Relative imports ignored via “browser” field cause absolute paths in source maps

[eelco]

While looking into why our builds were not perfectly reproducing between different systems, I ended up finding that in some cases, absolute paths end up in the source map file, causing the hash to change.

Example (snippet from .map file):

"empty:/mnt/ramdisk/node_modules/paper/dist/node/self.js", "empty:/mnt/ramdisk/node_modules/paper/dist/node/extend.js", "../../../../node_modules/paper/dist/paper-core.js"

Turns out, this is caused by the browser field in the package.json of dependencies. In this case: https://github.com/paperjs/paper.js/blob/095671073b9cfed2221b48aea54776ca6f2aed70/package.json#L45-L52

  "browser": {
    "canvas": false,
    "jsdom": false,
    "jsdom/lib/jsdom/living/generated/utils": false,
    "source-map-support": false,
    "./dist/node/self.js": false,
    "./dist/node/extend.js": false
  },

I believe this is caused by the resolver, specifically:

esbuild/internal/resolver/resolver.go

Line 400 in 9580aa8

return &ResolveResult{PathPair: PathPair{Primary: logger.Path{Text: absPath, Namespace: BrowserFalseNamespace}}}

And

esbuild/internal/resolver/resolver.go

Lines 452 to 457 in 9580aa8

	if absolute, ok := r.loadNodeModules(importPath, kind, sourceDirInfo); ok {
	absolute.Primary = logger.Path{Text: absolute.Primary.Text, Namespace: BrowserFalseNamespace}
	if absolute.HasSecondary() {
	absolute.Secondary = logger.Path{Text: absolute.Secondary.Text, Namespace: BrowserFalseNamespace}
	}
	return &ResolveResult{PathPair: absolute}

I’m not sure why this has to be an absolute path, nor why these should be part of the sourcemap, and as this causes differences between builds on different systems, I’d love to get rid of it.

[evanw]

Thanks for the report. I intend to have output files be deterministic across different systems so this is a bug.

This happens because esbuild uses absolute paths internally to represent file identity, esbuild converts absolute file system paths to relative paths for output file determinism, and esbuild only interprets paths in the file namespace as file system paths. Paths in other namespaces could be in an arbitrary format that is up to whichever plugin made them, and esbuild deliberately doesn't change with those paths to avoid messing with the plugin.

The change to preserve non-file paths was made with the introduction of the plugin API. However, that broke this internal code which uses the empty namespace for these files. One way to fix this could be to use some other mechanism instead of the empty namespace to flag empty modules.

[eelco]

Thank you so much for the fast fix! 🙏