[I] Issue relativedelta midnight serialization [airflow]

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

azharizz opened a new issue, #70527:
URL: https://github.com/apache/airflow/issues/70527

   ### Under which category would you file this issue?
   
   Airflow Core
   
   ### Apache Airflow version
   
   Airflow 3.x
   
   ### What happened and how to reproduce it?
   
   Serializing a `dateutil.relativedelta` that uses absolute time fields set to `0` (e.g. `hour=0` for midnight) drops those fields. After the Dag is deserialized, the schedule no longer snaps to midnight and keeps the base datetime's clock time instead.
   
   Root cause is in `encode_relativedelta` (`airflow-core/src/airflow/serialization/encoders.py`):
   
   ```python
   encoded = {k: v for k, v in var.__dict__.items() if not k.startswith("_") and v}
   ```
   
   The `and v` filter treats every falsy value as "unset". That is correct for **relative** fields (`hours`, `days`, …), which default to `0`, but wrong for **absolute** fields (`hour`, `minute`, `second`, `day`, …), which default to `None`. For absolute fields, `0` is meaningful like e.g. `hour=0` means "snap to midnight".
   
   **Minimal repro:**
   
   ```python
   from datetime import datetime
   from dateutil.relativedelta import relativedelta, FR
   from airflow.serialization.serialized_objects import BaseSerialization
   
   delta = relativedelta(weekday=FR, hour=0, minute=0, second=0)
   base = datetime(2024, 1, 10, 15, 30, 45)
   
   print(base + delta)
   # expected: 2024-01-12 00:00:00
   
   round_tripped = BaseSerialization.deserialize(BaseSerialization.serialize(delta))
   print(base + round_tripped)
   # actual today:   2024-01-12 15:30:45  (hour/minute/second lost)
   ```
   
   Same bug hits any path that serializes intervals via `encode_relativedelta` / `encode_interval` (Dag schedules, timetables, serialized deltas).
   
   Example Dag schedule that silently changes meaning after parse → serialize → scheduler:
   
   ```python
   from datetime import datetime
   from dateutil.relativedelta import relativedelta, FR
   from airflow.sdk import DAG
   
   with DAG(
       dag_id="example_friday_midnight",
       start_date=datetime(2024, 1, 1),
       schedule=relativedelta(weekday=FR, hour=0, minute=0, second=0),
       catchup=False,
   ):
       ...
   ```
   
   After serialization, the absolute `hour`/`minute`/`second` fields are gone, so runs are planned at the previous interval's wall-clock time instead of 00:00:00.
   
   
   
   ### What you think should happen instead?
   
   This is for the example:
   Absolute fields with value `0` must survive the encode → decode round-trip. Relative fields that are `0` can still be dropped (they mean "unset").
   
   Expected encode for `relativedelta(weekday=FR, hour=0)`:
   
   ```python
   {"hour": 0, "weekday": [4]}
   ```
   
   not only `{"weekday": [4]}`.
   
   ### Operating System
   
   Debian
   
   ### Deployment
   
   Other
   
   ### Apache Airflow Provider(s)
   
   _No response_
   
   ### Versions of Apache Airflow Providers
   
   _No response_
   
   ### Official Helm Chart version
   
   main (development)
   
   ### Kubernetes Version
   
   _No response_
   
   ### Helm Chart configuration
   
   _No response_
   
   ### Docker Image customizations
   
   _No response_
   
   ### Anything else?
   
   _No response_
   
   ### Are you willing to submit PR?
   
   - [x] Yes I am willing to submit a PR!
   
   ### Code of Conduct
   
   - [x] I agree to follow this project's [Code of Conduct](https://github.com/apache/airflow/blob/main/CODE_OF_CONDUCT.md)
   


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