Async state and message handler

Have you read the Contributing Guidelines on issues?

• I have read the Contributing Guidelines on issues.

Description

Original suggestion by Val on Slack

To allow the state_handler and message_handler functions to handle asynchronous functions.

Motivation

These two functions are used to interface with code outside of Pact Python to setup/teardown states and generate messages. Especially in the case of the state setup and teardown, a common pattern would be to make a number of calls to (mocked or not) databases or other services to ensure they are in a particular state. These requests can be significantly sped up by running them in parallel, such as with an asyncio.gather call or using a task group.

Have you tried building it?

Not yet, but I will first open this issue and ask for anyone with specific examples to work towards.

While asynchronous function bring a number of advantages, they can also bring some complications. In particular:

• There are a number of underlying runtimes: asyncio, trio, curio. If possible, it would be ideal to support all of these.
• The use of threads has implications for ContextVar (if used)

Self-service

• I'd be willing to contribute this feature to Pact Python myself.

Start by locating the state_handler and message_handler entry points and reading how synchronous functions are currently invoked. Compare the requirements of asyncio, trio, and curio, including ContextVar and thread implications; done means the supported asynchronous handlers work without breaking existing synchronous handlers and are covered by tests.