Summary:
This diff cleans up some cruft and adds some features:
* It removes the usage of an env variable to control workers.
* It removes the lazy and handwavy calculation on how many workers to use for jest-haste-map. Jest itself uses the maximum amount of workers available and it has never been reported as an issue – especially since it is a one-time startup cost of about 3 seconds on a cold cache only.
* It adds a `--max-workers` flag to replace the env variable. This one is able to control both the number of workers for `jest-haste-map` as well as the transformers.
* It makes the transformers run in the parent process if 1 or fewer workers are are specified. This should help with debugging.
Once you approve this diff, I will publish a new version of metro to npm and update the version used in RN and remove the use of the env variable altogether: https://our.intern.facebook.com/intern/biggrep/?corpus=xplat&filename=&case=false&view=default&extre=&s=REACT_NATIVE_MAX_WORKERS&engine=apr_strmatch&context=false&filter[uninteresting]=false&filter[intern]=false&filter[test]=false&grep_regex=
Note: the process of adding a CLI option is really broken. Commander also has a weird API. We should consider building a better public API for Metro and then consider how to build a new CLI on top of it and simplify our internal integration. I really don't like how Metro is integrated across pieces of the RN cli in ways that is hard to manage. But that is a larger task for another time :)
Reviewed By: jeanlauliac
Differential Revision: D5217726
fbshipit-source-id: 74efddbb87755a9e744c816fbc62efa21f6a79bf
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