fix: apply custom serializer to source map endpoint in dev server

[EvanBacon]

Summary

• The source maps for Expo CLI are off by 1, but only in development. The cause is that source maps are generated differently between the server endpoint and the build function. In the Expo metro-config, we use the serializer to inject custom pre-modules for things like SSR and client-side environment variables. These changes aren't reflected in the source map endpoint.
• This PR addresses the differences by applying a custom serializer to the graph if one exists, and returning the map from the serializer if one is returned.
• The Expo CLI-side of this PR can be found here fix(cli, metro-config): Fix source map generation in development. expo/expo#29463
• This doesn't address the same issue occurring in error symbolication https://github.com/EvanBacon/metro/blob/f73d9f139343bdc67e03c78222ace3ec8f980304/packages/metro/src/Server.js#L1446

Changelog: [Fix] Apply custom serializer to source map graph in .map endpoint.

[motiz88]

This doesn't address the same issue occurring in error symbolication

Yeah we should definitely make sure those are all consistent.

[motiz88]

It makes sense to me to run the custom serializer consistently (and I vaguely remember seeing cases where this was broken), but what do you mean by "mutations to the graph"? When during bundling do those occur?

[EvanBacon]

If someone were to preModules.splice(0,0, { ... }) to add a new module inside of a custom serializer, then it would technically mutate the arguments used to generate the bundle. I can think of two cases like this:

• sentry (not exactly, but close)
• One I just wrote for Expo CLI as a fallback to returning the map: here

[motiz88]

Hmm. The serializer API wasn't designed with side-effectful serializers in mind, and the preModules parameter specifically is typed as being read-only. Is there a way to achieve what you're trying to do without mutating read-only arguments?

[EvanBacon]

Yeah but it's a little unstable. I'm sniffing if the sourceUrl has a .map pathname to determine if the serializer should return source maps. This works for the .map endpoint, but it doesn't account for /symbolicate where the result is getExplodedSourceMap and not a standard source map string.