Misleading panic message in backend/access/transam/xlog.c

Hi,

there was a report in #postgresql recently about a crash on Google Cloud
SQL with the somewhat misleading message "could not write to log file"
while in fact it was the xlog/wal:

|PANIC: could not write to log file 000000010000019600000054 at offset
| 13279232, length 245760: Cannot allocate memory 
|ERROR: could not write block 74666 in file "base/18031/48587": Cannot
| allocate memory 
|CONTEXT: writing block 74666 of relation base/18031/48587
|LOG: server process (PID 5160) was terminated by signal 9: Killed

The slightly longer logfile can be found here: http://dpaste.com/2T61PS9

I suggest to reword that message, e.g. "could not write to transaction
log file" or "could not write to wal file".

Also, the errno (ENOMEM) is curious (and the user wrote that Google
monitoring reported memory at 16/20GB at the time of the crash), but it
could be a due to running on a cloud-fork? As you have no access to
PGDATA, it sounds difficult to diagnose after the fact.

Michael

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