async function support (or at least better error message)

[getaaron]

I have a working lambda function and am trying to migrate to chalice. This lambda function includes a Python async function:

async def fetch(session, url):

When I run chalice deploy, the deploy fails cryptically:

$ chalice deploy
Loading function
Regen deployment package.
'xml_request'
$

(xml_request is a variable in my lambda function, not sure if that's a coincidence or if chalice has a similar variable name.)

If I comment out the async def function, the deploy goes smoothly.

• IMO, chalice should support async functions since lambda supports them (despite the comments in Support asynchronous function #637)
• if chalice doesn't support them, this should be (1) a documented limitation, and (2) there should be a friendly explanation that the function as written is incompatible with chalice and to either rewrite it or find another deployment solution

CC @rajasimon

[joguSD]

This is actually a bug in the policy generation analyzer. It's definitely not our intention to prohibit the use of async in general.