output.futureEmitAssets option breaks compatibility with source map upload plugins

[bengourley]

Bug report

The output.futureEmitAssets option added in v4.29 breaks BugsnagSourceMapUploaderPlugin (specific Webpack version tested: 4.29.6).

Full disclosure, I am the maintainer of webpack-bugsnag-plugins.

Related issues

bugsnag/webpack-bugsnag-plugins#28
honeybadger-io/honeybadger-webpack#85
thredup/rollbar-sourcemap-webpack-plugin#61

Current behaviour

The source map uploader implements an afterEmit plugin which makes use of source.existsAt to find the location of the output files and their accompanying source maps. When the new option output.futureEmitAssets is switched on (which I understand is to be the new default), the sources in the afterEmit hook no longer have the existsAt property. This property does not exist on the sources in the emit stage either, presumably because the files are not yet written to disk, but I assume the compiler must know where the files will end up being written?

Steps to reproduce

Add BugsnagSourceMapUploader to any Webpack project using >=4.29 and enable output.futureEmitAssets. Or check out this repro.

Expected behaviour

At some stage around emit/afterEmit I would expect there to be information on where a file exists on disk, so that the BugsnagSourceMapUploaderPlugin can retrieve it. Prior to v4.29 this was the case, but subsequently it does not.

Other relevant information:
webpack version: 4.29.x
Node.js version: any compatible
Operating System: any
Additional tools: webpack-bugsnag-plugins

[sokra]

You don't need existsAt. Join filename and output path to get the full path.

webpack/lib/Compiler.js

Lines 333 to 336 in 685a062

	const targetPath = this.outputFileSystem.join(
	outputPath,
	targetFile
	);

webpack/lib/Compiler.js

Line 443 in 685a062

outputPath = compilation.getPath(this.outputPath);

[bengourley]

Thanks! Switching to that kind of implementation worked for us. For anybody else interested here's the change we made: bugsnag/webpack-bugsnag-plugins#29.