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* No longer block while waiting for a lock on a store path. Instead

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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.
This commit is contained in:
Eelco Dolstra 2009-03-23 01:05:54 +00:00
parent 58969fa2bf
commit cacff1be88
5 changed files with 85 additions and 28 deletions
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17
src/libstore/pathlocks.cc
View file

@ -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;
}