Fix context between two async requests

[daniloisr]

This commit fixes a bug where a request can end without finishing its timings. See: #4

Bug cause: The structure that is used to register Miniprofiler providers, like (Postgres, HTTP, Redis, ...), because it overrides the original method (globally) and uses the request object to access the
Miniprofiler extensions to build the timings, and this doesn't work in a scenario with simultaneous requests using an async provider.

Here is an example using pg to try illustrating the failing scenario (check tests/concurrent-async-test.js test to see it running).

request A start (it's expected to run two DB queries)

• pg.Client.prototype.query holds a req object of requestA.
• It calls .query in a pg instance
  • A miniprofiler timing starts with the call to req.miniprofiler.timeQuery(...) (let's call this timing A-1)
  • The original method is called (async).

request B start (it's expected to run a single DB query)

• pg.Client.prototype.query holds a req object of request B.
• It calls .query in a pg instance
  • Start timing with req.miniprofiler.timeQuery(...) (timing B-1)
  • The original method is called (async).

request A resume

• The result of .query is returned (timing A-1 is stoped)
• A new call to .query is made
  • This time the req points to request B, this means that req.miniprofiler.timeQuery(...) starts a timing on request B. (timing B-2)
  • The original method is called (async)

request B resume

• The result of .query is returned (timing B-1 is stoped)
• All data was fetched, the request is ready to finish, so internally Miniprofile calls stopProfilling.
  • This fails because the timing (B-2) is started (by request A) but not finished, so calculating the diffs will fails because stop is undefined.

Solution

Using NodeJS "async_hooks" we can track the reference to the correct extension for each request, so calls to req.miniprofiler.timeQuery() will point to the correct miniprofiler extension.

To check some performance analysis see:
nodejs/node#14794 (comment)

The goal of the current commit isn't introduce breaking changes, so the miniprofiler reference is injected into the request using JS getters.

Another solution is changing the API for providers, where instead of receiving a req reference, they would receive a function to get the reference to the correct miniprofiler instance. But this will break API with all existing providers.

References

• https://medium.com/the-node-js-collection/async-hooks-in-node-js-illustrated-b7ce1344111f
• https://medium.com/@guysegev/async-hooks-a-whole-new-world-of-opportunities-a1a6daf1990a
• RFC: seeking feedback on async_hooks performance in production nodejs/node#14794 (comment)

[coveralls]

Coverage increased (+0.01%) to 99.746% when pulling 7f9fa5e on daniloisr:handle-async-context into 426fb18 on MiniProfiler:master.

[daniloisr]

@NickCraver thank you very much for merging this one 🙇‍♂️
One more question, do you know who can publish this update on NPM registry (https://www.npmjs.com/package/miniprofiler)?

[NickCraver]

@goenning @mjibson Can either of you please chime in here on what the build and deploy process to npm was and if you'd like to maintain it going forward?

[goenning]

@daniloisr sorry for the delay, I've add as you maintainer of miniprofiler package. AFAIK, the deploy process is manual