feat(outbound-pg): Reuse connection during request lifecycle

[etehtsea]

Also, pg_backend_pid endpoint into outbound-pg example.

Before the fix:

$ curl -i localhost:3000/pg_backend_pid
HTTP/1.1 500 Internal Server Error
content-length: 0
date: Tue, 27 Sep 2022 14:03:50 GMT

thread '<unnamed>' panicked at 'assertion failed: `(left == right)`
  left: `592913`,
  right: `592914`', src/lib.rs:112:5

After the fix:

$ curl -i localhost:3000/pg_backend_pid
HTTP/1.1 200 OK
content-length: 23
date: Tue, 27 Sep 2022 14:07:14 GMT

pg_backend_pid: 595194

Ideally, this fix has to be covered by an integration test rather than
manual testing through examples, but the testing environment has to be
set up first.

Refs: #667.
Signed-off-by: Konstantin Shabanov mail@etehtsea.me

[itowlson]

Thanks @etehtsea, looks good to me! Really appreciate the extra info and polish in the sample, and nice to have the connection reuse. Are you still pushing updates or is this okay to merge?

[etehtsea]

@itowlson yep, it should be ready to go. The only thing I was thinking about is the difference with redis connections map implementation - https://github.com/fermyon/spin/blob/main/crates/outbound-redis/src/lib.rs#L19-L21

My understanding is that as the state is built per request then it's single-threaded. Am I missing something?

[lann]

My understanding is that as the state is built per request then it's single-threaded. Am I missing something?

The difference with the outbound-redis example is that it is reusing connections between requests. Since this PR only caches within a single request it does not need to lock; individual requests are handled on a single thread.

Edit: We will probably be removing that inter-request caching from outbound-redis. Among other reasons, it has the potential to leak resources in a long-lived spin instance.

[itowlson]

@etehtsea Given Lann's edit to his comment, it looks like the pattern we should be adopting is the one you already have, so I'll merge this as it stands. Thanks for the contribution and for taking the time to cross-check!