fix(assertions): stack overflow while parsing template (#26767)

Closes #26766

The function `findCycle` tries to find a cycle by using a depth-first search (DFS). The DFS is implemented recursively in the recurse function. For each node, it tries to find a path that eventually leads back to the start of the path. If such a path is found, a cycle exists, and the nodes forming this cycle are returned.

One of the bugs in the current implementation is that it only checks whether the current dependency `dep` is equal to the first node of the current path `path[0]`. This means it will only detect a cycle if the cycle includes the first node of the search, which might not always be the case.

To fix this, the function should check whether the current dependency `dep` is already somewhere in the current path `path`. If it is, then a cycle has been found.

----

*By submitting this pull request, I confirm that my contribution is made under the terms of the Apache-2.0 license*

Author: Chriscbr

packages/aws-cdk-lib/assertions/lib/private/cyclic.ts

@@ -167,12 +167,12 @@ function findCycle(deps: ReadonlyMap<string, ReadonlySet<string>>): string[] {
 
   function recurse(node: string, path: string[]): string[] | undefined {
     for (const dep of deps.get(node) ?? []) {
-      if (dep === path[0]) { return [...path, dep]; }
+      if (path.includes(dep)) { return [...path, dep]; }
 
       const cycle = recurse(dep, [...path, dep]);
       if (cycle) { return cycle; }
     }
 
     return undefined;
   }
-}
+}

packages/aws-cdk-lib/assertions/test/template.test.ts

@@ -1345,6 +1345,29 @@ describe('Template', () => {
     }).toThrow(/dependency cycle/);
   });
 
+  test('throws when given a more complex template with cyclic dependencies', () => {
+    expect(() => {
+      Template.fromJSON({
+        Resources: {
+          Res1: {
+            Type: 'Foo',
+            Properties: {
+              Thing: { Ref: 'Res2' },
+            },
+          },
+          Res2: {
+            Type: 'Foo',
+            DependsOn: ['Res3'],
+          },
+          Res3: {
+            Type: 'Foo',
+            DependsOn: ['Res2'],
+          },
+        },
+      });
+    }).toThrow(/dependency cycle/);
+  });
+
   test('does not throw when given a template with cyclic dependencies if check is skipped', () => {
     expect(() => {
       Template.fromJSON({