feat: add DefaultDetector which supports RBAC role loop detection

[Copilot]

Implements DefaultDetector to detect role inheritance cycles (A→B→C→A) in RBAC graphs using depth-first search.

Implementation

DefaultDetector.java

• Iterative DFS (stack-based) to avoid stack overflow on deep graphs (10K+ roles)
• Detects cycles via visited/recursion-stack sets; returns "Cycle detected: A -> B -> C -> A" or null
• Accesses DefaultRoleManager.allRoles via reflection to build local adjacency list
• O(V+E) time complexity with thread-safe local data structures

DefaultDetectorTest.java

• 14 tests covering: simple cycles, self-loops, disconnected components, large graphs (10K roles)

Usage

RoleManager rm = new DefaultRoleManager(10);
rm.addLink("A", "B");
rm.addLink("B", "C");
rm.addLink("C", "A");

Detector detector = new DefaultDetector();
String result = detector.check(rm);
// Returns: "Cycle detected: A -> B -> C -> A"

Original prompt

---

This section details on the original issue you should resolve

<issue_title>feat(detector): implement DefaultDetector with RBAC cycle detection</issue_title>
<issue_description>Background:
Provide a default implementation for Detector to detect role inheritance cycles (A→B→C→A).

Tasks:

1. Add DefaultDetector.java under the org.casbin.jcasbin.detector package, implementing the Detector interface.

2. Use depth-first search (DFS) to traverse the passed-in RoleManager instance and detect whether any cycle exists.

3. Detailed steps:

   • Obtain the adjacency relationships of the current graph from APIs provided by RoleManager (such as getRoles(String name) and getUsers(String name)). Since DefaultRoleManager’s addLink method simply establishes parent-child relationships between Role objects
     github.com
     , you can build a directed graph by recursively traversing roles’ getAllRoles().
   • Maintain visited and stack sets; discovering a back edge is considered a cycle; once the first cycle is found, generate and return a description in the form "Cycle detected: A -> B -> C -> A".
   • If traversal finishes without finding any cycle, return null.

4. Do not modify the source code of RoleManager or Role, and do not change existing API signatures. All logic should be self-contained within DefaultDetector.

5. Pay attention to performance: even on large RBAC graphs (e.g., 10000 roles), DFS should run in linear time; avoid repeated traversal.

Constraints:

• The code should support multi-threaded access, but you may create data structures locally inside the method to avoid sharing references with the internal state of RoleManager.
• During implementation, do not catch and swallow exceptions; if an error occurs, throw a runtime exception directly.
  </issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes feat(detector): implement DefaultDetector with RBAC cycle detection #474

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[codecov-commenter]

Codecov Report

❌ Patch coverage is 90.76923% with 6 lines in your changes missing coverage. Please review.
✅ Project coverage is 67.80%. Comparing base (d79d4db) to head (aca0f3d).
⚠️ Report is 2 commits behind head on master.

Files with missing lines	Patch %	Lines
...a/org/casbin/jcasbin/detector/DefaultDetector.java	90.76%	4 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #483      +/-   ##
==========================================
+ Coverage   67.31%   67.80%   +0.49%     
==========================================
  Files          60       61       +1     
  Lines        3004     3069      +65     
  Branches      559      569      +10     
==========================================
+ Hits         2022     2081      +59     
- Misses        802      806       +4     
- Partials      180      182       +2     

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