Resolve cancel scope error in MCP session cleanup with lifetime task

[AnuradhaKaruppiah]

Description

Previously, session cleanup was calling client.aexit() from a different task context than where aenter() was called, violating anyio's CancelScope requirement that enter and exit must happen in the same task. This caused "Attempted to exit cancel scope in a different task" errors during session cleanup.

Solution:

• Introduce a per-client lifetime task that manages the entire client lifecycle
• The lifetime task enters the client context (async with client:) and waits for a stop_event signal before exiting
• Session cleanup now signals the stop_event and waits for the lifetime task to complete, ensuring aexit runs in the correct task context

This ensures proper cancel scope handling and prevents resource leaks while maintaining thread-safe session management.

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• When the PR is ready for review, new or existing tests cover these changes.
• When the PR is ready for review, the documentation is up to date with these changes.

Summary by CodeRabbit

• Bug Fixes

  • Sessions now shut down cleanly (stop signaling and fallback cleanup), reducing hangs and timeouts; inactive sessions are reliably removed.

• Refactor

  • Session lifecycle reworked to include explicit stop events and managed background lifetime tasks for more robust session management.

• Chores

  • Initialization and cleanup wiring improved to avoid orphaned tasks and lower resource use during idle/long runs.

• Tests

  • Test harness updated with session cleanup helpers to ensure per-session tasks are torn down and improve isolation.

[coderabbitai]

Walkthrough

Session lifecycle expanded: SessionData gains stop_event and lifetime_task; _create_session_client now returns (client, stop_event, lifetime_task) and starts a background lifetime task that manages the client's async context; _get_session_client stores these; _cleanup_inactive_sessions signals stop_event and awaits or falls back to direct close, then removes sessions.

Changes

Cohort / File(s)	Change Summary
Session lifecycle management packages/nvidia_nat_mcp/src/nat/plugins/mcp/client_impl.py	SessionData now includes stop_event: asyncio.Event and `lifetime_task: asyncio.Task
Session cleanup flow packages/nvidia_nat_mcp/src/nat/plugins/mcp/client_impl.py	_cleanup_inactive_sessions collects sessions to close, sets each session's stop_event (if present), awaits the lifetime_task when available, falls back to calling client._close() if no lifetime task exists, and removes sessions from tracking regardless of cleanup success.
Tests: session teardown helper tests/nat/mcp/test_mcp_session_management.py	Added cleanup_sessions(function_group) helper and imported SessionData; tests now call the helper to stop per-session tasks (stop_event, lifetime_task) and clear the in-memory _sessions mapping to prevent leaks between tests.

Sequence Diagram(s)

sequenceDiagram
    autonumber
    participant Caller
    participant ClientImpl as MCP ClientImpl
    participant Factory as _create_session_client
    participant Lifetime as lifetime_task
    participant MCP as MCPBaseClient

    Caller->>ClientImpl: request session client
    ClientImpl->>Factory: _create_session_client(session_id)
    activate Factory
    Factory->>MCP: instantiate client
    Factory->>Lifetime: spawn _lifetime(client, stop_event)
    Factory-->>ClientImpl: (client, stop_event, lifetime_task)
    deactivate Factory
    ClientImpl->>ClientImpl: store SessionData(client, stop_event, lifetime_task)
    ClientImpl-->>Caller: return client

    Note over Lifetime,MCP: _lifetime runs "async with client", awaits `stop_event`

    Caller->>ClientImpl: trigger cleanup inactive sessions
    ClientImpl->>ClientImpl: collect SessionData to_close
    ClientImpl->>Lifetime: stop_event.set()
    alt lifetime_task present
      ClientImpl->>Lifetime: await lifetime_task
    else no lifetime_task
      ClientImpl->>MCP: call client._close() (fallback)
    end
    Lifetime->>MCP: __aexit__() (client closed)
    Lifetime-->>ClientImpl: task completed

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Estimated code review effort

🎯 3 (Moderate) | ⏱️ ~25 minutes

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breaking

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Title Check	✅ Passed	The title uses imperative mood to summarize the primary fix of the cancel scope error by introducing a lifetime task in MCP session cleanup, is under the 72-character limit, and accurately reflects the core changes described in the pull request.
Docstring Coverage	✅ Passed	Docstring coverage is 90.00% which is sufficient. The required threshold is 80.00%.

