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

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

@@ -14,6 +14,7 @@ public static class LogEntryCodes
     public const string KeyInvalidSyntax = "KEY_INVALID_SYNTAX";
     public const string OutputFieldTypesNotMergeable = "OUTPUT_FIELD_TYPES_NOT_MERGEABLE";
     public const string ProvidesDirectiveInFieldsArg = "PROVIDES_DIRECTIVE_IN_FIELDS_ARG";
+    public const string QueryRootTypeInaccessible = "QUERY_ROOT_TYPE_INACCESSIBLE";
     public const string RootMutationUsed = "ROOT_MUTATION_USED";
     public const string RootQueryUsed = "ROOT_QUERY_USED";
     public const string RootSubscriptionUsed = "ROOT_SUBSCRIPTION_USED";

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

@@ -300,6 +300,19 @@ public static LogEntry ProvidesDirectiveInFieldsArgument(
             schema);
     }
 
+    public static LogEntry QueryRootTypeInaccessible(
+        INamedTypeDefinition type,
+        SchemaDefinition schema)
+    {
+        return new LogEntry(
+            string.Format(LogEntryHelper_QueryRootTypeInaccessible, schema.Name),
+            LogEntryCodes.QueryRootTypeInaccessible,
+            LogSeverity.Error,
+            new SchemaCoordinate(type.Name),
+            type,
+            schema);
+    }
+
     public static LogEntry RootMutationUsed(SchemaDefinition schema)
     {
         return new LogEntry(

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

@@ -0,0 +1,27 @@
+using HotChocolate.Fusion.Events;
+using static HotChocolate.Fusion.Logging.LogEntryHelper;
+
+namespace HotChocolate.Fusion.PreMergeValidation.Rules;
+
+/// <summary>
+/// Every source schema that contributes to the final composite schema must expose a public
+/// (accessible) root query type. Marking the root query type as <c>@inaccessible</c> makes it
+/// invisible to the gateway, defeating its purpose as the primary entry point for queries and
+/// lookups.
+/// </summary>
+/// <seealso href="https://graphql.github.io/composite-schemas-spec/draft/#sec-Query-Root-Type-Inaccessible">
+/// Specification
+/// </seealso>
+internal sealed class QueryRootTypeInaccessibleRule : IEventHandler<SchemaEvent>
+{
+    public void Handle(SchemaEvent @event, CompositionContext context)
+    {
+        var schema = @event.Schema;
+        var rootQuery = schema.QueryType;
+
+        if (rootQuery is not null && !ValidationHelper.IsAccessible(rootQuery))
+        {
+            context.Log.Write(QueryRootTypeInaccessible(rootQuery, schema));
+        }
+    }
+}

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

@@ -69,6 +69,9 @@
   <data name="LogEntryHelper_ProvidesDirectiveInFieldsArgument" xml:space="preserve">
     <value>The @provides directive on field '{0}' in schema '{1}' references field '{2}', which must not include directive applications.</value>
   </data>
+  <data name="LogEntryHelper_QueryRootTypeInaccessible" xml:space="preserve">
+    <value>The root query type in schema '{0}' must be accessible.</value>
+  </data>
   <data name="LogEntryHelper_RootMutationUsed" xml:space="preserve">
     <value>The root mutation type in schema '{0}' must be named 'Mutation'.</value>
   </data>

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

@@ -58,6 +58,7 @@ private CompositionResult<SchemaDefinition> MergeSchemaDefinitions(CompositionCo
         new KeyInvalidSyntaxRule(),
         new OutputFieldTypesMergeableRule(),
         new ProvidesDirectiveInFieldsArgumentRule(),
+        new QueryRootTypeInaccessibleRule(),
         new RootMutationUsedRule(),
         new RootQueryUsedRule(),
         new RootSubscriptionUsedRule()