fix: serialize concurrent cancel requests with a per-task lock - #1185
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ez-lbz wants to merge 2 commits into
Open
fix: serialize concurrent cancel requests with a per-task lock#1185ez-lbz wants to merge 2 commits into
ez-lbz wants to merge 2 commits into
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🧪 Code Coverage (vs
|
| Base | PR | Delta | |
|---|---|---|---|
| src/a2a/server/events/event_queue_v2.py | 91.79% | 91.28% | 🔴 -0.51% |
| src/a2a/server/request_handlers/default_request_handler.py | 98.13% | 98.26% | 🟢 +0.12% |
| src/a2a/utils/telemetry.py | 91.47% | 90.70% | 🔴 -0.78% |
| Total | 93.00% | 92.99% | ⚪️ -0.00% |
Generated by coverage-comment.yml
The ref-counted per-task cancel lock was typed as a heterogeneous list (`list[asyncio.Lock | int]`), which the type checker cannot narrow by index (entry[0] vs entry[1]). Replace it with a small dataclass carrying the lock and the in-flight user count, and add the TOCTOU acronym to the spellcheck allow list.
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What changed
1. Serialize the cancel check-then-act with a per-task lock
Problem:
LegacyRequestHandler.on_cancel_task()(src/a2a/server/request_handlers/default_request_handler.py) performed "load task → check terminal state → callagent_executor.cancel()" without any synchronization. Two concurrent cancels of the same task could both pass the terminal-state check and both issue a cancel, duplicating the cancellation side effects (TOCTOU race).Fix (src/a2a/server/request_handlers/default_request_handler.py):
_cancel_locks) so the entire check-then-act critical section inon_cancel_taskis atomic per task. The first cancel to acquire the lock performs the cancel; any concurrent cancel waits, then re-reads the task state and fails withTaskNotCancelableErrorif the task is now terminal.DefaultRequestHandlerV2is unaffected: cancels there are already serialized perActiveTaskbyActiveTaskRegistry/ActiveTask.Testing
./.venv/Scripts/python -m pytest tests/server/request_handlers/test_default_request_handler.py -q→ 75 passed (includes newtest_on_cancel_task_serializes_concurrent_cancels: two concurrent cancels, the first blocks inside the executor, and the second must observe the resulting terminal state and raiseTaskNotCancelableErrorwithout ever reaching the executor; also asserts the per-task lock entry is cleaned up afterwards)../.venv/Scripts/python -m ruff checkon modified files: clean (the pre-existingtoo-many-positional-argumentsfinding on the handler__init__exists onmain).TaskNotCancelableErrorinstead of issuing a duplicate cancel.