[PATCH] xlog: fix fallocate vs read race

[Konstantin Osipov]

* 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. 

Please push this patch and then proceed to fix it according to the
review.
> 
> Fix this issue by using fallocate() with FALLOC_FL_KEEP_SIZE flag
> instead of posix_fallocate(). With the flag fallocate() won't increase
> the file size, it will only allocate disk space beyond EOF.
> 
> The test will be added shortly.

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