and if so, queues scans of all of them. That had been very
innefficient, but now doesn't matter.
Used by the assistant, rather than copy, this is faster because it avoids
using git ls-files, avoids checking the location log redundantly, and
runs in oneshot mode, avoiding making a commit to the git-annex branch
for every file transferred.