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🧬 Code graph analysis (1)

tests/nat/mcp/test_mcp_session_management.py (1)

packages/nvidia_nat_mcp/src/nat/plugins/mcp/client_impl.py (6)

• SessionData (40-49)
• cleanup_sessions (186-199)
• _create_session_client (347-413)
• _cleanup_inactive_sessions (201-256)
• _get_session_client (258-314)
• _session_usage_context (317-345)

🪛 Ruff (0.13.3)

tests/nat/mcp/test_mcp_session_management.py

599-599: Unused function argument: self

(ARG001)

---

607-607: Pattern passed to match= contains metacharacters but is neither escaped nor raw

(RUF043)

---

644-644: Unused function argument: self

(ARG001)

---

650-650: Unused function argument: self

(ARG001)

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🔇 Additional comments (12)

tests/nat/mcp/test_mcp_session_management.py (12)

28-28: LGTM!

The import of SessionData is necessary for tests that directly construct session data objects to verify lifecycle management.

---

34-50: LGTM with robust cleanup logic.

The helper method properly manages session cleanup by:

• Signaling stop events for each session's lifetime task
• Implementing timeout handling to prevent indefinite waits
• Gracefully handling task cancellation
• Ensuring the sessions mapping is cleared

This approach ensures test isolation and prevents resource leaks between tests.

---

127-127: LGTM - Proper test cleanup.

The cleanup calls ensure test isolation by properly stopping lifetime tasks and clearing sessions after each test. This prevents resource leaks and ensures tests don't interfere with each other.

Also applies to: 148-148, 174-174, 199-199, 251-251, 281-281, 308-308, 364-364, 414-414, 454-454, 552-552, 580-580, 675-675, 802-802, 844-844

---

554-581: LGTM - Comprehensive lifecycle verification.

The test properly verifies:

• Successful client initialization with lifetime task
• Correct types for returned values (client, stop_event, lifetime_task)
• aenter invocation during initialization
• Proper cleanup via stop_event signaling

---

582-592: LGTM - Error handling verification.

The test correctly verifies that initialization failures in __aenter__ are properly propagated with descriptive error messages.

---

610-632: LGTM - Stop event mechanism verification.

The test properly verifies:

• Lifetime task responds to stop_event signal
• Task completes after stop_event is set
• aexit is called with correct arguments (None, None, None) indicating normal exit

---

633-676: LGTM - Critical test for cancel scope compliance.

This test validates the core objective of the PR: ensuring __aenter__ and __aexit__ execute in the same task to respect anyio's CancelScope requirements. The verification of matching task IDs and proper task naming provides strong evidence that the fix prevents the "Attempted to exit cancel scope in a different task" error.

Note: The unused self parameters on lines 644 and 650 are acceptable as they match the async context manager method signatures.

---

677-705: LGTM - Cleanup integration verification.

The test properly verifies that the cleanup mechanism:

• Signals the lifetime task via stop_event
• Removes the session from tracking
• Ensures aexit is called during cleanup

---

706-739: LGTM - Active session protection.

The test correctly verifies that sessions with active references (ref_count > 0) are protected from cleanup, even when they're old. This ensures in-use sessions are not prematurely terminated.

---

740-769: LGTM - Edge case handling.

The test verifies that cleanup handles already-completed lifetime tasks gracefully, ensuring the cleanup logic is robust against race conditions where a task might complete before cleanup is triggered.

---

770-803: LGTM - End-to-end lifecycle test.

The test provides comprehensive verification of the complete session lifecycle:

• Session creation with lifetime task
• Context manager usage with proper ref_count tracking
• Graceful cleanup via stop_event

This integration test ensures all components work together correctly.

---

804-845: LGTM - Multi-session independence verification.

The test properly verifies that multiple sessions with lifetime tasks operate independently:

• Each session has its own lifecycle
• Stop events are handled independently
• Tasks complete without interfering with each other

The use of asyncio.gather with return_exceptions=True ensures both tasks complete even if one fails.

