* 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.

src/libstore/globals.hh

@@ -65,7 +65,7 @@ extern string thisSystem;
 /* The maximum time in seconds that a builer can go without producing
    any output on stdout/stderr before it is killed.  0 means
    infinity. */
-extern unsigned int maxSilentTime;
+extern time_t maxSilentTime;
 
 /* The substituters.  There are programs that can somehow realise a
    store path without building, e.g., by downloading it or copying it