Access to ConnectionPool.count should be thread safe

[ya-pulser]

ConnectionPool has a collection of providers registered with the pool and access to the collection is guarded by ConnectionPool.lock.

Size of the collection is exposed through count accessor without guarding by the lock.
https://github.com/swift-server/async-http-client/blob/1.2.2/Sources/AsyncHTTPClient/ConnectionPool.swift#L112

That leads to warnings in thread safety analyser.

To reproduce run with thread safety analyzer:

docker run --rm -v $(pwd):/src -w /src swift:5.3.1 swift test -c release --sanitize=thread --enable-test-discovery

Analysis of output shows that this count accessor is main source of thread safety warnings.

[ya-pulser]

PR #319 as a proposed solution

[Lukasa]

I believe we’re done here: see #319 (comment) for more reasoning.

[Lukasa]

@weissi has a good reference for a TSAN issue in 5.3 that manifests this way.

[weissi]

@fabianfett I think we can close this?

[fabianfett]

I'll add it to the http/2 milestone and close it as soon as the connection pool has landed.

[fabianfett]

Fixed in #319 and removed from an active code path in #427.