Fix for OData#592

Description of problem:
Suppose there is a function getCurrentCustomer() whose EntitySetPath is not defined, if the request URL is /getCurrentCustomer()/oneNavigationProperty, the ODataPathParser.Parse() will throw NullReferenceException as the current code requires EntitySetPath to be specified on Function that returns an enitty type.

Description of fix:
As the EdmNavigationSource is not required for NavigationPropertySegment, we do not need to require the TargetEdmNavigationSource.

src/Microsoft.OData.Core/UriParser/Parsers/ODataPathParser.cs

@@ -1162,6 +1162,7 @@ private bool TryCreateTypeNameSegment(ODataPathSegment previous, string identifi
         [SuppressMessage("Microsoft.Naming", "CA2204:Literals should be spelled correctly", MessageId = "IEdmModel", Justification = "The spelling is correct.")]
         private void CreatePropertySegment(ODataPathSegment previous, IEdmProperty property, string queryPortion)
         {
+            Debug.Assert(previous != null, "previous != null");
             if (property.Type.IsStream())
             {
                 // The server used to allow arbitrary key expressions after named streams because this check was missing.
@@ -1180,7 +1181,11 @@ private void CreatePropertySegment(ODataPathSegment previous, IEdmProperty prope
             if (property.PropertyKind == EdmPropertyKind.Navigation)
             {
                 var navigationProperty = (IEdmNavigationProperty)property;
-                IEdmNavigationSource navigationSource = previous.TargetEdmNavigationSource.FindNavigationTarget(navigationProperty);
+                IEdmNavigationSource navigationSource = null;
+                if (previous.TargetEdmNavigationSource != null)
+                {
+                    navigationSource = previous.TargetEdmNavigationSource.FindNavigationTarget(navigationProperty);
+                }
 
                 // Relationship between TargetMultiplicity and navigation property:
                 //  1) EdmMultiplicity.Many <=> collection navigation property

src/Microsoft.OData.Core/UriParser/SemanticAst/ODataPathSegment.cs

@@ -91,7 +91,8 @@ internal void AssertValid()
             Debug.Assert(Enum.IsDefined(typeof(RequestTargetKind), this.TargetKind), "enum value is not valid");
             Debug.Assert(
                 this.TargetKind != RequestTargetKind.Resource ||
-                this.TargetEdmNavigationSource != null || 
+                this.TargetEdmNavigationSource != null ||
+                this.TargetEdmType != null ||
                 this.TargetKind == RequestTargetKind.OpenProperty ||
                 this is OperationSegment ||
                 this is OperationImportSegment,

test/FunctionalTests/Microsoft.OData.Core.Tests/UriParser/ODataUriParserTests.cs

@@ -512,5 +512,56 @@ public void ParseCompositeKeyReference(TestUrlConvention testUrlConvention, stri
         }
 
         #endregion
+
+        #region Null EntitySetPath
+
+        [Fact]
+        public void ParsePathFunctionWithNullEntitySetPath()
+        {
+            var model = new EdmModel();
+
+            var customer = new EdmEntityType("Test", "Customer", null, false, false);
+            var customerId = customer.AddStructuralProperty("id", EdmPrimitiveTypeKind.String, false);
+            customer.AddKeys(customerId);
+            model.AddElement(customer);
+
+            var detail = new EdmEntityType("Test", "Detail", null, false, true);
+            detail.AddStructuralProperty("address", EdmPrimitiveTypeKind.String, true);
+            model.AddElement(detail);
+
+            var customerDetail = customer.AddUnidirectionalNavigation(
+                new EdmNavigationPropertyInfo()
+                {
+                    Name = "detail",
+                    Target = detail,
+                    TargetMultiplicity = EdmMultiplicity.One,
+                    ContainsTarget = true
+                });
+
+            // The test is to make sure the ODataUriParser works even though
+            // the entitySetPathExpression is null.
+            var getCurrentCustomer = new EdmFunction(
+                "Test",
+                "getCurrentCustomer",
+                new EdmEntityTypeReference(customer, false),
+                isBound: false,
+                entitySetPathExpression: null,
+                isComposable: true);
+            model.AddElement(getCurrentCustomer);
+
+            var container = new EdmEntityContainer("Test", "Container");
+            var getCurrentCustomerImport = container.AddFunctionImport(getCurrentCustomer);
+            model.AddElement(container);
+
+            var parser = new ODataUriParser(model, new Uri("http://host"), new Uri("http://host/getCurrentCustomer()/detail"));
+            var path = parser.ParsePath();
+            var pathSegmentList = path.ToList();
+            pathSegmentList.Count.Should().Be(2);
+            pathSegmentList[0].ShouldBeOperationImportSegment(getCurrentCustomerImport);
+            pathSegmentList[1].ShouldBeNavigationPropertySegment(customerDetail);
+        }
+
+        #endregion
+
     }
 }