Task Instance model: allow pool names >50 chars (apache#15203)

* Task Instance model: allow pool names >50 chars

* Add nullability in migration

airflow/migrations/versions/90d1635d7b86_increase_pool_name_size_in_taskinstance.py

@@ -0,0 +1,46 @@
+#
+# Licensed to the Apache Software Foundation (ASF) under one
+# or more contributor license agreements.  See the NOTICE file
+# distributed with this work for additional information
+# regarding copyright ownership.  The ASF licenses this file
+# to you under the Apache License, Version 2.0 (the
+# "License"); you may not use this file except in compliance
+# with the License.  You may obtain a copy of the License at
+#
+#   http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing,
+# software distributed under the License is distributed on an
+# "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+# KIND, either express or implied.  See the License for the
+# specific language governing permissions and limitations
+# under the License.
+
+"""Increase pool name size in TaskInstance
+
+Revision ID: 90d1635d7b86
+Revises: 2e42bb497a22
+Create Date: 2021-04-05 09:37:54.848731
+
+"""
+
+import sqlalchemy as sa
+from alembic import op
+
+# revision identifiers, used by Alembic.
+revision = '90d1635d7b86'
+down_revision = '2e42bb497a22'
+branch_labels = None
+depends_on = None
+
+
+def upgrade():
+    """Apply Increase pool name size in TaskInstance"""
+    with op.batch_alter_table('task_instance') as batch_op:
+        batch_op.alter_column('pool', type_=sa.String(256), nullable=False)
+
+
+def downgrade():
+    """Unapply Increase pool name size in TaskInstance"""
+    with op.batch_alter_table('task_instance') as batch_op:
+        batch_op.alter_column('pool', type_=sa.String(50), nullable=False)

airflow/models/taskinstance.py

@@ -272,7 +272,7 @@ class TaskInstance(Base, LoggingMixin):  # pylint: disable=R0902,R0904
     hostname = Column(String(1000))
     unixname = Column(String(1000))
     job_id = Column(Integer)
-    pool = Column(String(50), nullable=False)
+    pool = Column(String(256), nullable=False)
     pool_slots = Column(Integer, default=1, nullable=False)
     queue = Column(String(256))
     priority_weight = Column(Integer)