[patch] demote

Hi,

As Amul sent a patch about "ALTER SYSTEM READ ONLY"[1], with similar futur
objectives than mine, I decided to share the humble patch I am playing with to
step down an instance from primary to standby status.

I'm still wondering about the coding style, but as the discussion about this
kind of feature is rising, I share it in an early stage so it has a chance to
be discussed.

I'm opening a new discussion to avoid disturbing Amul's one. 

The design of my patch is similar to the crash recovery code, without resetting
the shared memory. It supports smart and fast demote. The only existing user
interface currently is "pg_ctl [-m smart|fast] demote". An SQL admin function,
eg. pg_demote(), would be easy to add.

Main difference with Amul's patch is that all backends must be disconnected to
process with the demote. Either we wait for them to disconnect (smart) or we
kill them (fast). This makes life much easier from the code point of view, but
much more naive as well. Eg. calling "SELECT pg_demote('fast')" as an admin
would kill the session, with no options to wait for the action to finish, as we
do with pg_promote(). Keeping read only session around could probably be
achieved using global barrier as Amul did, but without all the complexity
related to WAL writes prohibition.

There's still some questions in the current patch. As I wrote, it's an humble
patch, a proof of concept, a bit naive.

Does it worth discussing it and improving it further or do I miss something
obvious in this design that leads to a dead end?

Thanks.

Regards,

[1] https://www.postgresql.org/message-id/flat/CAAJ_b97KZzdJsffwRK7w0XU5HnXkcgKgTR69t8cOZztsyXjkQw%40mail.gmail.com