[Fusion] Added source schema validation rule "ExternalOverrideCollisionRule" (#8784)

Author: glen-84

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

@@ -13,6 +13,7 @@ public static class LogEntryCodes
     public const string ExternalArgumentDefaultMismatch = "EXTERNAL_ARGUMENT_DEFAULT_MISMATCH";
     public const string ExternalMissingOnBase = "EXTERNAL_MISSING_ON_BASE";
     public const string ExternalOnInterface = "EXTERNAL_ON_INTERFACE";
+    public const string ExternalOverrideCollision = "EXTERNAL_OVERRIDE_COLLISION";
     public const string ExternalUnused = "EXTERNAL_UNUSED";
     public const string FieldArgumentTypesNotMergeable = "FIELD_ARGUMENT_TYPES_NOT_MERGEABLE";
     public const string FieldWithMissingRequiredArgument = "FIELD_WITH_MISSING_REQUIRED_ARGUMENT";

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

@@ -260,6 +260,22 @@ public static LogEntry ExternalOnInterface(
             schema);
     }
 
+    public static LogEntry ExternalOverrideCollision(
+        MutableOutputFieldDefinition externalField,
+        ITypeDefinition type,
+        MutableSchemaDefinition schema)
+    {
+        var coordinate = new SchemaCoordinate(type.Name, externalField.Name);
+
+        return new LogEntry(
+            string.Format(LogEntryHelper_ExternalOverrideCollision, coordinate, schema.Name),
+            LogEntryCodes.ExternalOverrideCollision,
+            LogSeverity.Error,
+            coordinate,
+            externalField,
+            schema);
+    }
+
     public static LogEntry ExternalUnused(
         MutableOutputFieldDefinition externalField,
         ITypeDefinition type,

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

@@ -213,6 +213,9 @@
   <data name="LogEntryHelper_ExternalOnInterface" xml:space="preserve">
     <value>The interface field '{0}' in schema '{1}' must not be marked as external.</value>
   </data>
+  <data name="LogEntryHelper_ExternalOverrideCollision" xml:space="preserve">
+    <value>The external field '{0}' must not be annotated with the @override directive.</value>
+  </data>
   <data name="LogEntryHelper_ExternalUnused" xml:space="preserve">
     <value>The external field '{0}' in schema '{1}' is not referenced by a @provides directive in the schema.</value>
   </data>

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

@@ -114,6 +114,7 @@ public CompositionResult<MutableSchemaDefinition> Compose()
     [
         new DisallowedInaccessibleElementsRule(),
         new ExternalOnInterfaceRule(),
+        new ExternalOverrideCollisionRule(),
         new ExternalUnusedRule(),
         new InvalidShareableUsageRule(),
         new IsInvalidFieldTypeRule(),

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

@@ -0,0 +1,29 @@
+using HotChocolate.Fusion.Events;
+using HotChocolate.Fusion.Events.Contracts;
+using HotChocolate.Fusion.Extensions;
+using static HotChocolate.Fusion.Logging.LogEntryHelper;
+
+namespace HotChocolate.Fusion.SourceSchemaValidationRules;
+
+/// <summary>
+/// The <c>@external</c> directive indicates that a field is <b>defined</b> in a different source
+/// schema, and the current schema merely references it. Therefore, a field marked with
+/// <c>@external</c> must <b>not</b> simultaneously carry directives that assume local ownership or
+/// resolution responsibility, such as <c>@override</c>, which transfers ownership of the field’s
+/// definition from one schema to another.
+/// </summary>
+/// <seealso href="https://graphql.github.io/composite-schemas-spec/draft/#sec-External-Override-Collision">
+/// Specification
+/// </seealso>
+internal sealed class ExternalOverrideCollisionRule : IEventHandler<OutputFieldEvent>
+{
+    public void Handle(OutputFieldEvent @event, CompositionContext context)
+    {
+        var (field, type, schema) = @event;
+
+        if (field.HasExternalDirective() && field.HasOverrideDirective())
+        {
+            context.Log.Write(ExternalOverrideCollision(field, type, schema));
+        }
+    }
+}

src/HotChocolate/Fusion-vnext/src/Fusion.Composition/SourceSchemaValidationRules/ExternalOverrideCollisionRule.cs

@@ -0,0 +1,88 @@
+using System.Collections.Immutable;
+using HotChocolate.Fusion.Logging;
+using static HotChocolate.Fusion.CompositionTestHelper;
+
+namespace HotChocolate.Fusion.SourceSchemaValidationRules;
+
+public sealed class ExternalOverrideCollisionRuleTests
+{
+    private static readonly object s_rule = new ExternalOverrideCollisionRule();
+    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 validator = new SourceSchemaValidator(schemas, s_rules, _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 validator = new SourceSchemaValidator(schemas, s_rules, _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 == "EXTERNAL_OVERRIDE_COLLISION"));
+        Assert.True(_log.All(e => e.Severity == LogSeverity.Error));
+    }
+
+    public static TheoryData<string[]> ValidExamplesData()
+    {
+        return new TheoryData<string[]>
+        {
+            // In this scenario, "User.fullName" is overriding the field from schema A. Since
+            // @override is not combined with @external on the same field, no collision occurs.
+            {
+                [
+                    """
+                    type User {
+                        id: ID!
+                        fullName: String @override(from: "A")
+                    }
+                    """
+                ]
+            }
+        };
+    }
+
+    public static TheoryData<string[], string[]> InvalidExamplesData()
+    {
+        return new TheoryData<string[], string[]>
+        {
+            // Here, "amount" is marked with both @override and @external. This violates the rule
+            // because the field is simultaneously labeled as "override from another schema" and
+            // "external" in the local schema, producing an EXTERNAL_OVERRIDE_COLLISION error.
+            {
+                [
+                    """
+                    type Payment {
+                        id: ID!
+                        amount: Int @override(from: "A") @external
+                    }
+                    """
+                ],
+                [
+                    "The external field 'Payment.amount' must not be annotated with the @override directive."
+                ]
+            }
+        };
+    }
+}