[Fusion] Add support for provides

src/HotChocolate/Core/src/Types/Types/Composite/Directives/EntityKey.cs

@@ -58,8 +58,7 @@ public EntityKey(SelectionSetNode fields)
     public EntityKey(string fields)
     {
         ArgumentNullException.ThrowIfNull(fields);
-        fields = $"{{ {fields.Trim('{', '}')} }}";
-        Fields = Utf8GraphQLParser.Syntax.ParseSelectionSet(fields);
+        Fields = FieldSelectionSetType.ParseSelectionSet(fields);
     }
 
     /// <summary>

src/HotChocolate/Core/src/Types/Types/Composite/Directives/EntityKeyDescriptorExtensions.cs

@@ -39,8 +39,7 @@ public static IObjectTypeDescriptor EntityKey(
 
         try
         {
-            fields = $"{{ {fields.Trim('{', '}')} }}";
-            selectionSet = Utf8GraphQLParser.Syntax.ParseSelectionSet(fields);
+            selectionSet = FieldSelectionSetType.ParseSelectionSet(fields);
         }
         catch (SyntaxException ex)
         {
@@ -87,8 +86,7 @@ public static IInterfaceTypeDescriptor EntityKey(
 
         try
         {
-            fields = $"{{ {fields.Trim('{', '}')} }}";
-            selectionSet = Utf8GraphQLParser.Syntax.ParseSelectionSet(fields);
+            selectionSet = FieldSelectionSetType.ParseSelectionSet(fields);
         }
         catch (SyntaxException ex)
         {

src/HotChocolate/Core/src/Types/Types/Composite/Directives/Provides.cs

@@ -19,8 +19,7 @@ public Provides(SelectionSetNode fields)
     public Provides(string fields)
     {
         ArgumentNullException.ThrowIfNull(fields);
-        fields = $"{{ {fields.Trim('{', '}')} }}";
-        Fields = Utf8GraphQLParser.Syntax.ParseSelectionSet(fields);
+        Fields = FieldSelectionSetType.ParseSelectionSet(fields);
     }
 
     [GraphQLType<NonNullType<FieldSelectionSetType>>]
@@ -43,8 +42,7 @@ public static IObjectFieldDescriptor Provides(
 
         try
         {
-            fields = $"{{ {fields.Trim('{', '}')} }}";
-            selectionSet = Utf8GraphQLParser.Syntax.ParseSelectionSet(fields);
+            selectionSet = FieldSelectionSetType.ParseSelectionSet(fields);
         }
         catch (SyntaxException ex)
         {

src/HotChocolate/Core/src/Types/Types/Composite/Types/FieldSelectionSetType.cs

@@ -89,7 +89,11 @@ protected override StringValueNode OnValueToLiteral(SelectionSetNode runtimeValu
     /// </exception>
     internal static SelectionSetNode ParseSelectionSet(string s)
     {
-        s = $"{{ {s.Trim('{', '}')} }}";
+        s = s.Trim();
+        if (s.Length < 2 || s[0] != '{' || s[^1] != '}')
+        {
+            s = $"{{ {s} }}";
+        }
         return Utf8GraphQLParser.Syntax.ParseSelectionSet(s);
     }
 

src/HotChocolate/Fusion/src/Fusion.Execution.Types/Completion/CompositeSchemaBuilder.cs

@@ -793,7 +793,9 @@ private static SourceObjectFieldCollection BuildSourceOutputFieldCollection(
                     fieldDirective.SourceName ?? fieldDefinition.Name,
                     context.GetSchemaName(fieldDirective.SchemaKey),
                     requirements,
-                    CompleteType(fieldDef.Type, fieldDirective.SourceType, context)));
+                    CompleteType(fieldDef.Type, fieldDirective.SourceType, context),
+                    fieldDirective.IsExternal,
+                    fieldDirective.Provides));
         }
 
         return new SourceObjectFieldCollection(temp.ToImmutable());

src/HotChocolate/Fusion/src/Fusion.Execution.Types/Metadata/SourceOutputField.cs

@@ -1,3 +1,4 @@
+using HotChocolate.Language;
 using HotChocolate.Types;
 
 namespace HotChocolate.Fusion.Types.Metadata;
@@ -6,7 +7,9 @@ public sealed class SourceOutputField(
     string name,
     string schemaName,
     FieldRequirements? requirements,
-    IType type)
+    IType type,
+    bool isExternal,
+    SelectionSetNode? provides)
     : ISourceMember
 {
     public string Name { get; } = name;
@@ -17,5 +20,7 @@ public sealed class SourceOutputField(
 
     public IType Type { get; } = type;
 
-    public int BaseCost => 1;
+    public bool IsExternal { get; } = isExternal;
+
+    public SelectionSetNode? Provides { get; } = provides;
 }

src/HotChocolate/Fusion/src/Fusion.Execution/Planning/Partitioners/SelectionSetPartitioner.cs

