Test Context not available in Custom Reporter when run in Parallel

[DanLomanto]

Prerequisites

• [X] Checked that your issue hasn't already been filed by cross-referencing issues with the faq label
• [X] Checked next-gen ES issues and syntax problems by using the same environment and/or transpiler configuration without Mocha to ensure it isn't just a feature that actually isn't supported in the environment in question or a bug in your code.
• [X] 'Smoke tested' the code to be tested by running it outside the real test suite to get a better sense of whether the problem is in the code under test, your usage of Mocha, or Mocha itself
• [X] Ensured that there is no discrepancy between the locally and globally installed versions of Mocha. You can find them with: node node_modules/.bin/mocha --version(Local) and mocha --version(Global). We recommend that you not install Mocha globally.

Description

I'm trying to access a test's context (ctx) in a custom reporter that I'm using for some test-specific data. However, when I run tests in parallel, the context isn't available on the test object. When I run tests sequentially (--no-parallel), then the context is available.

Steps to Reproduce

I created a repro that reproduces this issue. https://github.com/DanLomanto/mocha-parallel-reporter-test-context

Steps to Reproduce:

1. Download the repo.
2. Run yarn install
3. To reproduce the failure, run yarn test (tests run in parallel)
4. To see the things working, run yarn test --no-parallel (tests run sequentially)

Expected behavior: The test context to be available in a custom reporter when I'm running tests in parallel or sequentially.

Actual behavior: The test context is only available in a custom reporter when running tests sequentially.

Reproduces how often: Every time.

Versions

• The output of mocha --version and node node_modules/.bin/mocha --version: 8.4.0
• The output of node --version: v14.16.0
• Your operating system
  • name and version: Mac OSX 10.15.7
  • architecture (32 or 64-bit): 64bit
• Your shell (e.g., bash, zsh, PowerShell, cmd): zsh
• Your browser and version (if running browser tests): N/A
• Any third-party Mocha-related modules (and their versions): chai (v4.3.3)
• Any code transpiler (e.g., TypeScript, CoffeeScript, Babel) being used (and its version): Typescript (v4.2.4)

[juergba]

In parallel mode the events have to be de-/serialized in order to be transmitted via IPC. Not all event properties can be considered as serialization is a slow process. In lib/test.js you can see that ctx is not one of the selected properties. This issue is not a bug, but made by design, otherwise with too many serialized properties the entire gain in performance would vanish. The parallel mode has its severe drawbacks.

[DanLomanto]

@juergba Thanks for the response!

Do you know of any other way to send test-specific metadata to a custom reporter then, by any chance?

Thanks!

[bicarbon8]

not only is the test.ctx missing, but also the events aren't run in order. You'll find that your EVENT_TEST_BEGIN listeners are called after your EVENT_TEST_PASS, EVENT_TEST_FAIL and EVENT_TEST_PENDING listeners for a given test. This essentially negates the usability of a custom Mocha Reporter when running in parallel as you not only lose contractual data (the context), but also lose ordered reporting.