Python: Keep a dropped run's finalizer from discarding state writes

[digit50]

Motivation & Context

closes #9473

RunnerImpl.run_until_convergence discards pending state in the finally of every superstep that did not commit, but the State object is shared by every run on a Workflow. If a stream is dropped mid superstep and a new run starts before the old generator is finalized, the late discard() removes the new run's staged writes. The new run still ends IDLE with its outputs, so the write is lost silently.

Description & Review Guide

• What are the major changes? The runner records a token for the superstep it is running and the finally only calls discard() when the finishing superstep still owns that token. Cancelling the iteration and event tasks is unchanged. This mirrors how owns_run in Workflow._run_core keeps a stale finalizer from clearing a successor's run lock and runtime storage.
• What is the impact of these changes? A failed or cancelled superstep still discards its own pending writes. A superstep from a dropped run that is finalized after a newer superstep has started no longer touches the newer run's writes. A new run also discards any pending writes left by a dropped run before it commits its own run kwargs, so a half-finished superstep is never committed into the successor run's state.
• What do you want reviewers to focus on? Whether scoping by superstep (rather than by run) is the right granularity for the token.

I added test_workflow_partial_stream_does_not_discard_successor_state_writes next to test_workflow_partial_stream_does_not_clobber_successor_active_run. It drops stream A while its executor has staged a write and is blocked, starts run B, which stages b_key, lets A's deferred finalizer run, then releases B. It checks that B finishes IDLE with its output and that b_key is committed. Without the change the last assertion fails with assert None == 'from-run-B'.

Related Issue

See the link under Motivation & Context.

Contribution Checklist

• The code builds clean without any errors or warnings
• All unit tests pass, and I have added new tests where possible
• The PR follows the Contribution Guidelines
• This PR links to an agreed issue with no competing open PR, or the Related Issue section documents a trivial-change or repository-automation exception.
• This is not a breaking change. If it is a breaking change, add the breaking change label (or add "[BREAKING]" to the title prefix, before or after any language prefix).

[Copilot]

🟡 Changes recommended

Ownership transfers too late to protect successor entry-checkpoint snapshots from stale cleanup.

1 open issue

• Transfer checkpoint ownership before successor preparation · New

What changed in this PR

Addresses #9473 by guarding shared workflow state against cleanup from a dropped stream.

Changes:

• Adds a superstep ownership token to guard pending-state discard.
• Adds regression coverage for preserving a successor run’s state writes.

File	Description
python/​packages/​core/​tests/​workflow/​test_workflow.py	Tests successor state preservation after deferred stream cleanup.
python/​packages/​core/​agent_framework/​_workflows/​_runner.py	Restricts discard to the owning superstep.

_{Review effort: Balanced}

[digit50]

You are right: the successor's entry checkpoint stages executor snapshots before its first superstep takes ownership, so the dropped run's finalizer could discard them while a save hook was suspended, and the entry checkpoint then held the previous run's snapshot. The new commit has checkpoint preparation take ownership of pending writes before it calls the executors' save hooks, so a stale finalizer no longer matches and leaves them alone. The test test_workflow_partial_stream_does_not_discard_successor_entry_checkpoint_snapshots drops a stream mid-superstep, suspends the second executor's on_checkpoint_save during the next run's entry checkpoint while the stale finalizer runs, and asserts the entry checkpoint stores the new writer snapshot.

[digit50]

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[Somay kousis (Somay-kousis)]

The token fix works for what it targets: run B's writes are no longer thrown away by A's late finally.

What I ran:

• Your two new tests against main's _runner.py: both fail (assert None == 'from-run-B', assert {'saves': 1} == {'saves': 2}). On your head both pass.

One thing in the same race that the PR doesn't cover, worth considering. The description says a failed or cancelled superstep still discards its own pending writes. In the scenario from your first test, A's write is never discarded. It gets committed when B starts, because _run_core calls self._runner.state.commit() right after setting the run kwargs (_workflow.py:636), and A's uncommitted a_key is still pending at that point. B's executor also reads it during its superstep.

Probe: your first test plus a read of a_key from B's handler and a dump of State at the end:

== your head
B saw a_key during its superstep: from-run-A
committed after B: b_key = from-run-B | a_key = from-run-A
== main
B saw a_key during its superstep: from-run-A
committed after B: b_key = None | a_key = from-run-A

So it's not new, main does it too. But after this PR the dropped run's half-finished superstep ends up committed next to B's writes, which is the opposite of what the description says.

I tried a small change on your branch: discard pending writes at the start of _run_core, before the run kwargs are set and committed:

# Pending writes here can only come from a superstep that never committed
# (e.g. a dropped stream whose finalizer has not run yet); do not commit them below.
self._runner.state.discard()

Results with it: the probe prints B saw a_key ...: None and committed after B: b_key = from-run-B | a_key = None. I'm not sure there is no legitimate case of pending writes at run start that I'm missing, so treat this as a suggestion. If you take it, an assert workflow._runner.state.get("a_key") is None at the end of your first test would cover it.

[digit50]

Good catch, thanks. A dropped run's half-finished superstep is now thrown away when the next run starts, before that run
commits its own setup. Your probe now prints B saw a_key during its superstep: None and
committed after B: b_key = from-run-B | a_key = None.
I checked for a legitimate case of pending writes at run start and found none: they can only come from a superstep or a
checkpoint save that never committed, and checkpoint restores already clear pending writes. The discard sits right
before the run's kwargs commit, so it also covers cancel_pending_requests, which starts a run through the same path.
The dropped-stream test now also asserts state.get("a_key") is None after run B.