Avoid unnecessary project enumeration in SolutionState.Branch

[mavasani]

Fixes #53769

[mavasani]

@vatsalyaagrawal @jinujoseph This is for 16.11

[CyrusNajmabadi]

seems sane. but i worry if it's possible to somehow have an Add+REmove processed at teh same time that woudl lead us to have the same count. @jasonmalinowski for validation that we do things in tiny steps, so this sort of optimization is ok.

[mavasani]

I audited the existing callsites to validate that they satisfy the first assertion. Definitely need @jasonmalinowski's confirmation on this one though.

[jasonmalinowski]

So for some reason we also have a ProjectIds collection we pass around, and that's immutable. I'm guessing you're safe to do the WithLanguages any time the ProjectId collection object reference changes. The check is won't have false negatives where we fail to rerun WithLanguages; it could theoretically have a false positive where might run it if we manipulate the list and end up with the same original semantic value but I don't think we do that anywhere. (Or if we do, it's fine enough to not care since we'll still get quite the win.)

[mavasani]

Ah, that seems to be a much better approach. Let me adjust.

[CyrusNajmabadi]

This LGTM as an optimization in theory. But i just want signoff from @jasonmalinowski that it's totally safe with how the WS forks.

[jasonmalinowski]

Can we put these invariants in the CheckInvariants method? That way it'll get ran through places other than Branch, but actually at the constructor level.

[mavasani]

Alright, we have lot of test failures with this change. It seems it is not that trivial to add the optimization to skip Options.WithLanguages call because of the way options can be set on a specific solution snapshot without actually adding a project of that language.

For example, if we start with a workspace with an empty solution, and then do workspace.CurrentSolution.Options.WithChangedOption(..., C#);, it gives a solution snapshot with a SerializableOptionSet whose _languages has count 1 with C# as the entry, but project IDs for the solution snapshot is empty. Now, if we invoke a Branch on this solution snapshot, the project IDs are same, but Options == Options.WithLanguages(GetRemoteSupportedProjectLanguages(idToProjectStateMap))) does not hold as it creates a new option set due to differing language count. Same is true even for my prior implementation.

Going to ponder a bit more on this, marking the PR as draft meanwhile.

[davkean]

@mavasani In #54162, I found half of GetRemoteSupportedProjectLanguages occurred in the "Add Project" case, from reading the code it doesn't appear to address this. Any algorithm that results in your walking every single project in the solution is going to result in badness opening huge solutions.

[sharwell]

Superseded by #54196