Pass depth through execution call chain instead of recomputing

Addresses review feedback: rather than walking the Path linked list
at every executeField invocation, thread the current field depth as
an explicit parameter through executeFields/executeField/completeValue
and its helpers. completeObjectValue increments depth when descending
into sub-selections; list items and abstract-type resolution keep the
same depth as their parent field.

This changes the depth check from O(depth) per field resolution to
O(1), while preserving identical semantics (list indices still do not
count toward depth).

Author: eddieran

src/execution/execute.ts

@@ -445,19 +445,34 @@ function executeOperation(
 
   switch (operation.operation) {
     case OperationTypeNode.QUERY:
-      return executeFields(exeContext, rootType, rootValue, path, rootFields);
+      return executeFields(
+        exeContext,
+        rootType,
+        rootValue,
+        path,
+        rootFields,
+        1,
+      );
     case OperationTypeNode.MUTATION:
       return executeFieldsSerially(
         exeContext,
         rootType,
         rootValue,
         path,
         rootFields,
+        1,
       );
     case OperationTypeNode.SUBSCRIPTION:
       // TODO: deprecate `subscribe` and move all logic here
       // Temporary solution until we finish merging execute and subscribe together
-      return executeFields(exeContext, rootType, rootValue, path, rootFields);
+      return executeFields(
+        exeContext,
+        rootType,
+        rootValue,
+        path,
+        rootFields,
+        1,
+      );
   }
 }
 
@@ -471,6 +486,7 @@ function executeFieldsSerially(
   sourceValue: unknown,
   path: Path | undefined,
   fields: Map<string, ReadonlyArray<FieldNode>>,
+  depth: number,
 ): PromiseOrValue<ObjMap<unknown>> {
   return promiseReduce(
     fields.entries(),
@@ -482,6 +498,7 @@ function executeFieldsSerially(
         sourceValue,
         fieldNodes,
         fieldPath,
+        depth,
       );
       if (result === undefined) {
         return results;
@@ -509,6 +526,7 @@ function executeFields(
   sourceValue: unknown,
   path: Path | undefined,
   fields: Map<string, ReadonlyArray<FieldNode>>,
+  depth: number,
 ): PromiseOrValue<ObjMap<unknown>> {
   const results = Object.create(null);
   let containsPromise = false;
@@ -522,6 +540,7 @@ function executeFields(
         sourceValue,
         fieldNodes,
         fieldPath,
+        depth,
       );
 
       if (result !== undefined) {
@@ -578,22 +597,6 @@ function checkAliasCount(
   }
 }
 
-/**
- * Counts the field depth represented by a Path. Only string keys are counted
- * (numeric keys represent list indices and should not increase the depth).
- */
-function pathDepth(path: Path | undefined): number {
-  let depth = 0;
-  let current = path;
-  while (current !== undefined) {
-    if (typeof current.key === 'string') {
-      depth++;
-    }
-    current = current.prev;
-  }
-  return depth;
-}
-
 /**
  * Implements the "Executing fields" section of the spec
  * In particular, this function figures out the value that the field returns by
@@ -606,16 +609,14 @@ function executeField(
   source: unknown,
   fieldNodes: ReadonlyArray<FieldNode>,
   path: Path,
+  depth: number,
 ): PromiseOrValue<unknown> {
   // Check depth limit before resolving the field.
-  if (exeContext.maxDepth !== undefined) {
-    const depth = pathDepth(path);
-    if (depth > exeContext.maxDepth) {
-      throw new GraphQLError(
-        `Query depth limit of ${exeContext.maxDepth} exceeded, found depth: ${depth}.`,
-        { nodes: fieldNodes },
-      );
-    }
+  if (exeContext.maxDepth !== undefined && depth > exeContext.maxDepth) {
+    throw new GraphQLError(
+      `Query depth limit of ${exeContext.maxDepth} exceeded, found depth: ${depth}.`,
+      { nodes: fieldNodes },
+    );
   }
 
