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node_modules/graphql/validation/rules/NoFragmentCycles.js.flow

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+// @flow strict
+
+import { GraphQLError } from '../../error/GraphQLError';
+
+import { type ASTVisitor } from '../../language/visitor';
+import { type FragmentDefinitionNode } from '../../language/ast';
+
+import { type ASTValidationContext } from '../ValidationContext';
+
+export function cycleErrorMessage(
+  fragName: string,
+  spreadNames: $ReadOnlyArray<string>,
+): string {
+  const via = spreadNames.length ? ' via ' + spreadNames.join(', ') : '';
+  return `Cannot spread fragment "${fragName}" within itself${via}.`;
+}
+
+export function NoFragmentCycles(context: ASTValidationContext): ASTVisitor {
+  // Tracks already visited fragments to maintain O(N) and to ensure that cycles
+  // are not redundantly reported.
+  const visitedFrags = Object.create(null);
+
+  // Array of AST nodes used to produce meaningful errors
+  const spreadPath = [];
+
+  // Position in the spread path
+  const spreadPathIndexByName = Object.create(null);
+
+  return {
+    OperationDefinition: () => false,
+    FragmentDefinition(node) {
+      detectCycleRecursive(node);
+      return false;
+    },
+  };
+
+  // This does a straight-forward DFS to find cycles.
+  // It does not terminate when a cycle was found but continues to explore
+  // the graph to find all possible cycles.
+  function detectCycleRecursive(fragment: FragmentDefinitionNode) {
+    if (visitedFrags[fragment.name.value]) {
+      return;
+    }
+
+    const fragmentName = fragment.name.value;
+    visitedFrags[fragmentName] = true;
+
+    const spreadNodes = context.getFragmentSpreads(fragment.selectionSet);
+    if (spreadNodes.length === 0) {
+      return;
+    }
+
+    spreadPathIndexByName[fragmentName] = spreadPath.length;
+
+    for (const spreadNode of spreadNodes) {
+      const spreadName = spreadNode.name.value;
+      const cycleIndex = spreadPathIndexByName[spreadName];
+
+      spreadPath.push(spreadNode);
+      if (cycleIndex === undefined) {
+        const spreadFragment = context.getFragment(spreadName);
+        if (spreadFragment) {
+          detectCycleRecursive(spreadFragment);
+        }
+      } else {
+        const cyclePath = spreadPath.slice(cycleIndex);
+        const fragmentNames = cyclePath.slice(0, -1).map(s => s.name.value);
+        context.reportError(
+          new GraphQLError(
+            cycleErrorMessage(spreadName, fragmentNames),
+            cyclePath,
+          ),
+        );
+      }
+      spreadPath.pop();
+    }
+
+    spreadPathIndexByName[fragmentName] = undefined;
+  }
+}