[Exist-development] FIXED (probably) Critical deadlock issue

[Patrick B. <pat...@jo...>]

Hi everyone,

*eXist/stable - 1.4.x

I've been working on a locking issue that was caused by
NativeBroker.openCollection(..) on line 699, and I think I have a solution.

Below is what I have changed the function too. I've tested it pretty
thoroughly and it seems to run well, but being such a core part of the
database I think it's important that it's reviewed before I commit it into
the code base.

private Collection openCollection(XmldbURI uri, long addr, int lockMode) {
        uri = prepend(uri.toCollectionPathURI());
        //We *must* declare it here (see below)
        Collection collection;
        final CollectionCache collectionsCache =
pool.getCollectionsCache();
        synchronized(collectionsCache) {
            collection = collectionsCache.get(uri);
            if(collection != null)
            {
                if (!collection.getURI().equalsInternal(uri)) {
                    LOG.error("The collection received from the cache is
not the requested: " + uri +
                        "; received: " + collection.getURI());
                }

                synchronized(collectionsCache) {
                    collectionsCache.add(collection);
                }
            }
        }

        if (collection == null) {
            final Lock lock = collectionsDb.getLock();
            try{
                lock.acquire(Lock.READ_LOCK);
                try {
                    VariableByteInput is;
                    if (addr == BFile.UNKNOWN_ADDRESS) {
                        Value key = new
CollectionStore.CollectionKey(uri.toString());
                        is = collectionsDb.getAsStream(key);
                    } else {
                        is = collectionsDb.getAsStream(addr);
                    }
                    if (is == null)
                        return null;

                    collection = new Collection(uri);
                    collection.read(this, is);

                    //TODO : manage this from within the cache -pb
                    if(!pool.isInitializing())
                        synchronized(collectionsCache) {
                            /*
                             * This is here on the off chance this
collection was loaded by another
                             * thread in the time we were loading it. We'll
take theirs.
                             */
                            Collection tempTestCollection =
collectionsCache.get(uri);
                            if (tempTestCollection != null) collection =
tempTestCollection;

                            collectionsCache.add(collection);

                        }

                    //TODO : rethrow exceptions ? -pb
                } catch (UnsupportedEncodingException e) {
                    LOG.error("Unable to encode '" + uri + "' in UTF-8");
                    return null;
                } catch (IOException e) {
                    LOG.error(e.getMessage(), e);
                    return null;
                } finally {
                    lock.release(Lock.READ_LOCK);
                }
            } catch (LockException e) {
                LOG.warn("Failed to acquire lock on " +
collectionsDb.getFile().getName());
                return null;
            }
        }


        //Important :
        //This code must remain ouside of the synchonized block
        //because another thread may already own a lock on the collection
        //This would result in a deadlock... until the time-out raises the
Exception
        //TODO : make an attempt to an immediate lock ?
        //TODO : manage a collection of requests for locks ?
        //TODO : another yet smarter solution ?
        if(lockMode != Lock.NO_LOCK) {
            try {
                collection.getLock().acquire(lockMode);
            } catch (LockException e) {
                LOG.warn("Failed to acquire lock on collection '" + uri +
"'");
            }
        }

        return collection;
    }


-- 
Patrick Bosek
Jorsek Software
Cell (585) 820 9634
Office (877) 492 2960
Jorsek.com