Re: BUG #17928: Standby fails to decode WAL on termination of primary

Thomas Munro <thomas(dot)munro(at)gmail(dot)com> writes:
> If you happened to run into zeroes where an xl_tot_len is wanted right
> at the end of a page (or any value not big enough to get you to the
> next page), we'll fall through to the single-page branch, and then go
> directly to the CRC check, but then ValidXLogRecord() subtracts
> SizeOfXLogRecord and gets a crazy big length. The CRC implementation
> routines on modern computers happened to use pointer arithmetic that
> terminates immediately without accessing any memory, which is why
> nothing was obviously wrong on most systems. The _sb8.c
> implementation for older ARM, MIPS etc use a length-based loop, and
> read off into deep space.

Ah-hah. Maybe there should be an Assert in the modern CRC code path
that detects the bogus arguments?

> Draft patch attached, including a new test for 039_end_of_wal.pl that
> fails on all systems without the above code.

Both the 009 and 039 tests pass on mamba with this patch. I did
not read the code, but testing-wise it seems good.

regards, tom lane