fix(cron): reset cron-delivery ContextVars to _UNSET after a job

[aldoeliacim]

What & why

run_job() sets the HERMES_CRON_AUTO_DELIVER_* ContextVars while a job runs
so the job's own send_message can detect a duplicate auto-delivery target.
The cleanup in the finally block reset them with var.set("").

The problem: in gateway/session_context.py, "" is the explicitly-cleared
state that intentionally suppresses the os.environ fallback in
get_session_env — it is not the same as the _UNSET "never set" sentinel.
So once a cron job finishes, the three auto-delivery vars stay at "" for the
lifetime of that thread/task, and every later caller in the same process reads
an empty cron-delivery value instead of falling back to os.environ.

Consequences:

• Production: in a long-lived scheduler process, a finished cron job can
  shadow a later same-thread caller's auto-delivery environment.
• Tests: it manifests as cross-file pollution — after
  tests/cron/test_scheduler.py runs, the duplicate-target detection in
  tests/tools/test_send_message_tool.py::test_cron_duplicate_target_is_skipped_and_explained
  fails with KeyError: 'skipped' because it reads the leaked empty ContextVar
  instead of the env it set. The test passes in isolation, which is the
  classic ordering-dependent symptom.

The fix

• Add session_context.reset_cron_delivery_vars(), which restores the three
  HERMES_CRON_AUTO_DELIVER_* vars to the _UNSET sentinel (re-enabling the
  os.environ fallback for later callers).
• Call it in run_job()'s finally cleanup instead of the set("") loop.

The per-job set("") at job start is deliberately unchanged: each job
should begin from a clean explicitly-empty state before its delivery target is
resolved, so a job that resolves no target can't inherit the previous job's.
Only the post-job cleanup is corrected.

How to test

# The fix in isolation:
pytest tests/gateway/test_session_env.py -q

# The ordering-dependent failure this resolves (red before, green after):
pytest tests/cron/test_scheduler.py \
       tests/tools/test_send_message_tool.py::TestSendMessageTool::test_cron_duplicate_target_is_skipped_and_explained -q

New regression tests in tests/gateway/test_session_env.py assert that
reset_cron_delivery_vars() returns the vars to _UNSET and that
get_session_env's os.environ fallback works again after a reset.

Local run: tests/cron/test_scheduler.py (136) + tests/gateway/test_session_env.py
(15, incl. 2 new) + the previously order-dependent send_message test all green
(152 passed together).

Platforms tested

Linux (Python 3.11). The change is pure ContextVar lifecycle management with no
OS-specific behavior.

Related

• Fix cron session context isolation #43370 — fixes the sibling HERMES_CRON_SESSION approval-mode leak via the
  set_session_vars/token mechanism. That PR does not touch the
  auto-delivery vars; this is the same bug class for the delivery-target path.
• Cron delivery target can be hijacked by current home channel even when deliver is explicitly set #8866 — cron delivery target reliability.

---

Rebased onto current main (2026-06-26). Conflict in tests/gateway/test_session_env.py: main added new executor tests (..._survives_default_executor_shutdown, ..._refuses_resurrection_after_shutdown) in the same region as this PR's reset_cron_delivery_vars tests; resolved by keeping both blocks. cron/scheduler.py + gateway/session_context.py auto-merged clean. Verified: session_env + cron suites = 594 passed.

[alt-glitch]

Related: #43370 (cron session-context isolation for the approval-mode/HERMES_CRON_SESSION vars) and #8866 (cron delivery-target reliability). Same _UNSET-vs-explicit-empty ContextVar bug class as the merged #17747 defensive clearing, applied to the auto-delivery path. Not a duplicate -- different vars, complementary fix.

[Copilot]

Pull request overview

This PR fixes a cron scheduler ContextVar lifecycle bug where post-job cleanup reset the cron auto-delivery ContextVars to "" (explicitly-cleared), unintentionally suppressing get_session_env’s os.environ fallback for later callers running in the same thread/task. The change restores the correct “never set” sentinel (_UNSET) after each job, preventing cross-run and cross-test pollution.

Changes:

• Add reset_cron_delivery_vars() in gateway/session_context.py to restore HERMES_CRON_AUTO_DELIVER_* ContextVars to _UNSET.
• Update cron/scheduler.py::run_job() to call reset_cron_delivery_vars() in finally instead of setting those ContextVars to "".
• Add regression tests in tests/gateway/test_session_env.py verifying _UNSET restoration and os.environ fallback behavior after reset.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 2 comments.

File	Description
gateway/session_context.py	Adds a dedicated reset helper to restore cron auto-delivery ContextVars to _UNSET (re-enabling env fallback).
cron/scheduler.py	Switches post-job cleanup to use the new reset helper to avoid leaking "" into later callers.
tests/gateway/test_session_env.py	Adds regression coverage for the sentinel vs empty-string behavior and env fallback restoration.

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.