[PATCH] xlog: fix fallocate vs read race

[Vladimir Davydov]

On Fri, Dec 14, 2018 at 02:07:41PM +0300, Konstantin Osipov wrote:
> * Vladimir Davydov <vdavydov.dev at gmail.com> [18/12/14 13:30]:
> > posix_fallocate(), which is used for preallocating disk space for WAL
> > files, increases the file size and fills the allocated space with zeros.
> > The problem is a WAL file may be read by a relay thread at the same time
> > it is written to. We try to handle the zeroed space in xlog_cursor (see
> > xlog_cursor_next_tx()), however this turns out to be not enough, because
> > transactions are written not atomically so it may occur that a writer
> > writes half a transaction when a reader reads it. Without fallocate, the
> > reader would stop at EOF until the rest of the transaction is written,
> > but with fallocate it reads zeroes instead and thinks that the xlog file
> > is corrupted while actually it is not.
> 
> You should use check_program_runs() not check_symbol_exists and
> avoid checks at runtime. 

At compile time we don't know if the filesystem that will be used for
storing WALs supports fallocate() so the runtime check is a must.