--enable-source-maps ignores index source maps (maps with sections) #66657

Version

v24.15.0 (also reproduced on v18.15.0, v20.17.0, v22.17.0, v23.11.0)

Platform

Darwin 27.0.0 arm64

Subsystem

source maps

What steps will reproduce the bug?

index.js:

function fail() {
  throw new Error('boom');
}
fail();
//# sourceMappingURL=index.js.map

index.js.map, an index map with a single section:

{
  "version": 3,
  "sections": [
    {
      "offset": {"line": 0, "column": 0},
      "map": {
        "version": 3,
        "sources": ["original.js"],
        "names": [],
        "mappings": "AAAA;AACA;AACA;AACA"
      }
    }
  ]
}

$ node --enable-source-maps index.js

How often does it reproduce? Is there a required condition?

Always, for any index map. The same mappings as a regular map (flat.js.map below) work.

What is the expected behavior? Why is that the expected behavior?

The stack trace is source mapped, as it is when the same mappings are in a regular map:

$ cat flat.js.map
{"version":3,"sources":["original.js"],"names":[],"mappings":"AAAA;AACA;AACA;AACA"}
$ node --enable-source-maps flat.js
...
    at fail (/tmp/node-index-map-repro/original.js:2:1)

Index maps are part of the source map spec, and module.SourceMap already supports them: new SourceMap(payload).findEntry(1, 2) on the map above returns originalSource: 'original.js'.

What do you see instead?

The stack trace is not mapped:

$ node --enable-source-maps index.js
...
    at fail (/tmp/node-index-map-repro/index.js:2:9)

module.findSourceMap() also returns undefined for index.js. With NODE_DEBUG=source_map, the map fails to load:

$ NODE_DEBUG=source_map node --enable-source-maps index.js
SOURCE_MAP 12345: TypeError: Cannot read properties of undefined (reading 'map')

Additional information

The error comes from sourcesToAbsolute() in lib/internal/source_map/source_map_cache.js, which runs data.sources.map(...) on the top-level map. Index maps have no top-level sources, so it throws, and the map isn't cached.

A possible fix is to resolve the sources of each section's map (with that map's sourceRoot) when the map has sections. Index maps appear to be tested only by constructing SourceMap directly (test/parallel/test-source-map-api.js with fixtures/source-map/disk-index.map), not through --enable-source-maps.

As a workaround, adding "sources": [] to the top level of the index map makes Node apply the sections. However, the sections' sources are then not resolved to absolute URLs, and sourceRoot is ignored, so stack traces show them verbatim (e.g. original.js:2:1 rather than an absolute path).

I ran into this in Parcel, which writes bundle source maps as index maps.