Fuzz tests have shown that git cat-file --batch sometimes stops running.
It's not yet known why (no error message; repo seems ok). But this is
something we can deal with in the CoProcess framework, since all 3 types of
long-running git processes should be restartable if they fail.
Note that, as implemented, only IO errors are caught. So an error thrown
by the reveiver, when it sees something that is not valid output from
git cat-file (etc) will not cause a restart. I don't want it to retry
if git commands change their output or are just outputting garbage.
This does mean that if the command did a partial output and crashed in the
middle, it would still not be restarted.
There is currently no guard against restarting a command repeatedly, if,
for example, it crashes repeatedly on startup.
The checkpresent hook can return either True or, False, or fail with a message
if it cannot successfully check the remote. Currently for glacier, when
--trust-glacier is not set, it always returns False. Crucially, in the case
when a file is in glacier, this is telling git-annex it's not there, so copy
re-uploads it. This is not desirable; it breaks using glacier-cli to retreive
that file later, and it wastes money/bandwidth.
What if it instead, when the glacier inventory is missing a
file, it returns False. And when the glacier inventory has a file, unless
--trust-glacier is set, it *fails*.
The result would be:
* `git annex copy --to glacier` would only send things not listed in inventory. If a file is listed in the inventory, `copy`
would complain that --trust-glacier` is not set, and not re-upload the file.
* `git annex drop` would only trust that glacier has a file when --trust-glacier is set. Behavior unchanged.
* `git annex move --to glacier`, when the file is not listed in inventory, would send the file, and delete it locally. Behavior unchanged.
* `git annex move --to glacier`, when the file is listed in inventory, would only trust that glacier has the file when --trust-glacier is set
* `git annex copy --from glacier` / `git annex get`, when the file is located in glacier, would trust the location log, and attempt to get the file from glacier.
