Concurrent calls of _hash_getnewbuf()

When doing my experiments with bucket split ([1]), I noticed a comment that
_hash_getnewbuf should not be called concurrently. However, there's no
synchronization of calls from _hash_splitbucket in HEAD. I could reproduce
such concurrent calls using gdb (multiple bucket splits in progress at a
time).

When the function is called from _hash_getovflpage, content lock of metapage
buffer seems to be (mis)used to synchronize the calls:

/*
* Fetch the page with _hash_getnewbuf to ensure smgr's idea of the
* relation length stays in sync with ours. XXX It's annoying to do this
* with metapage write lock held; would be better to use a lock that
* doesn't block incoming searches.
*/
newbuf = _hash_getnewbuf(rel, blkno, MAIN_FORKNUM);

I think it'd also be the easiest fix for _hash_splitbucket. Or should a
separate ("regular") lock be introduced and used and used in both cases?

[1] http://www.postgresql.org/message-id/32423.1427413442@localhost

--
Antonin Houska
Cybertec Schönig & Schönig GmbH
Gröhrmühlgasse 26
A-2700 Wiener Neustadt
Web: http://www.postgresql-support.de, http://www.cybertec.at