Fix zombie health check watcher threads on krkn runner failures

[WHOIM1205]

User description

Fix: Zombie Background Thread Leak in krkn_runner

Summary

This PR fixes a critical background thread leak in krkn_runner.py where the health_check_watcher thread could remain alive if the krkn subprocess crashes, hangs, or raises an exception.
The issue causes zombie threads, resource leaks, and non-terminating krkn-ai run executions in real production environments.

The fix ensures that all background threads are always stopped, even when failures occur.

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Problem Description

File: krkn_ai/runner/krkn_runner.py
Component: health_check_watcher lifecycle management

The runner starts a background health check watcher thread before launching the krkn subprocess.
However, if the subprocess crashes or exits unexpectedly, the watcher thread is never stopped.

Root Cause

The watcher lifecycle was not guarded by try...finally, so any exception or early exit skips the cleanup path.

This leads to:

• Orphaned threads
• Hung CLI execution
• Resource exhaustion over multiple runs
• CI pipelines never completing

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Why This Is Critical

• Affects real users running krkn-ai run in staging and production clusters
• Causes silent hangs instead of clean failures
• Makes chaos experiments non-deterministic
• Breaks automation, CI jobs, and long-running discovery runs
• Accumulates leaked threads over repeated executions

This is not theoretical — subprocess crashes and SIGINT/SIGTERM are common in chaos workflows.

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How to Reproduce (Realistic)

1. Run krkn-ai run with a valid scenario.
2. Force the krkn subprocess to crash or exit early:
   • Kill the process
   • Inject an invalid scenario
   • Trigger a runtime exception
3. Observe:
   • Main process exits
   • Background health_check_watcher thread remains alive
   • CLI hangs or never fully terminates

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Fix Applied

• Wrapped the runner execution logic in a try...finally block
• Ensured health_check_watcher.stop() is always called
• Guarantees cleanup on:
  • Exceptions
  • Subprocess crashes
  • User interrupts (SIGINT / SIGTERM)

This preserves existing behavior while making cleanup deterministic.

---

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Impact After Fix

• No more zombie background threads
• Clean shutdowns on failures
• Stable CI and automation
• Predictable chaos experiment behavior
• Improved reliability for long-running discovery runs

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Files Changed

• krkn_ai/runner/krkn_runner.py
• tests/test_krkn_runner_leak.py
• walkthrough.md (documentation)

---

Regression Test Added

New test: tests/test_krkn_runner_leak.py

The test:

• Simulates a krkn subprocess failure
• Verifies the watcher thread is stopped
• Fails on current main, passes with this fix

This prevents future regressions.

Severity

High — Production impacting

---

---

PR Type

Bug fix, Tests

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Description

• Wrap runner execution in try-finally block to ensure cleanup

• Prevent zombie health_check_watcher threads on subprocess failures

• Add regression test simulating subprocess crash scenarios

• Guarantee thread cleanup on exceptions, crashes, and interrupts

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Diagram Walkthrough

flowchart LR
  A["health_check_watcher.run()"] --> B["try block"]
  B --> C["run_shell command"]
  C --> D["Extract returncode"]
  D --> E["finally block"]
  C -->|Exception| E
  E --> F["health_check_watcher.stop()"]
  F --> G["Cleanup guaranteed"]

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File Walkthrough

	Relevant files
Bug fix	krkn_runner.py Add try-finally for guaranteed watcher cleanup                      krkn_ai/chaos_engines/krkn_runner.py Wrapped runner execution logic in try-finally block Moved health_check_watcher.stop() to finally clause Ensures cleanup on subprocess crashes, exceptions, and interrupts Preserves existing behavior while making cleanup deterministic +11/-9
Tests	test_krkn_runner_leak.py Add test for health check watcher cleanup                                tests/test_krkn_runner_leak.py New regression test for thread leak prevention Mocks HealthCheckWatcher and run_shell to simulate subprocess failure Verifies stop() is called even when run_shell raises exception Prevents future regressions of zombie thread issues +58/-0

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[qodo-code-review]

PR Compliance Guide 🔍

Below is a summary of compliance checks for this PR:

