[browser] throw PNSE from GetFunctionPointerForDelegate

[pavelsavara]

• throw meaningful exception
• WBT unit test

Fixes #104391
Fixes #39187

[dotnet-policy-service]

Tagging subscribers to 'arch-wasm': @lewing
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[Copilot]

Pull Request Overview

Adds support for throwing a PlatformNotSupportedException when GetFunctionPointerForDelegate is called on methods without an [UnmanagedCallersOnly] attribute, and wires up end-to-end tests in the Wasm Basic Test App and build tests.

• Introduces a null‐wrapper check in interp_create_method_pointer to produce a PNSE with a descriptive message.
• Extends main.js and the WebAssembly test app to invoke the new scenario.
• Adds both an in-browser unit test (MiscTests.cs) and a build‐time Xunit test.

Reviewed Changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
src/mono/mono/mini/interp/interp.c	Checks for missing native‐to‐managed wrapper and throws PNSE
src/mono/browser/runtime/runtime.c	Adds an assertion to ensure method is not null
src/mono/wasm/testassets/WasmBasicTestApp/App/wwwroot/main.js	Adds “GetFunctionPointerForDelegate” case to the browser harness
src/mono/wasm/testassets/WasmBasicTestApp/App/MiscTests.cs	JSExport method to call GetFunctionPointerForDelegate
src/mono/wasm/Wasm.Build.Tests/MiscTests.cs	Xunit test to verify the PNSE and its message

Comments suppressed due to low confidence (2)

src/mono/wasm/testassets/WasmBasicTestApp/App/wwwroot/main.js:292

• [nitpick] The variable name retMics is ambiguous; consider renaming it to something more descriptive (e.g., result or returnValue).

const retMics = testGetFunctionPointerForDelegate();

src/mono/mono/mini/interp/interp.c:3475

• The variable error is not defined in this scope (the parameter is named e). This should call mono_error_set_platform_not_supported(e, msg).

mono_error_set_platform_not_supported (error, msg);

[jkotas]

throw PNSE from GetFunctionPointerForDelegate for methods without UnmanagedCallersOnly

Should GetFunctionPointerForDelegate throw PNSE unconditionally on browser?

GetFunctionPointerForDelegate does not work on methods with UnmanagedCallersOnly either (outside browser at least). For example, the following will fail to compile with error CS8902: 'MyMethod()' is attributed with 'UnmanagedCallersOnly' and cannot be converted to a delegate type.:

using System.Runtime.InteropServices;

Marshal.GetFunctionPointerForDelegate(new Action(MyMethod));

[UnmanagedCallersOnly]
static void MyMethod()
{
}

[pavelsavara]

Should GetFunctionPointerForDelegate throw PNSE unconditionally on browser?

Hmmm, good point.

I'm looking at mono/mono#20076 but I'm not any wiser.

@lewing do you know why we have

runtime/src/mono/mono/mini/interp/interp.c

Lines 3468 to 3499 in e5866fe

	#ifdef HOST_WASM
	if (method->wrapper_type == MONO_WRAPPER_NATIVE_TO_MANAGED) {
	WrapperInfo *info = mono_marshal_get_wrapper_info (method);
	MonoMethod *orig_method = info->d.native_to_managed.method;
	/*
	* These are called from native code. Ask the host app for a trampoline.
	*/
	MonoFtnDesc *ftndesc = g_new0 (MonoFtnDesc, 1);
	ftndesc->addr = entry_func;
	ftndesc->arg = imethod;
	addr = mono_wasm_get_native_to_interp_trampoline (orig_method, ftndesc);
	if (addr) {
	mono_memory_barrier ();
	imethod->jit_entry = addr;
	return addr;
	}
	/*
	* The runtime expects a function pointer unique to method and
	* the native caller expects a function pointer with the
	* right signature, so fail right away.
	*/
	char *s = mono_method_get_full_name (orig_method);
	char *msg = g_strdup_printf ("No native to managed transition for method '%s', missing [UnmanagedCallersOnly] attribute.", s);
	mono_error_set_platform_not_supported (error, msg);
	g_free (s);
	g_free (msg);
	return NULL;
	}
	#endif

Is this for interp->AOT transitions only ?

[pavelsavara]

#56883 introduced the message about UnmanagedCallersOnly

[lewing]

I'd need to a few minutes to investigate to be sure, but I suspect this just a case where we are ending up in interp_create_method_pointer from different paths. We do expect a native to interp trampoline to exist for UCO methods (if the native build is enabled), that does not mean we intended GetFunctionPointerForDelegate to succeed if the attribute exists.

[jkotas]

BTW: You can reuse message added in https://github.com/dotnet/runtime/pull/116010/files#diff-d9a9b46d55b2e1fddcd071cc75b0408f92f33befa5ea92d5b6aa0c67ad437071R4365 to make this API unsupported on wasm.

[jkotas]

dotnet/dotnet-api-docs#11401 is a docs updated related to this change:

[jkotas]

Should the Wasm compat check be disabled in .csproj files for libraries like Ldap instead?