Re: Printing LSN made easy

On Fri, Nov 27, 2020 at 04:10:27PM +0530, Ashutosh Bapat wrote:
> LSN_FORMAT_ARG expands to two comma separated arguments and is kinda
> open at both ends  but it's handy that way.

Agreed that's useful.

> Off list Peter Eisentraut pointed out that we can not use these macros
> in elog/ereport since it creates problems for translations. He
> suggested adding functions which return strings and use %s when doing
> so.

What's the problem with translations?  We already have equivalent
stuff with INT64_FORMAT that may map to %ld or %lld.  But here we have
a format that's set in stone.

> The patch has two functions pg_lsn_out_internal() which takes an LSN
> as input and returns a palloc'ed string containing the string
> representation of LSN. This may not be suitable in performance
> critical paths and also may leak memory if not freed.

I'd rather never use this flavor.  An OOM could mask the actual error
behind.

> Off list Craig Ringer suggested introducing a new format specifier
> similar to %m for LSN but I did not get time to take a look at the
> relevant code. AFAIU it's available only to elog/ereport, so may not
> be useful generally. But teaching printf variants about the new format
> would be the best solution. However, I didn't find any way to do that.

-1.  %m maps to errno, that is much more generic.  A set of macros
that maps to our internal format would be fine enough IMO.
--
Michael