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Actionable comments posted: 3

🧹 Nitpick comments (1)

packages/nvidia_nat_mcp/src/nat/plugins/mcp/client_impl.py (1)

47-49: Consider making lifetime_task non-optional for clarity.

The type hint lifetime_task: asyncio.Task | None = None suggests it's optional, but _create_session_client always creates and returns a task (line 377-382). In practice, lifetime_task should never be None after session creation. The optional type and None default exist only for dataclass initialization, but could confuse readers about the actual invariants.

Consider either:

1. Removing the | None from the type hint and using field(init=False) if a default is needed
2. Adding a comment explaining that lifetime_task is always set during session creation and should never be None in normal operation

</review_comment_end>

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📄 CodeRabbit inference engine (.cursor/rules/nat-test-llm.mdc)

**/*.{py,yaml,yml}: Configure response_seq as a list of strings; values cycle per call, and [] yields an empty string.
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When retrieving the test LLM wrapper, use builder.get_llm(name, wrapper_type=LLMFrameworkEnum.) and call the framework’s method (e.g., ainvoke, achat, call).

**/*.py: In code comments/identifiers use NAT abbreviations as specified: nat for API namespace/CLI, nvidia-nat for package name, NAT for env var prefixes; do not use these abbreviations in documentation
Follow PEP 20 and PEP 8; run yapf with column_limit=120; use 4-space indentation; end files with a single trailing newline
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Provide Google-style docstrings for every public module, class, function, and CLI command; first line concise and ending with a period; surround code entities with backticks
Validate and sanitize all user input, especially in web or CLI interfaces
Prefer httpx with SSL verification enabled by default and follow OWASP Top-10 recommendations
Use async/await for I/O-bound work; profile CPU-heavy paths with cProfile or mprof before optimizing; cache expensive computations with functools.lru_cache or external cache; leverage NumPy vectorized operations when beneficial

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🧬 Code graph analysis (1)

packages/nvidia_nat_mcp/src/nat/plugins/mcp/client_impl.py (2)

packages/nvidia_nat_mcp/src/nat/plugins/mcp/utils.py (1)

• truncate_session_id (24-37)

packages/nvidia_nat_mcp/src/nat/plugins/mcp/client_base.py (1)

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packages/nvidia_nat_mcp/src/nat/plugins/mcp/client_impl.py (2)

297-304: LGTM!

The unpacking of the triple return value and initialization of SessionData correctly handles the new session lifecycle management pattern.

</review_comment_end>

---

342-342: LGTM!

The function signature correctly reflects the new return type with proper type hints as required by the coding guidelines.

</review_comment_end>

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Actionable comments posted: 0

🧹 Nitpick comments (1)

packages/nvidia_nat_mcp/src/nat/plugins/mcp/client_impl.py (1)

370-413: Excellent implementation of lifetime task pattern!

The lifetime task implementation correctly solves the cancel scope violation by ensuring __aenter__ and __aexit__ occur in the same task context. Key strengths:

1. Prevents initialization hang: The try/except block (lines 378-384) ensures ready.set() is called even if __aenter__ fails, preventing indefinite blocking at line 391.

2. Timeout protection: Initialization waits with a timeout (lines 388-401), preventing resource leaks from stuck clients.

3. Failure detection: Lines 404-409 properly detect and propagate initialization failures by checking if the task completed before entering the wait state.

4. Session ID privacy: Task name uses truncate_session_id() (line 386) to avoid exposing sensitive information in debugging tools.

Optional refinement: At line 401, consider using from None to clarify the timeout is a new error context unrelated to any caught exception:

     raise RuntimeError(f"Session client initialization timed out after {timeout}s")
+    # Or more explicitly:
+    raise RuntimeError(f"Session client initialization timed out after {timeout}s") from None

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**/*.{py,yaml,yml}: Configure response_seq as a list of strings; values cycle per call, and [] yields an empty string.
Configure delay_ms to inject per-call artificial latency in milliseconds for nat_test_llm.

Files:

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**/*.py

📄 CodeRabbit inference engine (.cursor/rules/nat-test-llm.mdc)

**/*.py: Programmatic use: create TestLLMConfig(response_seq=[...], delay_ms=...), add with builder.add_llm("", cfg).
When retrieving the test LLM wrapper, use builder.get_llm(name, wrapper_type=LLMFrameworkEnum.) and call the framework’s method (e.g., ainvoke, achat, call).

