doesn't infinitely recurse on fragment cycle

Author: enisdenjo

src/validation/__tests__/MaxIntrospectionDepthRule-test.ts

@@ -536,4 +536,19 @@ describe('Validate: Max introspection nodes rule', () => {
     }
     `);
   });
+
+  it("doesn't infinitely recurse on fragment cycle", () => {
+    expectValid(`
+    query test {
+      __schema {
+        types {
+          ...Cycle
+        }
+      }
+    }
+    fragment Cycle on __Type {
+      ...Cycle
+    }
+    `);
+  });
 });

src/validation/rules/MaxIntrospectionDepthRule.ts

@@ -15,14 +15,36 @@ export function MaxIntrospectionDepthRule(
    * Counts the depth of list fields in "__Type" recursively and
    * returns `true` if the limit has been reached.
    */
-  function checkDepth(node: ASTNode, depth: number = 0): boolean {
+  function checkDepth(
+    node: ASTNode,
+    visitedFragments: {
+      [fragmentName: string]: true | undefined;
+    } = Object.create(null),
+    depth: number = 0,
+  ): boolean {
     if (node.kind === Kind.FRAGMENT_SPREAD) {
-      const fragment = context.getFragment(node.name.value);
+      const fragmentName = node.name.value;
+      if (visitedFragments[fragmentName]) {
+        // Fragment cycles are handled by `NoFragmentCyclesRule`.
+        return false;
+      }
+      const fragment = context.getFragment(fragmentName);
       if (!fragment) {
-        // missing fragments checks are handled by the `KnownFragmentNamesRule`
+        // Missing fragments checks are handled by the `KnownFragmentNamesRule`.
         return false;
       }
-      return checkDepth(fragment, depth);
+
+      // Rather than following an immutable programming pattern which has
+      // significant memory and garbage collection overhead, we've opted to
+      // take a mutable approach for efficiency's sake. Importantly visiting a
+      // fragment twice is fine, so long as you don't do one visit inside the
+      // other.
+      try {
+        visitedFragments[fragmentName] = true;
+        return checkDepth(fragment, visitedFragments, depth);
+      } finally {
+        visitedFragments[fragmentName] = undefined;
+      }
     }
 
     if (
@@ -43,7 +65,7 @@ export function MaxIntrospectionDepthRule(
     // handles fields and inline fragments
     if ('selectionSet' in node && node.selectionSet) {
       for (const child of node.selectionSet.selections) {
-        if (checkDepth(child, depth)) {
+        if (checkDepth(child, visitedFragments, depth)) {
           return true;
         }
       }