Add IsDelegatable and IsPageable flags to UDF class (#2459)

These flags will allow the binder to mark a UDF as delegatable/pageable
correctly based on the rule bound to it.

src/libraries/Microsoft.PowerFx.Core/Binding/Binder.cs

@@ -654,7 +654,7 @@ namespace Microsoft.PowerFx.Core.Binding
                 return false;
             }
 
-            var isServerDelegatable = function.IsServerDelegatable(node, this);
+            var isServerDelegatable = (function is not UserDefinedFunction udf || udf.Binding != null) && function.IsServerDelegatable(node, this);
             LogTelemetryForFunction(function, node, this, isServerDelegatable);
             return isServerDelegatable;
         }
@@ -1194,7 +1194,7 @@ namespace Microsoft.PowerFx.Core.Binding
             Contracts.AssertValue(func);
 
             // Server delegatable call always returns a pageable object.
-            if (func.SupportsPaging(node, this))
+            if ((func is not UserDefinedFunction udf || udf.Binding != null) && func.SupportsPaging(node, this))
             {
                 _isPageable.Set(node.Id, true);
             }
@@ -2261,7 +2261,7 @@ namespace Microsoft.PowerFx.Core.Binding
             if (function != null)
             {
                 // If the invocation is async then the whole call path is async.
-                if (markIfAsync && function.IsAsyncInvocation(node, this))
+                if (markIfAsync && (function is UserDefinedFunction udf && udf.Binding != null) && function.IsAsyncInvocation(node, this))
                 {
                     FlagPathAsAsync(node);
                 }

src/libraries/Microsoft.PowerFx.Core/Functions/UserDefinedFunction.cs

@@ -3,6 +3,7 @@
 
 using System;
 using System.Collections.Generic;
+using System.Data;
 using System.Linq;
 using Microsoft.CodeAnalysis;
 using Microsoft.PowerFx.Core.App;
@@ -11,6 +12,7 @@ using Microsoft.PowerFx.Core.Binding;
 using Microsoft.PowerFx.Core.Binding.BindInfo;
 using Microsoft.PowerFx.Core.Entities;
 using Microsoft.PowerFx.Core.Errors;
+using Microsoft.PowerFx.Core.Functions.FunctionArgValidators;
 using Microsoft.PowerFx.Core.Glue;
 using Microsoft.PowerFx.Core.IR;
 using Microsoft.PowerFx.Core.IR.Nodes;
@@ -23,6 +25,7 @@ using Microsoft.PowerFx.Core.Utils;
 using Microsoft.PowerFx.Syntax;
 using Microsoft.PowerFx.Types;
 using static Microsoft.PowerFx.Core.Localization.TexlStrings;
+using CallNode = Microsoft.PowerFx.Syntax.CallNode;
 
 namespace Microsoft.PowerFx.Core.Functions
 {
@@ -31,13 +34,26 @@ namespace Microsoft.PowerFx.Core.Functions
     /// This includings the binding (and hence IR for evaluation) - 
     /// This is conceptually immutable after initialization - if the body or signature changes, you need to create a new instance.
     /// </summary>
-    internal class UserDefinedFunction : TexlFunction
+    internal class UserDefinedFunction : TexlFunction, IExternalPageableSymbol, IExternalDelegatableSymbol
     {
         private readonly bool _isImperative;
         private readonly IEnumerable<UDFArg> _args;
         private TexlBinding _binding;
 
-        public override bool IsAsync => _binding?.IsAsync(UdfBody) ?? false;
+        public override bool IsAsync => _binding.IsAsync(UdfBody);
+
+        public bool IsPageable => _binding.IsPageable(_binding.Top);
+
+        public bool IsDelegatable => _binding.IsDelegatable(_binding.Top);
+
+        public override bool IsServerDelegatable(CallNode callNode, TexlBinding binding)
+        {
+            Contracts.AssertValue(callNode);
+            Contracts.AssertValue(binding);
+            Contracts.Assert(binding.GetInfo(callNode).Function is UserDefinedFunction udf && udf.Binding != null);
+
+            return base.IsServerDelegatable(callNode, binding) || IsDelegatable;
+        }
 
         public override bool SupportsParamCoercion => true;
 
@@ -47,6 +63,13 @@ namespace Microsoft.PowerFx.Core.Functions
 
         public override bool IsSelfContained => !_isImperative;
 
+        public TexlBinding Binding => _binding;
+
+        public bool TryGetExternalDataSource(out IExternalDataSource dataSource)
+        {
+            return ArgValidators.DelegatableDataSourceInfoValidator.TryGetValidValue(_binding.Top, _binding, out dataSource);
+        }
+
         /// <summary>
         /// Initializes a new instance of the <see cref="UserDefinedFunction"/> class.
         /// </summary>

src/tests/Microsoft.PowerFx.Interpreter.Tests.Shared/RecalcEngineTests.cs

@@ -9,6 +9,7 @@ using System.Text;
 using System.Threading;
 using System.Threading.Tasks;
 using Microsoft.PowerFx.Core;
+using Microsoft.PowerFx.Core.Functions;
 using Microsoft.PowerFx.Core.IR;
 using Microsoft.PowerFx.Core.Localization;
 using Microsoft.PowerFx.Core.Tests;
@@ -675,6 +676,39 @@ namespace Microsoft.PowerFx.Tests
             }
         }
 
+        [Fact]
+
+        public void DelegatableUDFTest()
+        {
+            var config = new PowerFxConfig();
+            config.EnableSetFunction();
+
+            var schema = DType.CreateTable(
+                new TypedName(DType.Guid, new DName("ID")),
+                new TypedName(DType.Number, new DName("Value")));
+            config.SymbolTable.AddEntity(new TestDataSource("MyDataSource", schema));
+            config.SymbolTable.AddType(new DName("MyDataSourceTableType"), FormulaType.Build(schema));
+
+            var recalcEngine = new RecalcEngine(config);
+
+            recalcEngine.AddUserDefinedFunction("A():MyDataSourceTableType = Filter(Sort(MyDataSource,Value), Value > 10);C():MyDataSourceTableType = A();", CultureInfo.InvariantCulture, symbolTable: recalcEngine.EngineSymbols, allowSideEffects: true);
+            var func = recalcEngine.Functions.WithName("A");
+
+            if (func is UserDefinedFunction udf)
+            {
+                Assert.True(udf.IsAsync);
+                Assert.True(udf.IsDelegatable);
+            }
+
+            func = recalcEngine.Functions.WithName("C");
+
+            if (func is UserDefinedFunction udf2)
+            {
+                Assert.True(udf2.IsAsync);
+                Assert.True(udf2.IsDelegatable);
+            }
+        }
+
         // Binding to inner functions does not impact outer functions. 
         [Fact]
         public async Task FunctionInner()