Fix broken sourcemaps when using esbuild --public-path

[richardkmiller]

When the --public-path option was added to esbuild it broke Sprockets's ability to resolve sourcemaps, even though they're still present.

Esbuild's sourcemap output with --public-path=assets:
//# sourceMappingURL=assets/application.js.map

Esbuild's sourcemap output without --public-path:
//# sourceMappingURL=application.js.map

It's desirable to keep --public-path=assets for properly resolving static assets in JavaScript files, but the leading directory in sourceMappingURL is undesirable, or at least it does not work with Sprockets.

This PR allows sourceMappingURL= to contain a leading, optional directory which is ignored as the path is resolved.

This is an alternative to #501. (Sorry, I didn't see it until after I found this solution.)

[capripot]

Are we still planning on merging this? Should we fix errors from tests?

[guillaumebriday-pa]

@dhh could you take a look please?

Thanks

[dhh]

Thanks for looking into this. Needs testing to verify behavior.

[joevandyk]

I am also running into this as well with esbuild, I have to remove the --public-path option from esbuild to get sourcemaps to work, otherwise sprockets-rails seems to remove the sourcemap comment that esbuild creates.

[joevandyk]

would the propshaft fix at rails/propshaft#170 be applicable here?

[nicklozon]

@richardkmiller would you like me to write test cases and maybe we can get this merged?

[richardkmiller]

@nicklozon Yes, that'd be great. Thanks for asking!

[nicklozon]

@richardkmiller I wrote a test and opened a PR against your repo: richardkmiller#1

I think the change makes sense and works - sprockets-rails assumes the map file will exists in the same directory as the source file so it stands to reason that any prefixed path should be removed from the sourceMappingUrl.

[richardkmiller]

@nicklozon Thank you for these tests! I've merged them just now.

[jroes]

Would love to see this merged! Can confirm this is resolving some issues on my end.

[zacheryph]

Any ideas on when this would possibly get merged [and released] ?