skip source maps for replaced scripts

[vzaidman]

Keep source mapping of replaced scripts when source maps load out of order

Summary

When a newer script replaces code from an earlier script (e.g. on hot reload), both scripts' source maps claim the same original file but disagree on its content. CompilerScriptMapping.sourceMapAttached resolves such conflicts in favor of whichever source map finishes loading last.

That works while DevTools is attached during the replacement, because the replacement script's source map attaches last. But when DevTools attaches (or reloads) after the replacement happened, all scripts are reported at once in parse order. The small source map of the replacement script can then finish loading before the large source map of the original bundle. The bundle's stale source map then takes over the original file, and the replacement script loses its mapping. Breakpoints and debugger; statements in the replaced code then pause in the generated script instead of the original source.

In React Native this reproduces reliably:

1. Add a debugger; statement and let Fast Refresh apply it.
2. Reload DevTools.
3. The pause shows the generated bundle instead of the source file.

This change makes conflict resolution depend on script parse order rather than source map load order. A source map no longer takes over a URL when that URL is already provided by an incompatible source map of a newer script. Compatible source maps (same content, e.g. code splitting) are still merged into one UISourceCode as before.

Test plan

Manually reloaded DevTools with a "debugger;" statement, after a hot reload on the same script.

• Before- landing on "debugger;" on the full bundle.
• After- landing on "debugger;" in the source mapped source code.

Added unit tests in CompilerScriptMapping.test.ts. Both delay the older script's source map so that it finishes loading last:

• supports script replacement when source maps finish loading out of order: the replacement script keeps mapping to app.js, the bundle's stale app.js mapping is not used, and the bundle still maps its other sources. It fails without this change.
• shares UISourceCodes with compatible source maps that finish loading out of order: compatible source maps loading out of order still share the UISourceCode.

Ran the models/bindings and models/breakpoints unit tests in headless Chrome: all CompilerScriptMapping tests pass, and the remaining results are identical with and without this change. ESLint and clang-format are clean.

• This change maintains backwards compatibility with previous Local Storage data (if modifying settings, experiments, or other persisted client state).

Upstreaming plan

• This commit should be sent as a patch to the upstream devtools-frontend repo following the contribution guide for Meta employees OR contribution guide.
• This commit is React Native-specific and cannot be upstreamed.

[motiz88]

In this particular case I would love to see the upstream fix (or at least a steer / ack from the Chrome team regarding the bug) before we patch core source map machinery. Also worth checking if this has perhaps already been fixed.

[motiz88]

The cause is that Metro's main bundle map omits sourcesContent, while hot-update maps embed it, so the two maps are always considered incompatible.

That seems independently worth looking into on the Metro side.

[vzaidman]

The cause is that Metro's main bundle map omits sourcesContent, while hot-update maps embed it, so the two maps are always considered incompatible.

That seems independently worth looking into on the Metro side.

This is on purpose to fix yet another issue: D68631591

[vzaidman]

The cause is that Metro's main bundle map omits sourcesContent, while hot-update maps embed it, so the two maps are always considered incompatible.

That seems independently worth looking into on the Metro side.

Actually, now I remembered that I already looked at it. Filed https://issues.chromium.org/issues/395661481 which resulted in tc39/ecma426#208.

[vzaidman]

Also- I removed it from the description:

The cause is that Metro's main bundle map omits sourcesContent, while hot-update maps embed it, so the two maps are always considered incompatible.

This is not the cause in anyway. The cause is simply the race condition between the big bundle map and the small hot reload map.