* p2phttp: Allow unauthenticated users to lock content by default.
* p2phttp: Added --unauth-nolocking option to prevent unauthenticated
users from locking content.
The rationalle for this is that locking is not really a write operation, so
makes sense to allow in a repository that only allows read-only access. Not
supporting locking in that situation will prevent the user from dropping
content from a special remote they control in cases where the other copy of
the content is on the p2phttp server.
Also, when p2phttp is configured to also allow authenticated access,
lockcontent was resulting in a password prompt for users who had no way to
authenticate. And there is no good way to distinguish between the two types
of users client side.
--unauth-nolocking anticipates that this might be abused, and seems better
than disabling unauthenticated access entirely if a server is being
attacked. It may be that rate limiting locking by IP address or similar
would be an effective measure in such a situation. Or just limiting the
number of locks by anonymous users that can be live at any one time. Since
the impact of such an DOS attempt is limited to preventing dropping content
from the server, it seems not a very appealing target anyway.
p2phttp: Support serving unauthenticated users while requesting
authentication for operations that need it. Eg, --unauth-readonly can be
combined with --authenv.
Drop locking currently needs authentication so it will prompt for that.
That still needs to be addressed somehow.
This bypasses the usual haskell file locking used to prevent opening a
file for read that is being written to.
This is unfortunately a bit of a hack. But it seems fairly unlikely to
get broken by changes to ghc. I hope. Using fdToHandle' will also work.
This does not work on windows because it uses openFd from posix. It
would probably be possible to implement it for windows too, just opening
the FD using the Win32 library instead. However, whether windows will
allow reading from a file that is also being written to I don't know,
and since in the git-annex case the writer could be another process (eg
external special remote), that might be doing its own locking in
windows, that seems a can of worms I'd prefer not to open.
