This fixes a race whereby:
- User hits an endpoint.
- No cached transaction so executes main code.
- User hits same endpoint.
- No cache transaction so executes main code.
- Main code finishes executing and caches response and returns.
- Main code finishes executing and caches response and returns.
This race is common in the wild when Synapse is struggling under load.
This commit fixes the race by:
- User hits an endpoint.
- Caches the promise to execute the main code and executes main code.
- User hits same endpoint.
- Yields on the same promise as the first request.
- Main code finishes executing and returns, unblocking both requests.
Rather than reimplementing the token parsing in the various places.
This will make it easier to change the token parsing to allow access_tokens
in HTTP headers.
