[Fusion] Added post-merge validation rule "EmptyMergedInterfaceTypeRule" (#7980)

Author: danielreynolds1

src/HotChocolate/Fusion-vnext/src/Fusion.Composition/Logging/LogEntryCodes.cs

@@ -4,6 +4,7 @@ public static class LogEntryCodes
 {
     public const string DisallowedInaccessible = "DISALLOWED_INACCESSIBLE";
     public const string EmptyMergedEnumType = "EMPTY_MERGED_ENUM_TYPE";
+    public const string EmptyMergedInterfaceType = "EMPTY_MERGED_INTERFACE_TYPE";
     public const string EmptyMergedObjectType = "EMPTY_MERGED_OBJECT_TYPE";
     public const string EmptyMergedUnionType = "EMPTY_MERGED_UNION_TYPE";
     public const string EnumValuesMismatch = "ENUM_VALUES_MISMATCH";

src/HotChocolate/Fusion-vnext/src/Fusion.Composition/Logging/LogEntryHelper.cs

@@ -113,6 +113,19 @@ public static LogEntry EmptyMergedEnumType(
             schema);
     }
 
+    public static LogEntry EmptyMergedInterfaceType(
+        InterfaceTypeDefinition interfaceType,
+        SchemaDefinition schema)
+    {
+        return new LogEntry(
+            string.Format(LogEntryHelper_EmptyMergedInterfaceType, interfaceType.Name),
+            LogEntryCodes.EmptyMergedInterfaceType,
+            LogSeverity.Error,
+            new SchemaCoordinate(interfaceType.Name),
+            interfaceType,
+            schema);
+    }
+
     public static LogEntry EmptyMergedObjectType(
         ObjectTypeDefinition objectType,
         SchemaDefinition schema)

src/HotChocolate/Fusion-vnext/src/Fusion.Composition/PostMergeValidationRules/EmptyMergedInterfaceTypeRule.cs

@@ -0,0 +1,32 @@
+using HotChocolate.Fusion.Events;
+using HotChocolate.Fusion.Events.Contracts;
+using HotChocolate.Fusion.Extensions;
+using static HotChocolate.Fusion.Logging.LogEntryHelper;
+
+namespace HotChocolate.Fusion.PostMergeValidationRules;
+
+/// <summary>
+/// TODO: Summary
+/// </summary>
+/// <seealso href="https://graphql.github.io/composite-schemas-spec/draft/#sec-Empty-Merged-Interface-Type">
+/// Specification
+/// </seealso>
+internal sealed class EmptyMergedInterfaceTypeRule : IEventHandler<InterfaceTypeEvent>
+{
+    public void Handle(InterfaceTypeEvent @event, CompositionContext context)
+    {
+        var (interfaceType, schema) = @event;
+
+        if (interfaceType.HasInaccessibleDirective())
+        {
+            return;
+        }
+
+        var accessibleFields = interfaceType.Fields.Where(f => !f.HasInaccessibleDirective());
+
+        if (!accessibleFields.Any())
+        {
+            context.Log.Write(EmptyMergedInterfaceType(interfaceType, schema));
+        }
+    }
+}

src/HotChocolate/Fusion-vnext/src/Fusion.Composition/Properties/CompositionResources.Designer.cs

@@ -48,6 +48,9 @@
   <data name="LogEntryHelper_EmptyMergedEnumType" xml:space="preserve">
     <value>The merged enum type '{0}' is empty.</value>
   </data>
+  <data name="LogEntryHelper_EmptyMergedInterfaceType" xml:space="preserve">
+    <value>The merged interface type '{0}' is empty.</value>
+  </data>
   <data name="LogEntryHelper_EmptyMergedObjectType" xml:space="preserve">
     <value>The merged object type '{0}' is empty.</value>
   </data>

src/HotChocolate/Fusion-vnext/src/Fusion.Composition/Properties/CompositionResources.resx