Security Compliance
🟢	No security concerns identified No security vulnerabilities detected by AI analysis. Human verification advised for critical code.
Ticket Compliance
⚪	🎫 No ticket provided Create ticket/issue
Codebase Duplication Compliance
⚪	Codebase context is not defined Follow the guide to enable codebase context checks.
Custom Compliance
🟢	Generic: Comprehensive Audit Trails Objective: To create a detailed and reliable record of critical system actions for security analysis and compliance. Status: Passed Learn more about managing compliance generic rules or creating your own custom rules
	Generic: Meaningful Naming and Self-Documenting Code Objective: Ensure all identifiers clearly express their purpose and intent, making code self-documenting Status: Passed Learn more about managing compliance generic rules or creating your own custom rules
	Generic: Security-First Input Validation and Data Handling Objective: Ensure all data inputs are validated, sanitized, and handled securely to prevent vulnerabilities Status: Passed Learn more about managing compliance generic rules or creating your own custom rules
⚪	Generic: Robust Error Handling and Edge Case Management Objective: Ensure comprehensive error handling that provides meaningful context and graceful degradation Status: Broad exception catch: The new test catches a broad Exception and only prints it, which can mask unexpected failure modes and does not assert the expected exception behavior explicitly. Referred Code print("\n[TEST] Executing runner.run() with failing run_shell...") try: runner.run(scenario, 1) except Exception as e: print(f"[INFO] Caught expected exception: {e}") Learn more about managing compliance generic rules or creating your own custom rules
	Generic: Secure Error Handling Objective: To prevent the leakage of sensitive system information through error messages while providing sufficient detail for internal debugging. Status: Exception printed to stdout: The test prints the caught exception message to stdout, which could expose internal error details into CI logs depending on what exceptions include in real runs. Referred Code print("\n[TEST] Executing runner.run() with failing run_shell...") try: runner.run(scenario, 1) except Exception as e: print(f"[INFO] Caught expected exception: {e}") Learn more about managing compliance generic rules or creating your own custom rules
	Generic: Secure Logging Practices Objective: To ensure logs are useful for debugging and auditing without exposing sensitive information like PII, PHI, or cardholder data. Status: Unstructured console output: The test introduces multiple print(...) statements which produce unstructured log output and may inadvertently include sensitive details if exception messages change. Referred Code print("\n[TEST] Executing runner.run() with failing run_shell...") try: runner.run(scenario, 1) except Exception as e: print(f"[INFO] Caught expected exception: {e}") # Verification print("[VERIFY] Checking if HealthCheckWatcher.stop() was called...") # Assert that stop was called mock_watcher.stop.assert_called_once() print("[SUCCESS] stop() WAS called. Thread leak prevented.") Learn more about managing compliance generic rules or creating your own custom rules

Compliance status legend

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🟡 - Partial Compliant
🔴 - Not Compliant
⚪ - Requires Further Human Verification
🏷️ - Compliance label

[qodo-code-review]

PR Code Suggestions ✨

Explore these optional code suggestions:

Category	Suggestion	Impact
Possible issue	Ensure cleanup on watcher startup failure Move the health_check_watcher.run() call inside the try block. This ensures health_check_watcher.stop() is always called in the finally block, even if the watcher fails to start, preventing potential resource leaks. krkn_ai/chaos_engines/krkn_runner.py [115-130] -# Start watching application urls for health checks -health_check_watcher.run() +try: + # Start watching application urls for health checks + health_check_watcher.run() -try: # Run command (show logs when verbose mode is enabled) log, returncode = run_shell( self.process_es_env_string(command, True), do_not_log=not is_verbose() ) # Extract return code from run log which is part of telemetry data present in the log returncode, run_uuid = self.__extract_returncode_from_run(log, returncode) logger.info("Krkn scenario return code: %d", returncode) finally: # Stop watching application urls for health checks health_check_watcher.stop() Apply / Chat Suggestion importance[1-10]: 8 __ Why: This suggestion correctly identifies a resource leak edge case where health_check_watcher.run() could fail, which is not covered by the PR. Applying this change makes the cleanup logic more robust, fully addressing the class of issue the PR intends to fix.	Medium
General	Use idiomatic exception testing pattern In test_run_shell_exception_cleanup, replace the try...except block with the with self.assertRaises(Exception): context manager. This ensures the test correctly fails if the expected exception is not raised, making the test more robust. tests/test_krkn_runner_leak.py [41-51] print("\n[TEST] Executing runner.run() with failing run_shell...") -try: +with self.assertRaises(Exception) as cm: runner.run(scenario, 1) -except Exception as e: - print(f"[INFO] Caught expected exception: {e}") +print(f"[INFO] Caught expected exception: {cm.exception}") # Verification print("[VERIFY] Checking if HealthCheckWatcher.stop() was called...") # Assert that stop was called mock_watcher.stop.assert_called_once() Apply / Chat Suggestion importance[1-10]: 7 __ Why: The suggestion correctly points out a flaw in the new test case and proposes using the idiomatic self.assertRaises context manager. This improves the test's reliability by ensuring it fails if the expected exception is not raised.	Medium
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[rh-rahulshetty]

LGTM! Thank you