fix: support serverRoot with hmr serializer chunks

[EvanBacon]

Summary

We use unstable_serverRoot heavily in Expo to support monorepos and consistently have issues where saving a file will log an error in the console:

Error: Unable to resolve module ./app/index from /Users/evanbacon/Documents/GitHub/expo/.: 

None of these files exist:
  * ../../app/index(.web.ts|.ts|.web.tsx|.tsx|.web.mjs|.mjs|.web.js|.js|.web.jsx|.jsx|.web.json|.json|.web.cjs|.cjs|.web.scss|.scss|.web.sass|.sass|.web.css|.css|.web.cjs|.cjs)
  * ../../app/index/index(.web.ts|.ts|.web.tsx|.tsx|.web.mjs|.mjs|.web.js|.js|.web.jsx|.jsx|.web.json|.json|.web.cjs|.cjs|.web.scss|.scss|.web.sass|.sass|.web.css|.css|.web.cjs|.cjs)
    at ModuleResolver.resolveDependency (/Users/evanbacon/Documents/GitHub/expo/node_modules/metro/src/node-haste/DependencyGraph/ModuleResolution.js:114:15)
    at DependencyGraph.resolveDependency (/Users/evanbacon/Documents/GitHub/expo/node_modules/metro/src/node-haste/DependencyGraph.js:277:43)
    at /Users/evanbacon/Documents/GitHub/expo/node_modules/metro/src/lib/transformHelpers.js:169:21
    at Server._resolveRelativePath (/Users/evanbacon/Documents/GitHub/expo/node_modules/metro/src/Server.js:1045:12)
    at Server.requestProcessor [as _processSourceMapRequest] (/Users/evanbacon/Documents/GitHub/expo/node_modules/metro/src/Server.js:449:37)
    at Server._processRequest (/Users/evanbacon/Documents/GitHub/expo/node_modules/metro/src/Server.js:396:7)

This is because the HMR sourceMappingURL is being calculated relative to the projectRoot instead of the serverRoot, but all files are being resolved relative to the serverRoot.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (e2d6419) 83.09% compared to head (a7ae0d4) 83.09%.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1137   +/-   ##
=======================================
  Coverage   83.09%   83.09%           
=======================================
  Files         206      206           
  Lines       10549    10550    +1     
  Branches     2619     2620    +1     
=======================================
+ Hits         8766     8767    +1     
  Misses       1783     1783           

Files	Coverage Δ
.../metro/src/DeltaBundler/Serializers/hmrJSBundle.js	82.92% <100.00%> (+0.42%)	⬆️

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[facebook-github-bot]

@robhogan has imported this pull request. If you are a Meta employee, you can view this diff on Phabricator.

[robhogan]

Apologies for the delay with this, I wanted to try this to make sure I understood how it all fits together - can confirm the root cause is incorrectly generated source (map) URLs, and this LGTM.

Before

After

[robhogan]

We use unstable_serverRoot heavily in Expo to support monorepos

I'm keen to promote some of these unstable_ features to stable given how load-bearing Expo's use makes them, but tbh I'm not totally sure what the concrete use case is for serverRoot in new projects.

Why not set projectRoot to the workspace root instead?

[facebook-github-bot]

@robhogan merged this pull request in 402dc0b.