aspnetcore: Regression - PWA manifest version no longer varies

A while ago, we fixed https://github.com/dotnet/aspnetcore/issues/19882 by adding a hash to service-worker.js, computed by combining the hashes of all the files listed in the assets manifest, during publish.

Unfortunately it looks like our logic for this has regressed. During one of the refactorings related to precompression just before we released, we changed the metadata name from %(FileHash) to %(Integrity), but this line was not updated:

https://github.com/dotnet/aspnetcore/blob/9b98da360a3051a4d75a716048960840fb1b9b6d/src/Components/WebAssembly/Build/src/targets/ServiceWorkerAssetsManifest.targets#L166

The result is that, instead of combining all the hashes, it now combines a set of empty strings, which results in another empty string, and then we hash that to produce a fixed value (47DEQpj8). Since this value never changes, the browser doesn’t try to fetch updates.

It’s troubling that this was not caught by tests. We do have build/publish tests that check that some hash was produced, but we can’t really check it’s the right hash without actually replicating all the build logic. What we should probably do is extend the build/publish test to publish once, then modify a source file, publish again, and verify that the hash actually changed. We do have some publish tests that do this kind of thing.