   const fieldDef = getFieldDef(exeContext.schema, parentType, fieldNodes[0]);
@@ -655,7 +656,15 @@ function executeField(
     let completed;
     if (isPromise(result)) {
       completed = result.then((resolved) =>
-        completeValue(exeContext, returnType, fieldNodes, info, path, resolved),
+        completeValue(
+          exeContext,
+          returnType,
+          fieldNodes,
+          info,
+          path,
+          resolved,
+          depth,
+        ),
       );
     } else {
       completed = completeValue(
@@ -665,6 +674,7 @@ function executeField(
         info,
         path,
         result,
+        depth,
       );
     }
 
@@ -755,6 +765,7 @@ function completeValue(
   info: GraphQLResolveInfo,
   path: Path,
   result: unknown,
+  depth: number,
 ): PromiseOrValue<unknown> {
   // If result is an Error, throw a located error.
   if (result instanceof Error) {
@@ -771,6 +782,7 @@ function completeValue(
       info,
       path,
       result,
+      depth,
     );
     if (completed === null) {
       throw new Error(
@@ -794,6 +806,7 @@ function completeValue(
       info,
       path,
       result,
+      depth,
     );
   }
 
@@ -813,6 +826,7 @@ function completeValue(
       info,
       path,
       result,
+      depth,
     );
   }
 
@@ -825,6 +839,7 @@ function completeValue(
       info,
       path,
       result,
+      depth,
     );
   }
   /* c8 ignore next 6 */
@@ -846,6 +861,7 @@ function completeListValue(
   info: GraphQLResolveInfo,
   path: Path,
   result: unknown,
+  depth: number,
 ): PromiseOrValue<ReadonlyArray<unknown>> {
   if (!isIterableObject(result)) {
     throw new GraphQLError(
@@ -872,6 +888,7 @@ function completeListValue(
             info,
             itemPath,
             resolved,
+            depth,
           ),
         );
       } else {
@@ -882,6 +899,7 @@ function completeListValue(
           info,
           itemPath,
           item,
+          depth,
         );
       }
 
@@ -937,6 +955,7 @@ function completeAbstractValue(
   info: GraphQLResolveInfo,
   path: Path,
   result: unknown,
+  depth: number,
 ): PromiseOrValue<ObjMap<unknown>> {
   const resolveTypeFn = returnType.resolveType ?? exeContext.typeResolver;
   const contextValue = exeContext.contextValue;
@@ -958,6 +977,7 @@ function completeAbstractValue(
         info,
         path,
         result,
+        depth,
       ),
     );
   }
@@ -976,6 +996,7 @@ function completeAbstractValue(
     info,
     path,
     result,
+    depth,
   );
 }
 
@@ -1044,12 +1065,15 @@ function completeObjectValue(
   info: GraphQLResolveInfo,
   path: Path,
   result: unknown,
+  depth: number,
 ): PromiseOrValue<ObjMap<unknown>> {
   // Collect sub-fields to execute to complete this value.
   const subFieldNodes = collectSubfields(exeContext, returnType, fieldNodes);
 
   checkAliasCount(exeContext, subFieldNodes);
 
+  const subDepth = depth + 1;
+
   // If there is an isTypeOf predicate function, call it with the
   // current result. If isTypeOf returns false, then raise an error rather
   // than continuing execution.
@@ -1067,6 +1091,7 @@ function completeObjectValue(
           result,
           path,
           subFieldNodes,
+          subDepth,
         );
       });
     }
@@ -1076,7 +1101,14 @@ function completeObjectValue(
     }
   }
 
-  return executeFields(exeContext, returnType, result, path, subFieldNodes);
+  return executeFields(
+    exeContext,
+    returnType,
+    result,
+    path,
+    subFieldNodes,
+    subDepth,
+  );
 }
 
 function invalidReturnTypeError(