The P2P protocol's LOCKCONTENT assumes that the P2P connection does not get closed unexpectedly. If the P2P connection does close before the drop happens, the remote's lock will be released, but the git-annex that is doing the dropping does not have a way to find that out.
This in particular affects drops from remotes. Drops from the local repository have a ContentRemovalLock that doesn't have this problem.
This was discussed in 73a6b9b51455f2ae8483a86a98e9863fffe9ebac (2016). There I concluded:
Probably this needs to be fixed by eg, making lockContentWhile catch any
exceptions due to the connection closing, and in that case, wait a
significantly long time before dropping the lock.
I'm inclined to agree with past me. While the P2P protocol could be extended with a way to verify that the connection is still open, there is a point where git-annex has told the remote to drop, and is relying on the locks remaining locked until the drop finishes. --Joey
Worst case, I can imagine that the local git-annex process takes the remote locks. Then it's put to sleep for a day. Then it wakes up and drops from the other remote. The P2P connections for the locks have long since closed. Consider for example, a ssh password prompt on connection to the remote to drop the content, and the user taking a long time to respond.
It seems that LOCKCONTENT needs to guarantee that the content remains locked for some amount of time. Then local git-annex would know it has at most that long to drop the content. But it's the remote that's dropping that really needs to know. So, extend the P2P protocol with a REMOVE-BEFORE Timestamp Key and a GETTIMESTAMP.
How long to lock for? 10 minutes is arbitrary, but seems in the right ballpark. Since this will cause drops to fail if they timeout sitting at a ssh password prompt, it needs to be more than a few minutes. But making it too long, eg an hour can result in content being stuck locked on a remote for a long time, preventing a later legitimate drop. It could be made configurable, if needed, by extending the P2P protocol so LOCKCONTENT was passed the amount of time.
Having lockContentWhile catch all exceptions and keep the content locked
for the time period won't work though. Systemd reaps processes on ssh
connection close. And if the P2P protocol is served by git annex
remotedaemon for tor, or something similar for future P2P over HTTP
(either a HTTP daemon or a CGI script), nothing guarantees that such a
process is kept running. An admin may bounce the HTTP server at any point,
or the whole system reboot.
retention locking
So, this needs a way to make lockContentShared guarentee it remains locked for an amount of time even after the process has exited.
In a v10 repo, the content lock file is separate from the content file, and it is currently an empty file. So a timestamp could be put in there. It seems ok to only fix this in v10, because by the time the fixed git-annex gets installed, a user is likely to have been using git-annex 10.x long enough (1 year) for their repo to have been upgraded to v10.
OTOH putting the timestamp in the lock file may be hard (eg on Windows).
Status: Content retention files implemented on
p2p_lockingbranch. P2P LOCKCONTENT uses a 10 minute retention in case it gets killed, but other values can be used in the future safely.
clusters
How to handle this when proxying to a cluster? In a cluster, each node has a different clock. So GETTIMESTAMP will return a bunch of times. The cluster can get its own current time, and return that to the client. Then REMOVE Key Timestamp can have the timestamp adjusted when it's sent out to each client, by calling GETTIMESTAMP again and applying the offsets between the cluster's clock and each node's clock.
done
future flag day
There is a potential future flag day where p2pDefaultLockContentRetentionDuration is not assumed, but is probed using the P2P protocol, and peers that don't support it can no longer produce a LockedCopy. And P2P.Protocol.remove does not fall back to REMOVE when the peer does not support REMOVE-WHEN and there's a proof expiry time.
Until that flag day, when git-annex is communicating with older peers there is a risk of data loss when a ssh connection closes during LOCKCONTENT.
I think that now is not the right time for that flag day, because it will cause disruption. Everyone would have to upgrade remote git-annex versions in order to drop content from those remotes, or with content locked on those remotes. This problem is not likely enough to occur to seem worth that disruption.
A flag day might be worth doing in a couple of years though. --Joey