fix: Preserve original exception cause in timeout context managers (closes #16299)

[botbikamordehai2-sketch]

What

When a timeout occurs in the timeout_async or timeout context managers, the original CancelledError is caught and re-raised as a generic TimeoutError, losing the underlying cause (e.g., an asyncpg database timeout). This makes debugging difficult, as seen in issue #16299 where database communication timeouts are masked.

Fix

Use raise ... from e to chain the original CancelledError as the cause of the new timeout_exc_type exception. This preserves the full traceback and allows users to see the original error that triggered the timeout.

Closes #16299

[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Preserve the cause for sync timeouts too

This change only chains the CancelledError in timeout_async, but the sync timeout context manager below still raises an unchained TimeoutError. Sync tasks use timeout directly (src/prefect/task_engine.py:1008) and sync flows select it when self.flow.isasync is false (src/prefect/flow_engine.py:1274), so those timeout failures continue to lose the underlying cancellation cause this fix is meant to preserve. Please mirror the except CancelledError as e / raise ... from e change in the sync path as well.

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[codspeed-hq]

Merging this PR will not alter performance

✅ 8 untouched benchmarks
⏩ 1 skipped benchmark¹

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_{Comparing botbikamordehai2-sketch:fix/issue-16299-1781182461 (46b5e40) with main (4c3d694)}

Footnotes

1. 1 benchmark was skipped, so the baseline result was used instead. If it was deleted from the codebase, click here and archive it to remove it from the performance reports. ↩

[zzstoatzz]

hi @botbikamordehai2-sketch thanks for opening a PR

this seems incomplete and afaict over-claims the fix. it only chains the async timeout_async path; sync timeout is still the same, so sync flow/task timeouts would still lose their cancellation cause.

also, #16299 appears to be about DB latency/locking/server starvation more than timeout traceback formatting. this may improve traceback readability, but I don’t think it resolves the linked issue. I’d suggest adding sync coverage + tests asserting __cause__

[github-actions]

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