Fix sync_compatible resolution in synchronous transaction hooks

[hsusul]

Summary

This PR fixes an issue where @sync_compatible functions (such as Block.load or prefect.variables.get) returned unawaited coroutine objects instead of synchronous values when called inside synchronous transaction hooks (on_rollback, on_commit, on_completion) during an asynchronous flow run.

Problem

When a synchronous transaction hook (e.g. @task.on_rollback) is executed during an async flow, the transaction runner executes the hook in a synchronous thread (or directly as a sync function call). However, @sync_compatible functions inspect the enclosing run context (FlowRunContext), see that the parent flow is asynchronous (flow.isasync == True), and assume that the caller will await the returned coroutine. Because synchronous hook functions are not async def and cannot await coroutines, the hook receives a raw unawaited coroutine object (causing AttributeError or TypeError when accessing properties on the returned value).

Root cause

1. sync_compatible checked parent_obj.isasync without checking if the current thread has an active event loop or if RUNNING_ASYNC_FLAG was explicitly set to False.
2. RUNNING_ASYNC_FLAG defaulted to False, making RUNNING_ASYNC_FLAG.get() is False indistinguishable from the default unset state.
3. Transaction hook handlers did not set RUNNING_ASYNC_FLAG to False when invoking synchronous hook callables.

Fix

1. Updated RUNNING_ASYNC_FLAG default to None so that RUNNING_ASYNC_FLAG.get() is False uniquely identifies contexts explicitly marked as synchronous.
2. Updated sync_compatible to verify that an active event loop exists in the calling thread and respect RUNNING_ASYNC_FLAG.get() is False.
3. Updated Transaction.run_hook and AsyncTransaction.run_hook to execute synchronous hooks with RUNNING_ASYNC_FLAG.set(False) (using run_sync_in_worker_thread for async transaction runners).

Testing

• uv run pytest tests/test_transactions.py tests/utilities/test_asyncutils.py — Passed (148 passed, 8 skipped).
• Verified deterministic reproduction script repros/17656.py fails before the fix and passes cleanly after the fix.

Compatibility

Preserves backward compatibility across all client and server APIs.

Related issue

Closes #17656

[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Keep synchronous transaction hooks shielded from cancellation

When an AsyncTransaction is unwound inside an active cancellation scope (for example, during a flow or task timeout), this helper uses abandon_on_cancel=True, so cancellation stops awaiting the synchronous commit or rollback hook while its worker thread continues running. The previous anyio.to_thread.run_sync call retained the default shielding behavior and waited for the hook to finish; this change can therefore abort rollback()/commit() and reset() while cleanup side effects continue after the transaction context has unwound. Preserve the prior non-abandoning cancellation behavior while propagating the synchronous context flag.

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[codspeed-hq]

Merging this PR will not alter performance

✅ 8 untouched benchmarks
⏩ 1 skipped benchmark¹

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_{Comparing hsusul:fix-transactions-sync-rollback-hooks (68b3847) with main (d2d873b)}

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. ↩