[PR] Fix scheduler refusing to start when [scheduler] only_idle is enabled [airflow]

["Eason09053360 (via GitHub)" <gi...@apache.org>]

Eason09053360 opened a new pull request, #73141:
URL: https://github.com/apache/airflow/pull/73141

   ## Why
   
   Enabling the documented `[scheduler] only_idle` option leaves the deployment without a scheduler. `ARG_ONLY_IDLE` resolved its default from config at import time, so the guard in `scheduler_command.scheduler()` could not tell an explicitly typed `--only-idle` from a config-supplied default. Since `[scheduler] num_runs` defaults to `-1`, the guard tripped on every invocation:
   
   ```console
   $ AIRFLOW__SCHEDULER__ONLY_IDLE=True airflow scheduler
   The --only-idle flag requires --num-runs to be set to a positive number.   # exit 1
   ```
   
   The same import-time default made the option impossible to switch off: `action="store_true"` only pushes towards `True`, so once config enabled it, no command line could get back to `False` — the opposite of the usual command-line-beats-config precedence.
   
   The guard itself is only about usability. `SchedulerJobRunner` already ignores `only_idle` when the run limit is not positive (`run_count >= self.num_runs > 0`), so a non-positive `num_runs` is inert rather than dangerous.
   
   ## What
   
   `airflow-core/src/airflow/cli/cli_config.py` — `ARG_ONLY_IDLE` becomes tri-state: `default=None` with `argparse.BooleanOptionalAction`, so `None` means "user said nothing", and `--no-only-idle` can override a config-enabled default.
   
   `airflow-core/src/airflow/cli/commands/scheduler_command.py` — `_resolve_only_idle()` reads `[scheduler] only_idle` when the command runs instead of when the parser is built. An explicit `--only-idle` without a positive `--num-runs` still raises `SystemExit` with the same message; the config-supplied case now logs a warning and starts. The warning names both `--num-runs` and `[scheduler] num_runs`, since the non-positive limit can come from either.
   
   `airflow-core/src/airflow/config_templates/config.yml` — record that `only_idle` has no effect unless `num_runs` is positive.
   
   Tests cover the three-way flag, that a config-enabled `only_idle` starts the scheduler and reaches `SchedulerJobRunner`, and that `--no-only-idle` wins over config. `test_only_idle_help_survives_rich_markup_rendering` asserts against rendered `--help` output rather than the raw `help=` kwarg, because the Rich-based help formatter silently swallows `[section]` text — the reason the help string spells the option `scheduler.only_idle`.
   
   ---
   
   ##### Was generative AI tooling used to co-author this PR?
   
   - [X] Yes — Claude Code (Opus 5)
   
   Generated-by: Claude Code (Opus 5) following [the guidelines](https://github.com/apache/airflow/blob/main/contributing-docs/05_pull_requests.rst#gen-ai-assisted-contributions)
   


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Re: [PR] Fix scheduler refusing to start when [scheduler] only_idle is enabled [airflow]

["Eason09053360 (via GitHub)" <gi...@apache.org>]

Eason09053360 commented on PR #73141:
URL: https://github.com/apache/airflow/pull/73141#issuecomment-5673165857

   > LGTM overall, could you please add a newsfragment?
   
   Added — `airflow-core/newsfragments/73141.bugfix.rst` . 
   Thanks for the review!
   


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Re: [PR] Fix scheduler refusing to start when [scheduler] only_idle is enabled [airflow]

["Eason09053360 (via GitHub)" <gi...@apache.org>]

Eason09053360 commented on PR #73141:
URL: https://github.com/apache/airflow/pull/73141#issuecomment-5844211444

   cc @shahar1 friendly ping — I’ve added the newsfragment and rebased onto ‎`main`. Would you mind taking another look when you get a chance? 
   All checks are green. Thanks for reviewing!


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