* 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.

1 files changed, 13 insertions, 4 deletions

diff --git a/src/libstore/pathlocks.cc b/src/libstore/pathlocks.cc
index f8753bcb6..cad893726 100644
--- a/src/libstore/pathlocks.cc
+++ b/src/libstore/pathlocks.cc
@@ -141,9 +141,9 @@ PathLocks::PathLocks(const PathSet & paths, const string & waitMsg)
 }
 
 
-void PathLocks::lockPaths(const PathSet & _paths, const string & waitMsg)
+bool PathLocks::lockPaths(const PathSet & _paths,
+    const string & waitMsg, bool wait)
 {
-    /* May be called only once! */
     assert(fds.empty());
     
     /* Note that `fds' is built incrementally so that the destructor
@@ -174,8 +174,15 @@ void PathLocks::lockPaths(const PathSet & _paths, const string & waitMsg)
 
             /* Acquire an exclusive lock. */
             if (!lockFile(fd, ltWrite, false)) {
-                if (waitMsg != "") printMsg(lvlError, waitMsg);
-                lockFile(fd, ltWrite, true);
+                if (wait) {
+                    if (waitMsg != "") printMsg(lvlError, waitMsg);
+                    lockFile(fd, ltWrite, true);
+                } else {
+                    /* Failed to lock this path; release all other
+                       locks. */
+                    unlock();
+                    return false;
+                }
             }
 
             debug(format("lock acquired on `%1%'") % lockPath);
@@ -199,6 +206,8 @@ void PathLocks::lockPaths(const PathSet & _paths, const string & waitMsg)
         fds.push_back(FDPair(fd.borrow(), lockPath));
         lockedPaths.insert(lockPath);
     }
+
+    return true;
 }