feature: Adds NoFragmentCyclesRule

Author: NeedleInAJayStack

Sources/GraphQL/Validation/Rules/NoFragmentCyclesRule.swift

@@ -0,0 +1,79 @@
+
+/**
+ * No fragment cycles
+ *
+ * The graph of fragment spreads must not form any cycles including spreading itself.
+ * Otherwise an operation could infinitely spread or infinitely execute on cycles in the underlying data.
+ *
+ * See https://spec.graphql.org/draft/#sec-Fragment-spreads-must-not-form-cycles
+ */
+func NoFragmentCyclesRule(context: ValidationContext) -> Visitor {
+    // Tracks already visited fragments to maintain O(N) and to ensure that cycles
+    // are not redundantly reported.
+    var visitedFrags = Set<String>()
+
+    // Array of AST nodes used to produce meaningful errors
+    var spreadPath = [FragmentSpread]()
+
+    // Position in the spread path
+    var spreadPathIndexByName = [String: Int]()
+
+    // 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.
+    func detectCycleRecursive(fragment: FragmentDefinition) {
+        if visitedFrags.contains(fragment.name.value) {
+            return
+        }
+
+        let fragmentName = fragment.name.value
+        visitedFrags.insert(fragmentName)
+
+        let spreadNodes = context.getFragmentSpreads(node: fragment.selectionSet)
+        if spreadNodes.count == 0 {
+            return
+        }
+
+        spreadPathIndexByName[fragmentName] = spreadPath.count
+
+        for spreadNode in spreadNodes {
+            let spreadName = spreadNode.name.value
+            let cycleIndex = spreadPathIndexByName[spreadName]
+
+            spreadPath.append(spreadNode)
+            if let cycleIndex = cycleIndex {
+                let cyclePath = Array(spreadPath[cycleIndex ..< spreadPath.count])
+                let viaPath = cyclePath[0 ..< max(cyclePath.count - 1, 0)]
+                    .map { "\"\($0.name.value)\"" }.joined(separator: ", ")
+
+                context.report(
+                    error: GraphQLError(
+                        message: "Cannot spread fragment \"\(spreadName)\" within itself" +
+                            (viaPath != "" ? " via \(viaPath)." : "."),
+                        nodes: cyclePath
+                    )
+                )
+            } else {
+                if let spreadFragment = context.getFragment(name: spreadName) {
+                    detectCycleRecursive(fragment: spreadFragment)
+                }
+            }
+            spreadPath.removeLast()
+        }
+
+        spreadPathIndexByName[fragmentName] = nil
+    }
+
+    return Visitor(
+        enter: { node, _, _, _, _ in
+            if node is OperationDefinition {
+                return .skip
+            }
+            if let fragmentDefinition = node as? FragmentDefinition {
+                detectCycleRecursive(fragment: fragmentDefinition)
+                return .skip
+            }
+            return .continue
+        }
+    )
+}

Sources/GraphQL/Validation/SpecifiedRules.swift

@@ -15,7 +15,7 @@ public let specifiedRules: [(ValidationContext) -> Visitor] = [
     KnownFragmentNamesRule,
     NoUnusedFragmentsRule,
     PossibleFragmentSpreadsRule,
-//    NoFragmentCyclesRule,
+    NoFragmentCyclesRule,
 //    UniqueVariableNamesRule,
 //    NoUndefinedVariablesRule,
     NoUnusedVariablesRule,