Make the default TI pool slots '1'

[kaxil]

This is caused by the new add pool_slots field to task_instance
database migration, which adds a new pool_slots database column,
which is nullable (hence it is filled with NULLs after the migration).

This Bug was introduced in #7160 and is a blocker for 1.10.10

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[kaxil]

I have tested this manually on my machine. I installed 1.10.9 ran a few examples DAGs, upgraded to 1.10.10rc2, Visited old Task Instance details of old tasks and was able to see the error, also could see the error in Scheduler if the TIs were still queued before I upgraded to 1.10.10rc2 and applied migrations. I applied the fix in the PR and ran the Webserver and scheduler again and both ran fine and I was able to see Task Instance details.

Note: I am not updating all the Null values in TI table for old records as this table can be pretty huge which was also the intention in the original PR: #7160 to keep old behaviour and let the column be Nullable.

[codecov-io]

Codecov Report

Merging #8153 into master will decrease coverage by 5.83%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #8153      +/-   ##
==========================================
- Coverage   66.39%   60.55%   -5.84%     
==========================================
  Files         935      935              
  Lines       45170    45170              
==========================================
- Hits        29990    27353    -2637     
- Misses      15180    17817    +2637     

Impacted Files	Coverage Δ
airflow/models/taskinstance.py	94.70% <100.00%> (+31.88%)	⬆️
airflow/providers/exasol/operators/exasol.py	0.00% <0.00%> (-100.00%)	⬇️
airflow/providers/amazon/aws/hooks/kinesis.py	0.00% <0.00%> (-100.00%)	⬇️
airflow/providers/apache/livy/sensors/livy.py	0.00% <0.00%> (-100.00%)	⬇️
airflow/providers/amazon/aws/sensors/redshift.py	0.00% <0.00%> (-100.00%)	⬇️
airflow/providers/mysql/operators/s3_to_mysql.py	0.00% <0.00%> (-100.00%)	⬇️
airflow/providers/postgres/operators/postgres.py	0.00% <0.00%> (-100.00%)	⬇️
airflow/providers/microsoft/azure/operators/adx.py	0.00% <0.00%> (-100.00%)	⬇️
...irflow/providers/amazon/aws/hooks/batch_waiters.py	0.00% <0.00%> (-100.00%)	⬇️
...ow/providers/amazon/aws/sensors/cloud_formation.py	0.00% <0.00%> (-100.00%)	⬇️
... and 803 more

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[zhongjiajie]

Thanks @kaxil , quick response, code looking good. One more thing I am sure. Is it the new TI object column pool_slots with default value 1?

[ashb]

Is this the right fix, or is changing the migration to have a default of 1 better?

[kaxil]

Thanks @kaxil , quick response, code looking good. One more thing I am sure. Is it the new TI object column pool_slots with default value 1?

Yes

[kaxil]

Is this the right fix, or is changing the migration to have a default of 1 better?

@ashb I didn't update the migration code as the TI table can be large for many users (which the original PR intended #7160 ). I was thinking we could do that as part of Airflow 2.0 to update NULLs to 1 and make the column not nullable. WDYT?

[ashb]

Is this the right fix, or is changing the migration to have a default of 1 better?

@ashb I didn't update the migration code as the TI table can be large for many users (which the original PR intended #7160 ). I was thinking we could do that as part of Airflow 2.0 to update NULLs to 1 and make the column not nullable. WDYT?

Oh yeah, adding a new column with non null default would be a table-locking migration, good point.