VisualStudio (17.4. Preview 5) fails on language features for projects referencing Fsharp.Compiler.Services

[T-Gro]

When opening the VisualFsharp.sln, build works just fine.
Language services within the Fsharp.Compiler.Services work just fine as well.

However, when going into a project that directly or indirectly depends on Fsharp.Compiler.Service, language features stopped to work, demonstrated by a Object reference exception coming out of it.

When debugging the issue, the first failure happens on a pattern match of an Entity that that is unlinked (a pattern match, even though it has a | _ -> , tries to access .get_Tag and crashes on that).
Note that even when I tried to defensively cover the cases on Entity properties, the error just happens a little later or something else. The issue is more likely in the existance of such unlinked entity itself.

Test case:

• Open VisualFsharp.sln\
• Go to any test file in component tests project, e.g. "EmptyArray.fs"
• Any references to Test.Utilities are failing
• Within the Test.Utilities project, the first error (coloring stops) is at FileSystemShim usage - but that just happens to be the first usage of any type in Test.Utilities.

[vzarytovskii]

One thing to look at - #13671

Since it's related to pickling

[T-Gro]

This does not happen at classical build.
The code path being hit when loading a project references that results into thousands of Unlinked entities is via Incremental build:

type RawFSharpAssemblyDataBackedByLanguageService (tcConfig, tcGlobals, generatedCcu: CcuThunk, outfile, topAttrs, assemblyName, ilAssemRef) =

    let exportRemapping = MakeExportRemapping generatedCcu generatedCcu.Contents

    let sigData =
        let _sigDataAttributes, sigDataResources = EncodeSignatureData(tcConfig, tcGlobals, exportRemapping, generatedCcu, outfile, true)
        [ for r in sigDataResources  do
            GetResourceNameAndSignatureDataFunc r
        ]

[dsyme]

@T-Gro Since it might involve compressed data, could you list the exact VS versions involved here? Are you running RoslynDev prefix or VS preview etc.?

Also we'd better pin down whether metadata is actually being compressed in FSharp.Compiler.Service.dll

[dsyme]

If the error is indeed related to metadata compression, some thoughts are

• Try to remove metadata compression in-repo, so if it repros
• Remove it selectively for FCS, see if it repros.
• Work out if the VS it repros in supports compressed metadata at all. That is, .\build /deploy RoslynDev should, others will depend on release cycle

[T-Gro]

This fails for me on:
+17.4. Preview 5
+Latest dogfood VS
+as well as when launching VisualFsharpDebug on latest main, i.e. RoslynDev

I have tried removing compression everywhere, and it still reproduces => compression was a false suspect.

When inspecting the code path, metadata was not getting compressed.
The call to EncodeSignatureData really produced an uncompressed byte array (a big one), but for some reason GetResourceNameAndSignatureDataFunc cannot cope with it just a few milliseconds later.

[T-Gro]

And I still did not spot what is making the behaviour of RawFSharpAssemblyDataBackedByLanguageService so different from regular build (because regular build works just fine).

And, why it fails only for Fsharp.Compiler.Service when used as a reference, but not for other projects.
Size? Any capacity-limited buffers ? Overflow of some auto-increment byte/uint/.. perharps?

[T-Gro]

@dsyme :
Would you mind looking at this together, or suggesting other tips on how to spot issues around picking+unpickling?
Please let me know which way to connect is preferred by you.

[dsyme]

@T-Gro I have time now if you do, else put something on my calendar for tomorrow?