Avoid a second graph build when getting assets

[tido64]

Summary

@dannyvv recently discovered that Metro calls buildSubgraph() twice during bundling. Once during build(): https://github.com/facebook/metro/blob/15fef8ebcf5ae0a13e7f0925a22d4211dde95e02/packages/metro/src/index.flow.js#L431

And again, shortly after, in getAssets(): https://github.com/facebook/metro/blob/15fef8ebcf5ae0a13e7f0925a22d4211dde95e02/packages/metro/src/index.flow.js#L435

I think the second build should be mostly cached, but it would be better if we can avoid it altogether if possible.

Digging deeper down, it looks like we have the full graph inside build(), but throw away the work when returning. I've changed things so that it also returns the graph and made it possible to pass it to getAssets() to avoid the second graph build. I don't know if this is the best way to solve this, but it seems to work on my machine™️.

Changelog: [Performance] Avoid a second graph build when getting assets

Test plan

I've added console.log inside buildSubgraph to make sure it only gets called once per unique input.

[robhogan]

Hi @tido64 - thanks for spotting this. Indeed the transform results would normally be cached (where a cache is configured), but resolutions wouldn't, so this is a useful optimisation.

Is it particularly the use of the runBuild API that you're looking at? The reason I ask - I'd prefer to avoid leaking the whole Graph from the shared/output/bundle, but I think we can tweak things so that it can return assets directly (and also benefit from parallelising serialisation and asset enumeration). I'd add a withAssets: boolean to RequestOptions and encapsulate building assets within Server.build.

Does it still work for you if I pull this in and make those changes?

[tido64]

Does it still work for you if I pull this in and make those changes?

Yes, please! As long as we implement the optimization, please feel free to make any changes.

[facebook-github-bot]

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

[facebook-github-bot]

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

[facebook-github-bot]

@robhogan merged this pull request in 261b8f3.