git-annex/doc/todo/P2P_locking_connection_drop_safety.mdwn

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 [[!commit 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_locking` branch.
> 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.

TODO

## 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]]