Add GCHandle in native default ALC at creation

[CoffeeFlux]

This lets embedders have a handle to fetch and pass before the runtime is properly started up and the managed default ALC is not yet created. Once the managed counterpart is initialized, the handle's target is changed, but the handle stays the same.

Fixes #48610

cc: @grendello

[ghost]

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Issue Details

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This lets embedders have a handle to fetch and pass before the runtime is properly started up and the managed default ALC is not yet created. Once the managed counterpart is initialized, the handle's target is changed, but the handle stays the same.

cc: @grendello

Author:	CoffeeFlux
Assignees:	-
Labels:	area-AssemblyLoader-mono
Milestone:	-

[vargaz]

The failures are caused because of the code in native-library.c, which used to pass a 0 gchandle to the managed callbacks, and now it passes a non-0 one.

Adding something like:
if (mono_gchandle_get_target_internal (alc->gchandle) == NULL)
return NULL;

fixes it.

[lambdageek]

Failures look related.
I think these two checks need to also check that .Target is not null not just gchALC.

runtime/src/mono/System.Private.CoreLib/src/System/Runtime/Loader/AssemblyLoadContext.Mono.cs

Lines 121 to 147 in 897072a

	return Resolve(gchALC, new AssemblyName(assemblyName));
	}
	// Invoked by Mono to resolve using the Resolving event after
	// trying the Load override and default load context without
	// success.
	private static Assembly? MonoResolveUsingResolvingEvent(IntPtr gchALC, string assemblyName)
	{
	AssemblyLoadContext context;
	// This check exists because the function can be called early in startup, before the default ALC is initialized
	if (gchALC == IntPtr.Zero)
	context = Default;
	else
	context = (AssemblyLoadContext)(GCHandle.FromIntPtr(gchALC).Target)!;
	return context.ResolveUsingEvent(new AssemblyName(assemblyName));
	}
	// Invoked by Mono to resolve requests to load satellite assemblies.
	private static Assembly? MonoResolveUsingResolveSatelliteAssembly(IntPtr gchALC, string assemblyName)
	{
	AssemblyLoadContext context;
	// This check exists because the function can be called early in startup, before the default ALC is initialized
	if (gchALC == IntPtr.Zero)
	context = Default;
	else
	context = (AssemblyLoadContext)(GCHandle.FromIntPtr(gchALC).Target)!;
	return context.ResolveSatelliteAssembly(new AssemblyName(assemblyName));

And also the call to Resolve is passing the GC Handle blindly, but it's also not expecting a null target:

runtime/src/libraries/System.Private.CoreLib/src/System/Runtime/Loader/AssemblyLoadContext.cs

Line 607 in 897072a

AssemblyLoadContext context = (AssemblyLoadContext)(GCHandle.FromIntPtr(gchManagedAssemblyLoadContext).Target)!;

[CoffeeFlux]

Yeah, I need to go through more carefully and see where we're actually using the GCHandle. There aren't that many uses, so I can just check them all.