[mono][wasm] Handle delegates decorated with [UnmanagedFunctionPointe…

src/tasks/WasmAppBuilder/PInvokeTableGenerator.cs

@@ -79,6 +79,22 @@ private void CollectPInvokes(List<PInvoke> pinvokes, List<PInvokeCallback> callb
             }
         }
 
+        if (HasAttribute(type, new string[] {"System.Runtime.InteropServices.UnmanagedFunctionPointerAttribute"})) {
+            var method = type.GetMethod("Invoke");
+
+            if (method != null)
+            {
+                string? signature = SignatureMapper.MethodToSignature(method!);
+                if (signature == null)
+                    {
+                        throw new NotSupportedException($"Unsupported parameter type in method '{type.FullName}.{method.Name}'");
+                    }
+
+                Log.LogMessage(MessageImportance.Low, $"Adding pinvoke signature {signature} for method '{type.FullName}.{method.Name}'");
+                signatures.Add(signature);
+            }
+        }
+
         void CollectPInvokesForMethod(MethodInfo method)
         {
             if ((method.Attributes & MethodAttributes.PinvokeImpl) != 0)
@@ -151,6 +167,24 @@ static bool MethodHasCallbackAttributes(MethodInfo method)
         }
     }
 
+    private static bool HasAttribute(Type type, string[] attributeNames)
+    {
+        foreach (CustomAttributeData cattr in CustomAttributeData.GetCustomAttributes(type))
+        {
+            try
+            {
+                for (int i = 0; i < attributeNames.Length; ++i)
+                    if (cattr.AttributeType.FullName == attributeNames [i])
+                        return true;
+            }
+            catch
+            {
+                // Assembly not found, ignore
+            }
+        }
+        return false;
+    }
+
     private void EmitPInvokeTable(StreamWriter w, Dictionary<string, string> modules, List<PInvoke> pinvokes)
     {
         w.WriteLine("// GENERATED FILE, DO NOT MODIFY");