pgsql: Redesign archive modules

Redesign archive modules

A new callback named startup_cb, called shortly after a module is
loaded, is added. This makes possible the initialization of any
additional state data required by a module. This initial state data can
be saved in a ArchiveModuleState, that is now passed down to all the
callbacks that can be defined in a module. With this design, it is
possible to have a per-module state, aimed at opening the door to the
support of more than one archive module.

The initialization of the callbacks is changed so as
_PG_archive_module_init() does not anymore give in input a
ArchiveModuleCallbacks that a module has to fill in with callback
definitions. Instead, a module now needs to return a const
ArchiveModuleCallbacks.

All the structure and callback definitions of archive modules are moved
into their own header, named archive_module.h, from pgarch.h.
Command-based archiving follows the same line, with a new set of files
named shell_archive.{c,h}.

There are a few more items that are under discussion to improve the
design of archive modules, like the fact that basic_archive calls
sigsetjmp() by itself to define its own error handling flow. These will
be adjusted later, the changes done here cover already a good portion
of what has been discussed.

Any modules created for v15 will need to be adjusted to this new
design.

Author: Nathan Bossart
Reviewed-by: Andres Freund
Discussion: https://postgr.es/m/20230130194810.6fztfgbn32e7qarj@awork3.anarazel.de

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/35739b87dcfef9fc0186aca659f262746fecd778

Modified Files
--------------
contrib/basic_archive/basic_archive.c | 86 ++++++++++++++++++----
doc/src/sgml/archive-modules.sgml | 35 +++++++--
src/backend/Makefile | 2 +-
src/backend/archive/Makefile | 18 +++++
src/backend/archive/meson.build | 5 ++
.../{postmaster => archive}/shell_archive.c | 41 +++++++----
src/backend/meson.build | 1 +
src/backend/postmaster/Makefile | 1 -
src/backend/postmaster/meson.build | 1 -
src/backend/postmaster/pgarch.c | 27 ++++---
src/backend/utils/misc/guc_tables.c | 1 +
src/include/archive/archive_module.h | 59 +++++++++++++++
src/include/archive/shell_archive.h | 24 ++++++
src/include/postmaster/pgarch.h | 39 ----------
src/tools/pgindent/typedefs.list | 1 +
15 files changed, 252 insertions(+), 89 deletions(-)