[Fusion] Added pre-merge validation rule "RootMutationUsedRule"

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

@@ -7,4 +7,5 @@ public static class LogEntryCodes
     public const string ExternalMissingOnBase = "EXTERNAL_MISSING_ON_BASE";
     public const string ExternalUnused = "EXTERNAL_UNUSED";
     public const string OutputFieldTypesNotMergeable = "OUTPUT_FIELD_TYPES_NOT_MERGEABLE";
+    public const string RootMutationUsed = "ROOT_MUTATION_USED";
 }

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

@@ -164,4 +164,14 @@ public static LogEntry OutputFieldTypesNotMergeable(
             field,
             schemaA);
     }
+
+    public static LogEntry RootMutationUsed(SchemaDefinition schema)
+    {
+        return new LogEntry(
+            string.Format(LogEntryHelper_RootMutationUsed, schema.Name),
+            LogEntryCodes.RootMutationUsed,
+            severity: LogSeverity.Error,
+            member: schema,
+            schema: schema);
+    }
 }

src/HotChocolate/Fusion-vnext/src/Fusion.Composition/PreMergeValidation/Events.cs

@@ -35,6 +35,8 @@ internal record OutputFieldGroupEvent(
     ImmutableArray<OutputFieldInfo> FieldGroup,
     string TypeName) : IEvent;
 
+internal record SchemaEvent(SchemaDefinition Schema) : IEvent;
+
 internal record TypeEvent(
     INamedTypeDefinition Type,
     SchemaDefinition Schema) : IEvent;

src/HotChocolate/Fusion-vnext/src/Fusion.Composition/PreMergeValidation/PreMergeValidator.cs

@@ -26,6 +26,8 @@ private void PublishEvents(CompositionContext context)
 
         foreach (var schema in context.SchemaDefinitions)
         {
+            PublishEvent(new SchemaEvent(schema), context);
+
             foreach (var type in schema.Types)
             {
                 PublishEvent(new TypeEvent(type, schema), context);

src/HotChocolate/Fusion-vnext/src/Fusion.Composition/PreMergeValidation/Rules/RootMutationUsedRule.cs

@@ -0,0 +1,31 @@
+using HotChocolate.Fusion.Events;
+using static HotChocolate.Fusion.Logging.LogEntryHelper;
+
+namespace HotChocolate.Fusion.PreMergeValidation.Rules;
+
+/// <summary>
+/// This rule enforces that, for any source schema, if a root mutation type is defined, it must be
+/// named <c>Mutation</c>. Defining a root mutation type with a name other than <c>Mutation</c> or
+/// using a differently named type alongside a type explicitly named <c>Mutation</c> creates
+/// inconsistencies in schema design and violates the composite schema specification.
+/// </summary>
+/// <seealso href="https://graphql.github.io/composite-schemas-spec/draft/#sec-Root-Mutation-Used">
+/// Specification
+/// </seealso>
+internal sealed class RootMutationUsedRule : IEventHandler<SchemaEvent>
+{
+    public void Handle(SchemaEvent @event, CompositionContext context)
+    {
+        var schema = @event.Schema;
+        var rootMutation = schema.MutationType;
+
+        if (rootMutation is not null && rootMutation.Name != WellKnownTypeNames.Mutation)
+        {
+            context.Log.Write(RootMutationUsed(schema));
+        }
+
+        // An object type named 'Mutation' will be set as the root mutation type if it has not yet
+        // been defined, so it's not necessary to check for this type in the absence of a root
+        // mutation type.
+    }
+}

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

@@ -48,4 +48,7 @@
   <data name="LogEntryHelper_OutputFieldTypesNotMergeable" xml:space="preserve">
     <value>Field '{0}' has a different type shape in schema '{1}' than it does in schema '{2}'.</value>
   </data>
+  <data name="LogEntryHelper_RootMutationUsed" xml:space="preserve">
+    <value>The root mutation type in schema '{0}' must be named 'Mutation'.</value>
+  </data>
 </root>

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

@@ -50,6 +50,7 @@ private CompositionResult<SchemaDefinition> MergeSchemaDefinitions(CompositionCo
         new ExternalArgumentDefaultMismatchRule(),
         new ExternalMissingOnBaseRule(),
         new ExternalUnusedRule(),
-        new OutputFieldTypesMergeableRule()
+        new OutputFieldTypesMergeableRule(),
+        new RootMutationUsedRule()
     ];
 }

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

@@ -0,0 +1,6 @@
+namespace HotChocolate.Fusion;
+
+internal static class WellKnownTypeNames
+{
+    public const string Mutation = "Mutation";
+}

src/HotChocolate/Fusion-vnext/test/Fusion.Composition.Tests/PreMergeValidation/Rules/RootMutationUsedRuleTests.cs

@@ -0,0 +1,97 @@
+using HotChocolate.Fusion.Logging;
+using HotChocolate.Fusion.PreMergeValidation;
+using HotChocolate.Fusion.PreMergeValidation.Rules;
+
+namespace HotChocolate.Composition.PreMergeValidation.Rules;
+
+public sealed class RootMutationUsedRuleTests : CompositionTestBase
+{
+    private readonly PreMergeValidator _preMergeValidator = new([new RootMutationUsedRule()]);
+
+    [Theory]
+    [MemberData(nameof(ValidExamplesData))]
+    public void Examples_Valid(string[] sdl)
+    {
+        // arrange
+        var context = CreateCompositionContext(sdl);
+
+        // act
+        var result = _preMergeValidator.Validate(context);
+
+        // assert
+        Assert.True(result.IsSuccess);
+        Assert.True(context.Log.IsEmpty);
+    }
+
+    [Theory]
+    [MemberData(nameof(InvalidExamplesData))]
+    public void Examples_Invalid(string[] sdl, string[] errorMessages)
+    {
+        // arrange
+        var context = CreateCompositionContext(sdl);
+
+        // act
+        var result = _preMergeValidator.Validate(context);
+
+        // assert
+        Assert.True(result.IsFailure);
+        Assert.Equal(errorMessages, context.Log.Select(e => e.Message).ToArray());
+        Assert.True(context.Log.All(e => e.Code == "ROOT_MUTATION_USED"));
+        Assert.True(context.Log.All(e => e.Severity == LogSeverity.Error));
+    }
+
+    public static TheoryData<string[]> ValidExamplesData()
+    {
+        return new TheoryData<string[]>
+        {
+            // Valid example.
+            {
+                [
+                    """
+                    schema {
+                        mutation: Mutation
+                    }
+
+                    type Mutation {
+                        createProduct(name: String): Product
+                    }
+
+                    type Product {
+                        id: ID!
+                        name: String
+                    }
+                    """
+                ]
+            }
+        };
+    }
+
+    public static TheoryData<string[], string[]> InvalidExamplesData()
+    {
+        return new TheoryData<string[], string[]>
+        {
+            // The following example violates the rule because `RootMutation` is used as the root
+            // mutation type, but a type named `Mutation` is also defined.
+            {
+                [
+                    """
+                    schema {
+                        mutation: RootMutation
+                    }
+
+                    type RootMutation {
+                        createProduct(name: String): Product
+                    }
+
+                    type Mutation {
+                        deprecatedField: String
+                    }
+                    """
+                ],
+                [
+                    "The root mutation type in schema 'A' must be named 'Mutation'."
+                ]
+            }
+        };
+    }
+}