Re: COPYable logs status

Tom Lane wrote:
> Alvaro Herrera <alvherre@commandprompt.com> writes:
>   
>> The idea of one pipe per process is not really workable, because it
>> would mean having as many pipes as backends which does not sound very
>> good.  But how about a mixed approach -- like have the all the backends
>> share a pipe, controlled by an LWLock, and the auxiliary process have a
>> separate pipe each?
>>     
>
> Multiple pipes seem like a mess, and in any case the above still doesn't
> work for stderr output produced by non-cooperative software (dynamic
> loader for instance).
>
> The only solution that I can see is to invent some sort of simple
> protocol for the syslogger pipe.  Assume that the kernel honors PIPE_BUF
> (this assumption may need proving, see other message).  We could imagine
> having elog.c divvy up its writes to the pipe into chunks of less than
> PIPE_BUF bytes, where each chunk carries info sufficient to let it be
> reassembled.  Perhaps something on the order of
>
>     \0 \0 2-byte-length source-PID end-flag text...
>
> The syslogger reassembles these by joining messages with the same
> origination PID, until it gets one with the end-flag set.  It would need
> enough code to track multiple in-progress messages.
>
> The logger would have to also be able to deal with random text coming
> down the pipe (due to aforesaid non-cooperative software).  I would be
> inclined to say just take any text not preceded by \0\0 as a standalone
> message, up to the next \0\0.  Long chunks of non-protocol text would
> risk getting treated as multiple messages, but there's probably not a
> lot of harm in that.
>
> BTW, exactly what is the COPYable-logs code going to do with random
> text?  I trust the answer is not "throw it away".
>
>             
>   

The CSVlog pipe is a separate pipe from the stderr pipe. Anything that 
goes to stderr now will continue to go to stderr, wherever that is.

I like this scheme for a couple of reasons:
. it will include the ability to tell the real end of a message
. it will let us handle non-protocol messages (although there shouldn't 
be any in the CSVlog pipe).

I'll try to get a patch out for just the stderr case, which should be 
back-patchable, then adjust the CSVlog patch to use it.

I'm thinking of handling the partial lines with a small dynahash of 
StringInfo buffers, which get discarded whenever we don't have a partial 
line for the PID.

cheers

andrew