Experiments with Postgres and SSL

I had a conversation a while back with Heikki where he expressed that
it was annoying that we negotiate SSL/TLS the way we do since it
introduces an extra round trip. Aside from the performance
optimization I think accepting standard TLS connections would open the
door to a number of other opportunities that would be worth it on
their own.

So I took a look into what it would take to do and I think it would
actually be quite feasible. The first byte of a standard TLS
connection can't look anything like the first byte of any flavour of
Postgres startup packet because it would be the high order bits of the
length so unless we start having multi-megabyte startup packets....

So I put together a POC patch and it's working quite well and didn't
require very much kludgery. Well, it required some but it's really not
bad. I do have a bug I'm still trying to work out and the code isn't
quite in committable form but I can send the POC patch.

Other things it would open the door to in order from least
controversial to most....

* Hiding Postgres behind a standard SSL proxy terminating SSL without
implementing the Postgres protocol.

* "Service Mesh" type tools that hide multiple services behind a
single host/port ("Service Mesh" is just a new buzzword for "proxy").

* Browser-based protocol implementations using websockets for things
like pgadmin or other tools to connect directly to postgres using
Postgres wire protocol but using native SSL implementations.

* Postgres could even implement an HTTP based version of its protocol
and enable things like queries or browser based tools using straight
up HTTP requests so they don't need to use websockets.

* Postgres could implement other protocols to serve up data like
status queries or monitoring metrics, using HTTP based standard
protocols instead of using our own protocol.

Incidentally I find the logic in ProcessStartupPacket incredibly
confusing. It took me a while before I realized it's using tail
recursion to implement the startup logic. I think it would be way more
straightforward and extensible if it used a much more common iterative
style. I think it would make it possible to keep more state than just
ssl_done and gss_done without changing the function signature every
time for example.

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greg