[Prism] Guard ClearOrderedListState against uninitialized state maps

[prodriguezdefino]

Problem:
In TentativeData.ClearOrderedListState (sdks/go/pkg/beam/runners/prism/internal/engine/data.go), Prism indexed directly into d.state[stateID][w] and wrote the cleared slice back to kMap[string(uKey)] without checking whether d.state, d.state[stateID], or winMap[w] had been initialized:

    winMap := d.state[stateID]
    w := d.toWindow(wKey)
    kMap := winMap[w]
    data := kMap[string(uKey)]
    ...
    kMap[string(uKey)] = StateData{Bag: cleared} // panic: assignment to entry in nil map

SDK harnesses (such as the Python SDK's SynchronousOrderedListRuntimeState.commit) buffer ordered-list mutations in memory during a bundle and flush pending StateRequest_Clear range requests before StateRequest_Append requests at bundle commit. When a bundle clears an ordered-list state cell (or range) in a window for which no prior bundle has persisted state on the runner, TentativeData.state has no map for (stateID, window), kMap is nil, and Prism panics on line 336.

Fix:
Use the existing d.clearState(stateID, wKey) helper in ClearOrderedListState and return early when kMap is nil, matching ClearBagState, ClearMultimapState, and ClearMultimapKeysState.

Reproduction (Python SDK on PrismRunner):

import apache_beam as beam
from apache_beam.coders import VarIntCoder
from apache_beam.options.pipeline_options import PipelineOptions
from apache_beam.testing.util import assert_that, equal_to
from apache_beam.transforms.userstate import OrderedListStateSpec
from apache_beam.utils.timestamp import Timestamp

class ClearOrderedListDoFn(beam.DoFn):
    EVENTS = OrderedListStateSpec("events", VarIntCoder())

    def process(self, element, events=beam.DoFn.StateParam(EVENTS)):
        key, (ts, op) = element
        if op == "CLEAR":
            events.clear()
            yield f"cleared:{key}"
        else:
            events.add((Timestamp(micros=ts), op))
            entries = [f"{int(t.micros)}={v}" for t, v in events.read()]
            yield f"entry:{key}:{','.join(entries)}"

if __name__ == "__main__":
    options = PipelineOptions(["--runner=PrismRunner", "--environment_type=LOOPBACK"])
    with beam.Pipeline(options=options) as p:
        out = (
            p
            | beam.Create([("k", (30, 300)), ("k", (10, 100)), ("k", (0, "CLEAR"))])
            | beam.ParDo(ClearOrderedListDoFn())
        )
        assert_that(
            out,
            equal_to(["entry:k:30=300", "entry:k:10=100,30=300", "cleared:k"]),
        )

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 56.22%. Comparing base (76f6e6d) to head (ca7a555).
⚠️ Report is 200 commits behind head on master.

Additional details and impacted files

@@             Coverage Diff              @@
##             master   #40626      +/-   ##
============================================
+ Coverage     56.16%   56.22%   +0.05%     
============================================
  Files          1124      868     -256     
  Lines        178336   153512   -24824     
  Branches       1489        0    -1489     
============================================
- Hits         100170    86305   -13865     
+ Misses        75645    65367   -10278     
+ Partials       2521     1840     -681     

Flag	Coverage Δ
go	28.64% <100.00%> (-0.50%)	⬇️
java	?
python	79.93% <ø> (+0.37%)	⬆️

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[github-actions]

📋 Review Summary

This PR fixes a nil map panic in Prism's TentativeData.ClearOrderedListState when clearing ordered list state for an uninitialized state ID or window. By using the existing d.clearState(stateID, wKey) helper and returning early when the key map is nil, the implementation aligns with ClearBagState, ClearMultimapState, and ClearMultimapKeysState.

🔍 General Feedback

• The fix is clean, idiomatic, and consistent with the rest of TentativeData's state clearing methods.
• Unit tests in data_test.go thoroughly cover the uninitialized state map cases (d.state == nil, missing stateID, and missing window entry followed by an append).

---

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[github-actions]

Assigning reviewers:

R: @jrmccluskey for label go.
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[jrmccluskey]

LGTM, thanks!