Re: [Patch] pg_rewind: options to use restore_command from recovery.conf or command line

On Mon, Oct 29, 2018 at 12:09:21PM +0300, Alexey Kondratov wrote:
> Currently in the patch, with dry-run option (-n) pg_rewind only fetches
> missing WALs to be able to build file map, while doesn't touch any data
> files. So I guess it behaves exactly as you described and we do not need a
> separate tool.

Makes sense perhaps. Fetching only WAL segments which are needed for
the file map is critical, as you don't want to spend bandwidth for
nothing. Now, I look at your patch, and I can see things to complain
about, at least three at short glance:
- The TAP test added will fail on Windows.
- Simply copy-pasting RestoreArchivedWAL() from the backend code to
pg_rewind is not an acceptable option. You don't care about %r either
in this case.
- Reusing the GUC parser is something I would avoid as well. Not worth
the complexity.

Another thing I am wondering is: do we actually need something complex?
What we want to know is what data is necessary to build the file map, so
we could also add an option to pg_rewind which checks what segments are
necessary and lets the user know about them? This also avoids the
security-related problems of manipulating a command at option-level.
This kind of options makes folks willing to use more sensitive data on
command line, which is not always a good idea...
--
Michael