[Fusion] Added pre-merge validation rule "RootQueryUsedRule" (#7866)

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

@@ -8,4 +8,5 @@ public static class LogEntryCodes
     public const string ExternalUnused = "EXTERNAL_UNUSED";
     public const string OutputFieldTypesNotMergeable = "OUTPUT_FIELD_TYPES_NOT_MERGEABLE";
     public const string RootMutationUsed = "ROOT_MUTATION_USED";
+    public const string RootQueryUsed = "ROOT_QUERY_USED";
 }

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

@@ -174,4 +174,14 @@ public static LogEntry RootMutationUsed(SchemaDefinition schema)
             member: schema,
             schema: schema);
     }
+
+    public static LogEntry RootQueryUsed(SchemaDefinition schema)
+    {
+        return new LogEntry(
+            string.Format(LogEntryHelper_RootQueryUsed, schema.Name),
+            LogEntryCodes.RootQueryUsed,
+            severity: LogSeverity.Error,
+            member: schema,
+            schema: schema);
+    }
 }

src/HotChocolate/Fusion-vnext/src/Fusion.Composition/PreMergeValidation/Rules/RootQueryUsedRule.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 the root query type in any source schema must be named <c>Query</c>.
+/// Defining a root query type with a name other than <c>Query</c> or using a differently named type
+/// alongside a type explicitly named <c>Query</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-Query-Used">
+/// Specification
+/// </seealso>
+internal sealed class RootQueryUsedRule : IEventHandler<SchemaEvent>
+{
+    public void Handle(SchemaEvent @event, CompositionContext context)
+    {
+        var schema = @event.Schema;
+        var rootQuery = schema.QueryType;
+
+        if (rootQuery is not null && rootQuery.Name != WellKnownTypeNames.Query)
+        {
+            context.Log.Write(RootQueryUsed(schema));
+        }
+
+        // An object type named 'Query' will be set as the root query type if it has not yet been
+        // defined, so it's not necessary to check for this type in the absence of a root query
+        // type.
+    }
+}

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

@@ -51,4 +51,7 @@
   <data name="LogEntryHelper_RootMutationUsed" xml:space="preserve">
     <value>The root mutation type in schema '{0}' must be named 'Mutation'.</value>
   </data>
+  <data name="LogEntryHelper_RootQueryUsed" xml:space="preserve">
+    <value>The root query type in schema '{0}' must be named 'Query'.</value>
+  </data>
 </root>

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

@@ -51,6 +51,7 @@ private CompositionResult<SchemaDefinition> MergeSchemaDefinitions(CompositionCo
         new ExternalMissingOnBaseRule(),
         new ExternalUnusedRule(),
         new OutputFieldTypesMergeableRule(),
-        new RootMutationUsedRule()
+        new RootMutationUsedRule(),
+        new RootQueryUsedRule()
     ];
 }

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

@@ -3,4 +3,5 @@ namespace HotChocolate.Fusion;
 internal static class WellKnownTypeNames
 {
     public const string Mutation = "Mutation";
+    public const string Query = "Query";
 }

src/HotChocolate/Fusion-vnext/test/Fusion.Composition.Tests/PreMergeValidation/Rules/RootQueryUsedRuleTests.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 RootQueryUsedRuleTests : CompositionTestBase
+{
+    private readonly PreMergeValidator _preMergeValidator = new([new RootQueryUsedRule()]);
+
+    [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_QUERY_USED"));
+        Assert.True(context.Log.All(e => e.Severity == LogSeverity.Error));
+    }
+
+    public static TheoryData<string[]> ValidExamplesData()
+    {
+        return new TheoryData<string[]>
+        {
+            // Valid example.
+            {
+                [
+                    """
+                    schema {
+                        query: Query
+                    }
+
+                    type Query {
+                        product(id: ID!): 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 `RootQuery` is used as the root query
+            // type, but a type named `Query` is also defined.
+            {
+                [
+                    """
+                    schema {
+                        query: RootQuery
+                    }
+
+                    type RootQuery {
+                        product(id: ID!): Product
+                    }
+
+                    type Query {
+                        deprecatedField: String
+                    }
+                    """
+                ],
+                [
+                    "The root query type in schema 'A' must be named 'Query'."
+                ]
+            }
+        };
+    }
+}