Synchronize razor compiler assembly loading

[chsienki]

This PR ensures that the razor compiler is only loaded a single time between Roslyn and Razor. Whichever succeeds in first loading the assembly will provide their copy to the other.

There is a doc in the first commit that explains the strategy. You should read this before reviewing the rest of the PR.

Note this is blocked on dotnet/roslyn#76752 but it wont require code changes other than updating the roslyn version.

[chsienki]

@dotnet/razor-compiler and @dotnet/razor-tooling for reviews please :)

[333fred]

You mentioned in the doc that we'd need to error eventually in case Roslyn errored. When does that happen?

[chsienki]

Thats a good point. When writing the doc, I figured we'd need to do that, but at implementation I realized it's impossible to do so deterministically as-is, as its essentially a halting problem: we don't know if its not-loaded or failed to load and so another Yield might actually succeed. We could stick some arbitrary amount of retries in, but while unlikely, we could still prematurely assume failure if we hit the retry limit because the other thread just didn't get scheduled. We would need to add some other synchronization primitive to roslyn that allows you to query the status of a given assembly load.

The only way we could get into this state where it really has failed to load is if the assembly is missing or corrupted. Roslyn and razor both load the same assembly from disk, so even if we could detect that the load failed in Roslyn, it's just going to fail again in Razor anyway.

Given that its an error case either way, I'm inclined to just update the doc to note this and leave it as-is. If we think that's not acceptable let's file a bug and not block this work on it as its not trivial to do.

[davidwengier]

I don't think we need Remote.Razor to target netstandard any more, except possibly for tests. But also don't think its anything for you to worry about in this PR :)

[chsienki]

Moving this to draft while we try a different approach.

[chsienki]

@dotnet/razor-compiler this is ready for a second round of reviews. thanks!