Prove workflow-to-CHASM backfill cursor replay

[chaptersix]

Problem

Workflow-backed schedulers store each ongoing backfill's StartTime as an exclusive cursor. The CHASM migration converter currently clears the corresponding LastProcessedTime, so CHASM treats the transferred range as fresh and generates the already-processed boundary again.

Violated invariant

Migration must preserve the remaining occurrence set (exclusive cursor, inclusive end]. Every occurrence already accounted for by V1 must remain excluded, while every later occurrence must remain actionable.

Minimal reproduction

The deterministic test uses a one-minute schedule, cursor 2026-08-01T11:59:00Z, and end 12:00:00Z:

• Native CHASM continuation emits only 12:00.
• Migrated continuation emits 11:59 and 12:00.

Ordinary CI runs the native control and skips the known counterexample. Expose it with:

TEMPORAL_RUN_MIGRATION_COUNTEREXAMPLES=1 go test -tags test_dep ./chasm/lib/scheduler \
  -run '^TestMigrationBackfill_MigratedResumesAfterWatermark$' -count=1

Evidence

• Canonical bug note
• Deterministic regression and native control

The production trace is convertBackfillsLegacyToCHASM → BackfillerTaskHandler.processBackfill: the converter clears progress, then the backfiller subtracts one millisecond for a fresh inclusive range.

Scope

This layer contains evidence only; it intentionally makes no production change. The opposite CHASM-to-workflow fresh-range boundary is covered by upstream PR temporalio#11878 and is not duplicated here.

Validation

go test -tags test_dep ./chasm/lib/scheduler -run '^TestMigrationBackfill_' -count=1
TEMPORAL_RUN_MIGRATION_COUNTEREXAMPLES=1 go test -tags test_dep ./chasm/lib/scheduler \
  -run '^TestMigrationBackfill_MigratedResumesAfterWatermark$' -count=1

Stack

• Base: main
• Follow-up fix: Preserve workflow-to-CHASM backfill migration safety #43 (migration-backfill/preserve-cursor)

Rendered failure and sequence artifacts are attached below.

[chatgpt-codex-connector]

Codex Review Summary

This comment shows the latest Codex review activity on this pull request.

Review	Status	Commit	Review trigger
📝 Code Review	✅ Completed 2026-09-05T01:02:38.365158Z	c9b3351	Draft marked ready

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