fix(revert): "fix: add missing context switches for repeated state processing"#4552
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### What changes were proposed in this PR? Restores reliable state-output emission for Python operators after the #4552 revert. After this PR, both per-input-state outputs (`Operator.process_state(...)`) and the end-of-input-port output (`Operator.produce_state_on_finish(...)`) reach downstream channels. `MainLoop.process_input_state` previously did two `_switch_context()` calls with the read of `current_output_state` in between. The executor only writes that field during the *second* switch — so `MainLoop` always captured the previous cycle's value, and the finish-state set on `EndChannel` ended up in `current_output_state` after `MainLoop` had returned, never to be read again. This PR collapses the read to a single switch + read-after, drops the duplicate post-init and end-of-body switches in `DataProcessor.run`, and makes the run-loop's input dispatch peek-then-consume so `current_internal_marker` keeps the atomic single-consume semantics whose absence was the root cause of #4545. <details> <summary>History — third attempt at this fix</summary> - #4421 reported that a Python operator could process its first state input but not its second. - PR #4424 added three `_switch_context()` calls to keep `MainLoop` and `DataProcessor` in sync, closed #4421, but changed `current_internal_marker` lifetime and broke the source-propagation case in `ReconfigurationSpec` (#4545). - PR #4547 tried to restore atomic marker consumption on top of #4424 and re-enabled the source-propagation case in `ReconfigurationSpec`. CI continued to fail. - PR #4552 reverted #4424 outright as a stop-gap. State-processing is back to its pre-#4424 broken state — see #4559. </details> ### Any related issues, documentation, discussions? Fixes #4559. Follow-up to #4421 / #4424 / #4545 / #4547 / #4552. ### How was this PR tested? Existing `core/runnables/test_main_loop.py` tests pass unchanged. Added three new tests: - `test_process_state_can_emit_multiple_states` — stub-level coverage of the #4421 "second state not processed" scenario. - `test_main_loop_thread_can_process_state` — full real-thread coverage of state DataElements and `produce_state_on_finish` on `EndChannel`. Times out on plain `main` (#4559); passes on this branch. - `test_main_loop_thread_can_process_state_after_tuple` — coverage for the mixed `tuple → state` input sequence. `ReconfigurationSpec`'s source-propagation case (re-enabled in #4547) should be re-run on this branch to confirm the new handshake does not re-introduce #4545. ### Was this PR authored or co-authored using generative AI tooling? Generated-by: Anthropic Claude Opus 4.7
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Reverts #4424 since it might change the lifecycle of the Python engine.
Might be related to issue #4545
We don't know if #4424 really causes the issue, but reverting it for testing purposes.