**/*.py: In code comments/identifiers use NAT abbreviations as specified: nat for API namespace/CLI, nvidia-nat for package name, NAT for env var prefixes; do not use these abbreviations in documentation
Follow PEP 20 and PEP 8; run yapf with column_limit=120; use 4-space indentation; end files with a single trailing newline
Run ruff check --fix as linter (not formatter) using pyproject.toml config; fix warnings unless explicitly ignored
Respect naming: snake_case for functions/variables, PascalCase for classes, UPPER_CASE for constants
Treat pyright warnings as errors during development
Exception handling: use bare raise to re-raise; log with logger.error() when re-raising to avoid duplicate stack traces; use logger.exception() when catching without re-raising
Provide Google-style docstrings for every public module, class, function, and CLI command; first line concise and ending with a period; surround code entities with backticks
Validate and sanitize all user input, especially in web or CLI interfaces
Prefer httpx with SSL verification enabled by default and follow OWASP Top-10 recommendations
Use async/await for I/O-bound work; profile CPU-heavy paths with cProfile or mprof before optimizing; cache expensive computations with functools.lru_cache or external cache; leverage NumPy vectorized operations when beneficial

Files:

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packages/*/src/**/*.py

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**/*: # Code Review Instructions

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  PEP 20 and
  PEP 8 for style guidelines.
• Check for security vulnerabilities and potential issues. - Python methods should use type hints for all parameters and return values.
  Example:

  def my_function(param1: int, param2: str) -> bool:
      pass

• For Python exception handling, ensure proper stack trace preservation:
  • When re-raising exceptions: use bare raise statements to maintain the original stack trace,
    and use logger.error() (not logger.exception()) to avoid duplicate stack trace output.
  • When catching and logging exceptions without re-raising: always use logger.exception()
    to capture the full stack trace information.

Documentation Review Instructions - Verify that documentation and comments are clear and comprehensive. - Verify that the documentation doesn't contain any TODOs, FIXMEs or placeholder text like "lorem ipsum". - Verify that the documentation doesn't contain any offensive or outdated terms. - Verify that documentation and comments are free of spelling mistakes, ensure the documentation doesn't contain any

words listed in the ci/vale/styles/config/vocabularies/nat/reject.txt file, words that might appear to be
spelling mistakes but are listed in the ci/vale/styles/config/vocabularies/nat/accept.txt file are OK.

Misc. - All code (except .mdc files that contain Cursor rules) should be licensed under the Apache License 2.0,

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• Confirm that copyright years are up-to date whenever a file is changed.

Files:

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• packages/nvidia_nat_mcp/src/nat/plugins/mcp/client_impl.py

🧬 Code graph analysis (1)

packages/nvidia_nat_mcp/src/nat/plugins/mcp/client_impl.py (2)

packages/nvidia_nat_mcp/src/nat/plugins/mcp/utils.py (1)

• truncate_session_id (24-37)

packages/nvidia_nat_mcp/src/nat/plugins/mcp/client_base.py (2)

• MCPBaseClient (128-407)
• name (598-600)

🪛 Ruff (0.13.3)

packages/nvidia_nat_mcp/src/nat/plugins/mcp/client_impl.py

398-400: Use logging.exception instead of logging.error

Replace with exception

(TRY400)

---

401-401: Within an except clause, raise exceptions with raise ... from err or raise ... from None to distinguish them from errors in exception handling

(B904)

---

401-401: Avoid specifying long messages outside the exception class

(TRY003)

---

408-408: Use logging.exception instead of logging.error

Replace with exception

(TRY400)

---

409-409: Avoid specifying long messages outside the exception class

(TRY003)

🔇 Additional comments (3)

packages/nvidia_nat_mcp/src/nat/plugins/mcp/client_impl.py (3)

47-49: LGTM! Lifetime task fields properly integrated.

The stop_event and lifetime_task fields are correctly added with appropriate defaults. The Event factory ensures each SessionData instance gets its own event object, and lifetime_task being optional handles cases where it hasn't been created yet.

---

239-256: LGTM! Cleanup properly respects cancel scope boundaries.

