devblog

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+Been working today on getting git-annex to fall back from nice posix fcntl
+locks to pid locks when the former are not supported. There will be an
+`annex.pidlock` to control this. Mostly useful, I think for networked file
+systems like NFS and Lustre. While these *do* support posix locks, I
+guess it can be hard sometimes to get some big server configured
+appropriately, especially when you don't admin it and just want to use
+git-annex there.
+
+Of course, the fun part about pid locks is that it can be pretty hard to
+tell if one is stale or not. Especialy when using a networked filesystem,
+because then the pid in question can be running on a different computer.
+
+Even if you do figure out that a pid lock is stale, how do you then
+take over a stale pid lock, without racing with anther process that
+also wants to take it over? This was the truely tricky question of the
+day..
+
+I have a possibly slightly novel approach to solve that: 
+Put a more modern lock file someplace else (eg, /dev/shm)
+and use that lock file to lock the pid lock file. Then you can tell if 
+a local pid lock file is stale quickly locally, and take it over safely.
+Of course, if the pid is not locked by a local process, this still
+has to fall back to the inevitable retry-and-timeout-and-fail.
+
+I hope the result will work pretty well, although git-annex will not
+support as fine-grained concurrency when using pid locks. Will find out
+tomorrow when I run today's code! ;)