Allow AIOHTTPClient to accept user-provided session or connector (#1736)

[skukutla-stripe]

Why?

Currently, AIOHTTPClient always creates its own aiohttp.ClientSession and TCPConnector internally, with no way for users to customize them. This means users cannot:

• Share a connection pool across multiple clients (e.g., Stripe + other services)
• Tune connection pool settings like limit, limit_per_host, or keepalive_timeout
• Provide a custom TCPConnector with their own SSL context or DNS caching configuration

The only workaround is setting the private _cached_session attribute after construction, which is an unstable interface.

RequestsClient already accepts an optional session parameter in its constructor (line 585), so there is existing precedent for this pattern. The need is stronger for AIOHTTPClient because async usage inherently involves concurrent requests, making connection pool sharing and tuning a common requirement for people using this client.

What?

• Added optional session and connector parameters to AIOHTTPClient.init
• When session is provided, it is used directly as the underlying client session
• When connector is provided, AIOHTTPClient creates a ClientSession using the user's connector (the SDK still controls the session itself)
• When neither is provided, behavior is identical to before (creates its own connector with Stripe's CA bundle and a new session)
• Added _owns_session tracking so that close_async() only closes the session if AIOHTTPClient created it. This avoids destroying a shared session that the user manages themselves (an improvement over RequestsClient.close(), which
  unconditionally closes even user-provided sessions)
• This is a backward-compatible change: both new parameters default to None, preserving existing behavior for all current users

See Also

• Allow AIOHTTPClient constructor to take user-provided ClientSession #1736

[xavdid-stripe]

nice! had a couple of small style suggestions. once you fix those, you can merge!