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authorEelco Dolstra <e.dolstra@tudelft.nl>2009-03-23T01·05+0000
committerEelco Dolstra <e.dolstra@tudelft.nl>2009-03-23T01·05+0000
commitcacff1be886ed975bbef1b17151b25c633711256 (patch)
treea7e466012a5df86433b7e348c446cf98f9f032c2 /tests/locking.nix
parent58969fa2bf9d5e662c372bdf970470b8226bd4c7 (diff)
* No longer block while waiting for a lock on a store path. Instead
  poll for it (i.e. if we can't acquire the lock, then let the main
  select() loop wait for at most a few seconds and then try again).
  This improves parallelism: if two nix-store processes are both
  trying to build a path at the same time, the second one shouldn't
  block; it should first see if it can build other goals.  Also, it
  prevents the deadlocks that have been occuring in Hydra lately,
  where a process waits for a lock held by another process that's
  waiting for a lock held by the first.

  The downside is that polling isn't really elegant, but POSIX doesn't
  provide a way to wait for locks in a select() loop.  The only
  solution would be to spawn a thread for each lock to do a blocking
  fcntl() and then signal the main thread, but that would require
  pthreads.

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