"File URL path must be absolute" on Windows

• Operating System: Windows
• Node Version: 13.*
• NPM Version: yarn 1.21.1 (does not matter)
• webpack Version: 4.*
• source-map-loader Version: 1.0.0

Problem description (actual behavior)

On Windows, cryptic warnings are emitted, saying:

WARNING in d:/my/library/dist/index.js Module Warning (from …/node_modules/source-map-loader/dist/cjs.js): File URL path must be absolute @ ./myProgram.js 8:19-42

Expected Behavior

No warning. Does not happen on Mac.

How to reproduce?

No way to easily produce a minimal example of this problem, since the given warning is too cryptic.

First Analysis

Step 1

I found out that the error message indicates one of two possible problems:

1. Incorrect/unrecognized file/path for path/file conversion (as seen here)
2. An incorrect file URL on Windows, as explained in node documentation (here):

On Windows, file: URLs with a host name convert to UNC paths, while file: URLs with drive letters convert to local absolute paths. file: URLs without a host name nor a drive letter will result in a throw

Step 2: Debugging source-map-loader

I went into dist/index.js and supplemented the two calls to this.emitWarning(error) with console.error(error). After snooping around a bit more, I found that this makes the problem entirely visible:

I added console.error('fetchFromUrl failed at', >>${source}<<, ':\n', error); to the catch block around line 93 of the original source (NOTE: the source variable apparently is the url to the sourcemapped file); yielding error messages like this:

fetchFromUrl failed at >>file:///webpack/universalModuleDefinition<< : TypeError [ERR_INVALID_FILE_URL_PATH]: File URL path must be absolute at getPathFromURLWin32 (internal/url.js:1310:13) at Object.fileURLToPath (internal/url.js:1342:22) at fetchFromURL (d:\my\project\node_modules\source-map-loader\dist\utils.js:151:40) at d:\my\project\node_modules\source-map-loader\dist\index.js:89:41 at Array.map (<anonymous>) at Object.loader (d:\my\project\node_modules\source-map-loader\dist\index.js:77:57)

Analysis Results

It does not seem to handle all kinds of “special” webpack urls. More invalid url examples include:

file:///webpack/universalModuleDefinition file:///webpack/bootstrap file:///external “colors/safe” file:///external “core-js/modules/es.array.concat” file:///external “lodash/isObject” file:///external “core-js/modules/web.timers”

and many more…

Also, possibly related to the fact that the imported library is umd?

Solutions?

Not sure what to do here. But getting rid of those annoying warnings would be a great start (as I am generally not too concerned about source maps to these sorts of files to begin with).