Add migration for username column db schema + data

lib/galaxy/model/migrations/alembic/versions_gxy/c63848676caf_update_username_column_schema_and_data.py

@@ -0,0 +1,53 @@
+"""Update username column schema and data
+
+Revision ID: c63848676caf
+Revises: c14a3c93d66b
+Create Date: 2024-06-11 13:41:36.411803
+
+"""
+
+from alembic import op
+from sqlalchemy import (
+    or_,
+    select,
+    update,
+)
+
+from galaxy.managers.users import username_from_email_with_connection
+from galaxy.model import User
+from galaxy.model.migrations.util import (
+    alter_column,
+    transaction,
+)
+
+# revision identifiers, used by Alembic.
+revision = "c63848676caf"
+down_revision = "c14a3c93d66b"
+branch_labels = None
+depends_on = None
+
+table_name = "galaxy_user"
+column_name = "username"
+
+
+def upgrade():
+    with transaction():
+        _generate_missing_usernames()
+        alter_column(table_name, column_name, nullable=False)
+
+
+def downgrade():
+    with transaction():
+        # The data migration is one-way: we can change back the database schema,
+        # but we can't remove the generated username values.
+        alter_column(table_name, column_name, nullable=True)
+
+
+def _generate_missing_usernames():
+    stmt = select(User.id, User.email).where(or_(User.username.is_(None), User.username == ""))
+    connection = op.get_bind()
+    users = connection.execute(stmt).all()
+    for id, email in users:
+        new_username = username_from_email_with_connection(connection, email)
+        stmt = update(User).where(User.id == id).values(username=new_username)
+        connection.execute(stmt)