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
PRE-REMOVE step. After receiving PRE-REMOVE N Key, a REMOVE of that key is only
allowed until N seconds later. Sending PRE-REMOVE first, followed by
LOCKCONTENT will guarantee the content remains locked for the full amount
of time.

How long? 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.

----

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.

----

Extending the P2P protocol is a bit tricky, because the same P2P
protocol connection could be used for several different things at
the same time. A PRE-REMOVE N Key might be followed by removals of other
keys, and eventually a removal of the requested key. There are
sometimes pools of P2P connections that get used like this.
So the server would need to cache some number of PRE-REMOVE timestamps.
How many?

Certainly care would need to be taken to send PRE-REMOVE to the same
connection as REMOVE. How?

Could this be done without extending the REMOVE side of the P2P protocol?

1. check start time
2. LOCKCONTENT
3. prepare to remove
4. in checkVerifiedCopy, 
   check current time.. fail if more than 10 minutes from start
5. REMOVE

The issue with this is that git-annex could be paused for any amount of
time between steps 4 and 5. Usually it won't pause.. 
mkSafeDropProof calls checkVerifiedCopy and constructs the proof,
and then it immediately sends REMOVE. But of course sending REMOVE
could take arbitrarily long. Or git-annex could be paused at just the wrong
point.

Ok, let's reconsider... Add GETTIMESTAMP which causes the server to
return its current timestamp. The same timestamp must be returned on any
connection to the server, eg the server must have a single clock.
That can be called before LOCKCONTENT.
Then REMOVE Key Timestamp can fail if the current time is past the
specified timestamp. 

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.

This approach would need to use a monotonic clock!