@@ -239,12 +239,78 @@ void CompleteSelection<T>(T original, T? resolvable, T? unresolvable, int index)
         }
 
         static SelectionSetNode? GetProvidedSelectionSet(
-            ITypeDefinition _1,
-            FusionSchemaDefinition _2,
+            ITypeDefinition type,
+            FusionSchemaDefinition schema,
             SelectionSetNode? providedSelectionSetNode)
         {
-            // todo match correct inline fragment
-            return providedSelectionSetNode;
+            if (providedSelectionSetNode is null)
+            {
+                return null;
+            }
+
+            List<ISelectionNode>? flattened = null;
+            var hasFragment = false;
+
+            for (var i = 0; i < providedSelectionSetNode.Selections.Count; i++)
+            {
+                var selection = providedSelectionSetNode.Selections[i];
+
+                if (selection is InlineFragmentNode fragment)
+                {
+                    hasFragment = true;
+
+                    if (fragment.TypeCondition is null)
+                    {
+                        flattened ??= CopyUpTo(providedSelectionSetNode.Selections, i);
+                        flattened.AddRange(fragment.SelectionSet.Selections);
+                        continue;
+                    }
+
+                    if (!schema.Types.TryGetType(fragment.TypeCondition.Name.Value, out var fragmentType))
+                    {
+                        flattened ??= CopyUpTo(providedSelectionSetNode.Selections, i);
+                        continue;
+                    }
+
+                    if (fragmentType.IsAssignableFrom(type))
+                    {
+                        flattened ??= CopyUpTo(providedSelectionSetNode.Selections, i);
+                        flattened.AddRange(fragment.SelectionSet.Selections);
+                    }
+                    else if (type.IsAssignableFrom(fragmentType))
+                    {
+                        flattened ??= CopyUpTo(providedSelectionSetNode.Selections, i);
+                        flattened.Add(fragment);
+                    }
+                    else
+                    {
+                        flattened ??= CopyUpTo(providedSelectionSetNode.Selections, i);
+                    }
+                }
+                else
+                {
+                    flattened?.Add(selection);
+                }
+            }
+
+            if (!hasFragment)
+            {
+                return providedSelectionSetNode;
+            }
+
+            return flattened is null
+                ? providedSelectionSetNode
+                : new SelectionSetNode(flattened);
+        }
+
+        static List<ISelectionNode> CopyUpTo(IReadOnlyList<ISelectionNode> selections, int exclusiveEnd)
+        {
+            var copy = new List<ISelectionNode>(selections.Count);
+            for (var j = 0; j < exclusiveEnd; j++)
+            {
+                copy.Add(selections[j]);
+            }
+            return copy;
         }
 
         static bool IsTypeNameSelection(ISelectionNode selection)
@@ -385,29 +451,29 @@ private static SelectionSetNode WrapInExtraConditions(
         FieldNode? providedFieldNode)
     {
         var field = complexType.Fields.GetField(fieldNode.Name.Value, allowInaccessibleFields: true);
+        field.Sources.TryGetMember(context.SchemaName, out var source);
+
+        // The field is resolvable from the current schema when either the parent's provides
+        // scope covers it, or the schema declares a non-external source for it.
+        var isResolvable = providedFieldNode is not null || source is { IsExternal: false };
 
-        if (providedFieldNode is null)
+        if (!isResolvable)
         {
-            // if the field is not available in the current schema we return null
-            // which will remove the field from the rewritten selection set.
-            if (!field.Sources.TryGetMember(context.SchemaName, out var source))
-            {
-                return (null, fieldNode);
-            }
+            return (null, fieldNode);
+        }
 
-            if (source.Requirements is not null)
-            {
-                context.FieldsWithRequirements =
-                    context.FieldsWithRequirements.Push(
-                        new ConditionedFieldSelection(
-                            new FieldSelection(
-                                context.GetId((SelectionSetNode)context.Nodes.Peek()),
-                                fieldNode,
-                                field,
-                                context.BuildPath()),
-                            context.SnapshotConditions()));
-                return (null, null);
-            }
+        if (providedFieldNode is null && source?.Requirements is not null)
+        {
+            context.FieldsWithRequirements =
+                context.FieldsWithRequirements.Push(
+                    new ConditionedFieldSelection(
+                        new FieldSelection(
+                            context.GetId((SelectionSetNode)context.Nodes.Peek()),
+                            fieldNode,
+                            field,
+                            context.BuildPath()),
+                        context.SnapshotConditions()));
+            return (null, null);
         }
 
         var selectionSet = fieldNode.SelectionSet;
@@ -420,7 +486,7 @@ private static SelectionSetNode WrapInExtraConditions(
                 context,
                 field.Type.AsTypeDefinition(),
                 selectionSet,
-                providedFieldNode?.SelectionSet);
+                MergeProvidedSelectionSets(providedFieldNode?.SelectionSet, source?.Provides));
 
             context.Nodes.Pop();
 
@@ -437,6 +503,26 @@ private static SelectionSetNode WrapInExtraConditions(
         return (fieldNode, null);
     }
 
+    private static SelectionSetNode? MergeProvidedSelectionSets(
+        SelectionSetNode? inherited,
+        SelectionSetNode? fromSource)
+    {
+        if (inherited is null)
+        {
+            return fromSource;
+        }
+
+        if (fromSource is null)
+        {
+            return inherited;
+        }
+
+        var merged = new List<ISelectionNode>(inherited.Selections.Count + fromSource.Selections.Count);
+        merged.AddRange(inherited.Selections);
+        merged.AddRange(fromSource.Selections);
+        return new SelectionSetNode(merged);
+    }
+
     private (InlineFragmentNode?, InlineFragmentNode?) RewriteFragmentNode(
         Context context,
         ITypeDefinition type,