The cleanup logic correctly addresses the core issue by:

1. Signaling stop_event to trigger graceful shutdown in the lifetime task
2. Awaiting the lifetime task to ensure __aexit__ runs in the same task that called __aenter__
3. Including fallback handling for edge cases where the lifetime task is missing

This prevents the "Attempted to exit cancel scope in a different task" error while maintaining thread-safe session management.

---

302-309: LGTM! Session creation properly wired.

The unpacking and SessionData initialization correctly handles the new return type from _create_session_client, storing all lifecycle components needed for proper cleanup.

[coderabbitai]

Actionable comments posted: 1

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• tests/nat/mcp/test_mcp_session_management.py (12 hunks)

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**/*.{py,yaml,yml}

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**/*.{py,yaml,yml}: Configure response_seq as a list of strings; values cycle per call, and [] yields an empty string.
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• tests/nat/mcp/test_mcp_session_management.py

**/*.py

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**/*.py: Programmatic use: create TestLLMConfig(response_seq=[...], delay_ms=...), add with builder.add_llm("", cfg).
When retrieving the test LLM wrapper, use builder.get_llm(name, wrapper_type=LLMFrameworkEnum.) and call the framework’s method (e.g., ainvoke, achat, call).

**/*.py: In code comments/identifiers use NAT abbreviations as specified: nat for API namespace/CLI, nvidia-nat for package name, NAT for env var prefixes; do not use these abbreviations in documentation
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• tests/nat/mcp/test_mcp_session_management.py

tests/**/*.py

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• tests/nat/mcp/test_mcp_session_management.py

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• tests/nat/mcp/test_mcp_session_management.py

{tests/**/*.py,examples/*/tests/**/*.py}

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Files:

• tests/nat/mcp/test_mcp_session_management.py

**/*

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• tests/nat/mcp/test_mcp_session_management.py

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tests/nat/mcp/test_mcp_session_management.py (1)

packages/nvidia_nat_mcp/src/nat/plugins/mcp/client_impl.py (6)

• SessionData (40-49)
• cleanup_sessions (186-199)
• _create_session_client (347-413)
• _cleanup_inactive_sessions (201-256)
• _get_session_client (258-314)
• _session_usage_context (317-345)

🪛 Ruff (0.13.3)

tests/nat/mcp/test_mcp_session_management.py

599-599: Unused function argument: self

(ARG001)

---

607-607: Pattern passed to match= contains metacharacters but is neither escaped nor raw

(RUF043)

---

644-644: Unused function argument: self

(ARG001)

---

650-650: Unused function argument: self

(ARG001)

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🔇 Additional comments (5)

tests/nat/mcp/test_mcp_session_management.py (5)

34-49: LGTM! Well-structured test cleanup helper.

The cleanup logic properly handles the lifetime task lifecycle: signals stop_event, waits with timeout, and gracefully handles cancellation. The defensive hasattr checks ensure robustness.

---

126-127: Good test hygiene with cleanup calls.

Adding explicit cleanup at the end of tests that create sessions ensures proper resource management and prevents leaks between tests.

Also applies to: 147-148, 173-174, 198-199, 250-251, 280-281, 307-308, 363-364, 413-414, 453-454, 551-552

---

554-580: LGTM! Comprehensive test of successful initialization.

The test properly verifies the lifetime task is created, running, and that __aenter__ is called. The cleanup sequence (set stop_event, await task) is correct.

---

582-591: LGTM! Proper failure handling test.

The test correctly verifies that initialization failures are propagated with appropriate error messages.

---

610-844: Excellent comprehensive test coverage for lifetime task functionality.

These tests thoroughly cover:

• Cleanup on stop_event signal
• Cancel scope task boundaries (verifying enter/exit in same task)
• Cleanup with lifetime tasks
• Preservation of active sessions
• Handling of already-completed tasks
• Complete session lifecycle
• Multiple independent sessions

The logic is correct, assertions are appropriate, and edge cases are well covered.

Note: Static analysis warnings about unused self parameters on lines 644 and 650 are false positives—these are intentional mock method signatures that require the parameter to match the async context manager protocol.

[AnuradhaKaruppiah]

/merge