[IILink] UnsafeAccessor work for CorLib

[pavelsavara]

Description

It seems that ILLink doesn't process UnsafeAccessorAttribute when used inside another method.

     internal static Task RegisterWasiPollable(IPoll.Pollable pollable) 
     { 
         return CallRegisterWasiPollable((Thread)null!, 123); 
  
         [UnsafeAccessor(UnsafeAccessorKind.StaticMethod, Name = "RegisterWasiPollable")] 
         static extern Task CallRegisterWasiPollable(Thread t, int handle); 
     } 

Reproduction Steps

See

runtime/src/libraries/System.Net.Http/src/System/Net/Http/WasiHttpHandler/WasiHttpHandler.cs

Lines 464 to 475 in d606c60

	private static class WasiEventLoop
	{
	internal static Task RegisterWasiPollable(IPoll.Pollable pollable)
	{
	var handle = pollable.Handle;
	pollable.Handle = 0;
	return CallRegisterWasiPollable((Thread)null!, handle);
	[UnsafeAccessor(UnsafeAccessorKind.StaticMethod, Name = "RegisterWasiPollable")]
	static extern Task CallRegisterWasiPollable(Thread t, int handle);
	}
	}

runtime/src/libraries/System.Private.CoreLib/src/System/Threading/Thread.Unix.cs

Lines 15 to 23 in d606c60

	internal static System.Threading.Tasks.Task RegisterWasiPollable(int handle)
	{
	return WasiEventLoop.RegisterWasiPollable(handle);
	}
	internal static void DispatchWasiEventLoop()
	{
	WasiEventLoop.DispatchWasiEventLoop();
	}

Expected behavior

[UnsafeAccessor] on the nested external method will protect Thread.RegisterWasiPollable from trimming

Actual behavior

System.AggregateException: One or more errors occurred. (Could not find RegisterWasiPollableHandle)
   ---> System.MissingMethodException: Could not find RegisterWasiPollable
     at System.Net.Http.WasiHttpHandler.WasiEventLoop.RegisterWasiPollable(Pollable )
     at System.Net.Http.WasiHttpHandler.SendRequestAsync(OutgoingRequest )
     at System.Net.Http.WasiHttpHandler.SendAsync(HttpRequestMessage , CancellationToken )
     at System.Net.Http.HttpClient.GetStringAsyncCore(HttpRequestMessage , CancellationToken )
     at Test.Work()

Regression?

No response

Known Workarounds

use ILLink.Descriptors.WASI.xml

Configuration

Net9 Prev 7

Other information

Discovered here

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Tagging subscribers to this area: @agocke, @sbomer, @vitek-karas
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[vitek-karas]

Hmm - I tried a simple repro in ILLink tests and there it works just fine. This has to be something a bit subtle. @pavelsavara how would one go about reproducing this (as in running the linker)?

Or alternatively - please use this https://github.com/vitek-karas/illinkrepro to capture a illink repro and share it (probably internally over email to me and Sven).

Also, did you check the target method is missing from the IL after trimming? It could be a runtime problem where the runtime doesn't find the method even if it's there...

[pavelsavara]

For a while I thought this was caused by UnsafeAccessor being in inside of another method, but it happens also when it's top level in class.

It reproduces for me when I build current main for WASI on my machine.
The methods are really gone, I looked at the DLLs.

And I confirmed that with CI test in my PR - Log

And so I'm preventing that on the PR by adding src/mono/System.Private.CoreLib/src/ILLink/ILLink.Descriptors.WASI.xml into src/mono/System.Private.CoreLib/System.Private.CoreLib.csproj

[vitek-karas]

Discussed offline with Pavel.

Trimmer behaves correctly, this is an artifact of library trimming in the runtime repo (Library Build). We run the trimmer separately on each assembly - so when it gets executed on CoreLib it doesn't get System.Net.Http as input and so it doesn't see the UnsafeAccessor and thus removes the target methods.

The correct "fix" for this is to root the methods in the ILLink.Descriptor.LibraryBuild.xml files as we already do for several other methods.

/cc @sbomer - closing as resolved, but please double check my reasoning here :-)