Must call compiler.close() to write webpack filesystem cache

[snickell]

Q	A
Bug fix?	yes
Breaking change?	no
New feature?	no
Deprecations?	no
Tests added?	no
Docs updated?	no
Fixed tickets	#246
License	MIT

As per #246, grunt-webpack has been observed to not work with webpack 5 filesystem caching. This is a result of not calling (and waiting for) compiler.close(callback). Webpack defaults to waiting until idle to flush its cache to the filesystem, so this does not impact dev/watch mode, but with compiler.run() you exit before hitting idle, and thus, never write the filesystem cache (bug!).

Details of PR

This PR modifies a few more lines than might be expected because we have to handles change in async flow control. I tried to keep the diff very short and easy to compare to before for easy verification of same-behavior.

This is a standard webpack 5 migration issue that grunt-webpack still needs

The need to invoke compiler.close() is specified in the webpack 5 migration guide:

Additionally you can find a resolved issue on the webpack issue tracker containing the same advice to remedy the same problem: webpack/webpack#12342 (comment)

Verification of PR

This has been tested and fixes outputting webpack cache data to the filesystem in code.org's fairly large webpack config.

Using this PR combined with a webpack.config.js with cache: { type: 'filesystem' } results in webpack cache being output to disk, whereas before this PR the build succeeds but no cache is written to disk.

Fixes: #246

[linux-foundation-easycla]

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• ✅ login: snickell / name: Seth Nickell (0b00669, 39e172b)

[danez]

Nice find, can you ensure the code is formatted with prettier. pnpm run lint --fix. An also fix the lint warnings.

[danez]

Thanks again, I wasn't able to push to your branch, so I created a new branch with your changes.