Re: [PATCH 4/8] add simple xlogdump tool

After some fooling around to provide the discussed backend functionality
to xlogdump (StringInfo atop PQExpBuffer and elog_start/elog_finish),
the following items still need work:

1. rmgr tables
We're linking rmgr.c so that we can obtain the appropriate rm_desc
function pointer for each rmgr. However the table also includes the
rm_redo, startup, etc function pointers, which the linker wants resolved
at xlogdump link time. The idea I have to handle this is to use a macro
similar to PG_KEYWORD: at compile time we define it differently on
xlogdump than on backend, so that the symbols we don't want are hidden.

2. ereport() functionality
Currently the xlogreader.c I'm using (the latest version posted by
Andres) has both elog() calls and ereport(). I have provided trivial
elog_start and elog_finish implementations, which covers the first. I
am not really sure about implementing the whole errstart/errfinish
stack, because that'd be pretty duplicative, though I haven't tried.
The other alternative suggested elsewhere is to avoid elog/ereport
entirely in xlogreader.c and instead pass a function pointer for error
reportage. The backend would normally use ereport(), but xlogdump could
do something simple with fprintf. I think that would end up being
cleaner overall.

3. timestamptz_to_str
xact_desc uses this, which involves a couple of messy backend files
(because there's palloc in them, among other problems). Alternatively
we could tweak xact_desc to use EncodeDateTime (probably through some
simple wrapper); given the constraints imposed on the values, that might
be simpler, and we can provide a simple implementation of EncodeDateTime
or of its hypothetical wrapper in xlogdump.

4. relpathbackend and pfree of its return value
This is messy. Maybe we should a caller-supplied buffer instead of
palloc to solve this.

--
Álvaro Herrera http://www.2ndQuadrant.com/
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