@@ -121,6 +121,7 @@ public CompositionResult<SchemaDefinition> Compose()
     private static readonly ImmutableArray<object> s_postMergeRules =
     [
         new EmptyMergedEnumTypeRule(),
+        new EmptyMergedInterfaceTypeRule(),
         new EmptyMergedObjectTypeRule(),
         new EmptyMergedUnionTypeRule()
     ];

src/HotChocolate/Fusion-vnext/src/Fusion.Composition/SchemaComposer.cs

@@ -0,0 +1,127 @@
+using System.Collections.Immutable;
+using HotChocolate.Fusion.Logging;
+
+namespace HotChocolate.Fusion.PostMergeValidationRules;
+
+public sealed class EmptyMergedInterfaceTypeRuleTests : CompositionTestBase
+{
+    private static readonly object s_rule = new EmptyMergedInterfaceTypeRule();
+    private static readonly ImmutableArray<object> s_rules = [s_rule];
+    private readonly CompositionLog _log = new();
+
+    [Theory]
+    [MemberData(nameof(ValidExamplesData))]
+    public void Examples_Valid(string[] sdl)
+    {
+        // arrange
+        var schemas = CreateSchemaDefinitions(sdl);
+        var merger = new SourceSchemaMerger(schemas);
+        var mergeResult = merger.Merge();
+        var validator = new PostMergeValidator(mergeResult.Value, s_rules, schemas, _log);
+
+        // act
+        var result = validator.Validate();
+
+        // assert
+        Assert.True(result.IsSuccess);
+        Assert.True(_log.IsEmpty);
+    }
+
+    [Theory]
+    [MemberData(nameof(InvalidExamplesData))]
+    public void Examples_Invalid(string[] sdl, string[] errorMessages)
+    {
+        // arrange
+        var schemas = CreateSchemaDefinitions(sdl);
+        var merger = new SourceSchemaMerger(schemas);
+        var mergeResult = merger.Merge();
+        var validator = new PostMergeValidator(mergeResult.Value, s_rules, schemas, _log);
+
+        // act
+        var result = validator.Validate();
+
+        // assert
+        Assert.True(result.IsFailure);
+        Assert.Equal(errorMessages, _log.Select(e => e.Message).ToArray());
+        Assert.True(_log.All(e => e.Code == "EMPTY_MERGED_INTERFACE_TYPE"));
+        Assert.True(_log.All(e => e.Severity == LogSeverity.Error));
+    }
+
+    public static TheoryData<string[]> ValidExamplesData()
+    {
+        return new TheoryData<string[]>
+        {
+            // TODO: Use examples from spec
+            {
+                [
+                    """
+                    # Schema A
+                    interface Author {
+                        name: String
+                        age: Int @inaccessible
+                    }
+                    """,
+                    """
+                    # Schema B
+                    interface Author {
+                        age: Int
+                        registered: Boolean
+                    }
+                    """
+                ]
+            },
+            // TODO: Check spec text
+            // If the @inaccessible directive is applied to an interface type itself, the entire
+            // merged interface type is excluded from the composite schema, and it is not required
+            // to contain any fields.
+            {
+                [
+                    """
+                    # Schema A
+                    interface Author @inaccessible {
+                        name: String
+                        age: Int
+                    }
+                    """,
+                    """
+                    # Schema B
+                    interface Author {
+                        registered: Boolean
+                    }
+                    """
+                ]
+            }
+        };
+    }
+
+    public static TheoryData<string[], string[]> InvalidExamplesData()
+    {
+        return new TheoryData<string[], string[]>
+        {
+            // This example demonstrates an invalid merged interface type. In this case, "Author" is
+            // defined in two source schemas, but all fields are marked as @inaccessible in at least
+            // one of the source schemas, resulting in an empty merged interface type.
+            {
+                [
+                    """
+                    # Schema A
+                    interface Author {
+                        name: String @inaccessible
+                        registered: Boolean
+                    }
+                    """,
+                    """
+                    # Schema B
+                    interface Author {
+                        name: String
+                        registered: Boolean @inaccessible
+                    }
+                    """
+                ],
+                [
+                    "The merged interface type 'Author' is empty."
+                ]
+            }
+        };
+    }
+}