Move DAG._schedule_interval logic out of DAG.__init__

[kaxil]

closes #8166

• Replace usage of DAG._schedule_interval with DAG.normalized_schedule_interval. This will even allow users to tests that they set correct Cron Preset in unit tests (instead of using an internal property before)
• Added tests for Schedule Interval & Normalized Schedule Interval
• Also solves the issue mentioned in Task Duration Bug: Doesn't respect number of runs #8166
• Removes confusion between schedule_interval and _schedule_interval

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[kaxil]

Please feel free to suggest a better name for the PR title/commit which looks good for the Changelog.

[kaxil]

Maybe @cached_property is better ?

[ashb]

Maybe, the downside to cached_property would be if dag.schedule_interval is changed, this parameter would be out of sync.

We could do it as:

    @property
    def normalized_schedule_interval(self) -> Optional[ScheduleInterval]:
        return self._normalized_schedule_interval(self.schedule_interval)

    @functools.lru_cache(maxsize=1)
    def _normalized_schedule_interval(self, schedule_interval) -> Optional[ScheduleInterval]: 
        ...

Might be overkill.

[ashb]

Change looks good. Up to you if you want to do the cached_property/lru_cache or not.

[codecov-io]

Codecov Report

Merging #8225 into master will decrease coverage by 0.72%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #8225      +/-   ##
==========================================
- Coverage   88.31%   87.58%   -0.73%     
==========================================
  Files         936      936              
  Lines       45272    45319      +47     
==========================================
- Hits        39980    39694     -286     
- Misses       5292     5625     +333     

Impacted Files	Coverage Δ
airflow/models/dag.py	91.68% <100.00%> (+0.03%)	⬆️
airflow/models/dagbag.py	89.83% <100.00%> (ø)
...flow/providers/apache/cassandra/hooks/cassandra.py	21.25% <0.00%> (-72.50%)	⬇️
...w/providers/apache/hive/operators/mysql_to_hive.py	35.84% <0.00%> (-64.16%)	⬇️
airflow/kubernetes/volume_mount.py	44.44% <0.00%> (-55.56%)	⬇️
airflow/providers/postgres/operators/postgres.py	47.82% <0.00%> (-52.18%)	⬇️
airflow/providers/redis/operators/redis_publish.py	50.00% <0.00%> (-50.00%)	⬇️
airflow/kubernetes/volume.py	52.94% <0.00%> (-47.06%)	⬇️
airflow/providers/mongo/sensors/mongo.py	53.33% <0.00%> (-46.67%)	⬇️
airflow/kubernetes/pod_launcher.py	47.18% <0.00%> (-45.08%)	⬇️
... and 15 more

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[KostyaEsmukov]

@kaxil thank you for this fix! I have backported it to 1.10 and applied on my installation (instead of a patch in #8166 (comment)) and can confirm that it indeed fixes the issue.

Will this be backported to v1-10?

[kaxil]

Thanks for confirming @KostyaEsmukov

Yes this will be backported to v1-10 and would
be available in 1.10.11 :)

[KostyaEsmukov]

Great, thank you!