This avoids some apparently otherwise unsolveable problems involving
races that resulted in the manifest listing bundles that were deleted.
Removed the annex-max-git-bundles config because it can't actually
result in deleting old bundles. It would still be possible to have a
config that controls how often to do a full push, which would avoid
needing to download too many bundles on clone, as well as needing to
checkpresent too many bundles in verifyManifest. But it would need a
different name and description.
Added a backup manifest key, which is used if the main manifest key is
not present. When uploading a new Manifest, it makes sure that it never
drops one key except when the other key is present.
It's entirely possible for the two manifest keys to get out of sync, due
to races. The main one wins when it's present, it is possible for the
main one being dropped to expose the backup one, which has a different
push recorded.
On push, first try to drop all outManifest keys listed in the current
manifest file, which resumes from an interrupted push that didn't
get a chance to delete those keys.
The new manifest gets its outManifest populated with the keys that were
in the old manifest, plus any of the keys that were unable to be
dropped.
Note that it would be possible for uploadManifest to skip dropping old
keys at all. The old keys would get dropped on the next push. But it
seems better to delete stuff immediately rather than waiting. And the
extra work is limited to push and typically is small.
A remote where dropKey always fails will result in an outManifest that
grows longer and longer. It would be possible to check if the remote
has appendonly = True and avoid populating the outManifest. Of course,
an appendonly remote will grow with every git push anyway. And currently
only Remote.GitLFS sets that, which can't be used as a git-remote-annex
remote anyway.
Implemented alternateJournal, which git-remote-annex
uses to avoid any writes to the git-annex branch while setting up
a special remote from an annex:: url.
That prevents the remote.log from being overwritten with the special
remote configuration from the url, which might not be 100% the same as
the existing special remote configuration.
And it prevents an overwrite deleting of other stuff that was
already in the remote.log.
Also, when the branch was created by git-remote-annex, only delete it
at the end if nothing else has been written to it by another command.
This fixes the race condition described in
797f27ab05, where git-remote-annex
set up the branch and git-annex init and other commands were
run at the same time and their writes to the branch were lost.
