[[!comment format=mdwn
 username="joey"
 subject="""design"""
 date="2015-11-11T17:54:24Z"
 content="""
* annex.pidlock config setting
* init can test if regular locks work and if not set annex.pidlock
* Use .git/annex/lock as the lock file; create with `O_EXCL` and write pid
  and program name to it. (Should be able to check for stale pid and break
  old lock.)
* Adapt Utility.LockPool to use that lock file and lock method when
  annex.pidlock is set. (How? It's a generic library..)
* Note that for sanity, whenever Utility.LockPool would create a
  fine-grained lock file, that should still happen when using
  annex.pidlock. Just avoid locking it and use the
  global lock. This prevents any bugs along the lines of some code
  depending on the fine-grained lock file having been created
  (in order to delete it etc).
* (We could possibly assume that, if a lock file is being created,
  it could be used as a pid lock file, and so use that instead of the
  single top-level lock file. This assumption might hold, but I don't
  really want to risk it. If some other code path uses the same lock file
  but does not allow it to be created, it would not be able to write the
  pid to it (because it might be eg an annex object file), then the two
  code paths would end up using different lock files for the same lock,
  which would be bad.)

This will always be an exclusive lock, and a single lock at that, unlike
git-annex's usual fine-grained, often shared locks. But, the LockPool
builds all that stuff at the thread level using STM anyway, so multiple
threads of the same process can still cooperate with shared locks etc.

Commands that don't need to take any lock (eg, query commands) will
interoperate as before. But, many commands that can normally run
concurrently won't be able to when using annex.pidlock, and will
have to either loop-wait on the lock file, or error out.
"""]]