* 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/pathlocks.hh

@@ -33,8 +33,9 @@ public:
     PathLocks();
     PathLocks(const PathSet & paths,
         const string & waitMsg = "");
-    void lockPaths(const PathSet & _paths,
-        const string & waitMsg = "");
+    bool lockPaths(const PathSet & _paths,
+        const string & waitMsg = "",
+        bool wait = true);
     ~PathLocks();
     void unlock();
     void setDeletion(bool deletePaths);