mocha --watch breaks the parser

[luxzeitlos]

this is very strange, but when I'm running mocha everything works fine, however when using mocha --watch only the first run will succeed, after then the parser will always return [].

Is this a known Issue? I can try to build a minimal example if it will help.

[bd82]

Interesting, I just reproduced this using one of the samples grammars.

[bd82]

Try adding:

require("chevrotain").clearCache();

before your tests, for example at the start of your test module or
using a "beforeAll".

This resolved the problem in my small reproduction.
I am guessing that mocha caches the commonjs modules and this somehow conflicts with
Chevrotain's own cache management.

Not sure how to get a deeper understanding here as I am not sure how to actually
debug in watch mode.

[bd82]

Maybe I should add a section in the faq about this.

[luxzeitlos]

Not sure how to get a deeper understanding here as I am not sure how to actually
debug in watch mode.

You can call mocha --watch --inspect and then go to chrome://inspect.

[bd82]

You can call mocha --watch --inspect and then go to chrome://inspect.

Thanks. I will try to debug is farther, perhaps it would be possible to avoid this workaround.

[bd82]

I've finally gotten around to debugging this.
the --inspect works great! 👍

What happens is that mocha reloads the user files during watch, but not the libraries inside
node_modules. Chevrotain manages cache related to user created parsers internally
but this cache does not get reset every time the watcher restarts the tests.

Effectively this creates multiple Parsers with the same name (see #715)
And in future versions will fail fast with a better error message.

I don't see a way to tell mocha to also reload chevrotain each time the tests are re-run
So the only workaround I can think of is to invoke clearCache (once), before the tests run.

[luxzeitlos]

Nice. Maybe add this to the documentation? Sounds like this just needs to be added to beforeEach for a testsuite?

[bd82]

I think it needs to be added to the beforeAll of the entire test suite.

But it depends on the granularity of mocha --watch.
If mocha re-runs only specific tests due to code changes than maybe a beforeEach is needed
But if it just re-runs everything a beforeAll will suffice.

running clearCache too often will slow down the tests as it will force chevrotain to recompute
many things redundantly, so it may defeat the purpose of the --watch flag.

[bd82]

I will add a section in the FAQ

[bd82]

Website will be updated once I republish it again.
I would probably only do so when I release a new version as otherwise it will mess up the
changelog a bit.

[bd82]

I am reopening this because solving #754 should also fix this issue.

[bd82]

This should be resolved in 3.7.0 that will be released soon.
As the global Cache has been removed mocha reloading user's modules will create a new Parser instance with instance level cache so there would be no conflicts.

[luxzeitlos]

Awesome! 👍

[bd82]

😄