alembic current command shouldn't mutate the database

[edigaryev]

I have a case where two types of Kubernetes applications are using the same database schema. Before they are started, the following happens:

• multiple writers run alembic current in loop waiting for migrations to be applied
• single reader runs alembic upgrade head before it starts

This "locking" mechanism causes problems due to how alembic current is implemented. It's description says:

Display the current revision for a database.

However besides displaying the revision it also tries to create an empty alembic_version table, which may cause race conditions when multiple alembic current commands are being run in parallel:

[...]
sqlalchemy.exc.IntegrityError: (psycopg2.errors.UniqueViolation) duplicate key value violates unique constraint "pg_type_typname_nsp_index"
DETAIL:  Key (typname, typnamespace)=(alembic_version, 2200) already exists.

In fact, it does not make sense to create anything since the output after creation looks like this (the same amount of information could be provided without creating anything):

$ alembic current
INFO  [alembic.runtime.migration] Context impl PostgresqlImpl.
INFO  [alembic.runtime.migration] Will assume transactional DDL.

It would be nice if alembic current would implement an early exit if alembic_version table does not exist.

[zzzeek]

agree, the creation of the table is part of a fixed workflow and some effort would need to be taken to figure out how to break this cycle in the case of a command like "current". some flag would need to be passed thorugh. any interest on working on a PR ?

[zzzeek]

this should do it :

diff --git a/alembic/command.py b/alembic/command.py
index 7d19e3c..34c1075 100644
--- a/alembic/command.py
+++ b/alembic/command.py
@@ -511,7 +511,7 @@ def current(config, verbose=False, head_only=False):
 
         return []
 
-    with EnvironmentContext(config, script, fn=display_version):
+    with EnvironmentContext(config, script, fn=display_version, dont_mutate=True):
         script.run_env()
 
 
diff --git a/alembic/runtime/migration.py b/alembic/runtime/migration.py
index 48408a4..5c8590d 100644
--- a/alembic/runtime/migration.py
+++ b/alembic/runtime/migration.py
@@ -497,7 +497,9 @@ class MigrationContext(object):
         else:
             heads = self.get_current_heads()
 
-            if not self.as_sql and not heads:
+            dont_mutate = self.opts.get("dont_mutate", False)
+
+            if not self.as_sql and not heads and not dont_mutate:
                 self._ensure_version_table()
 
         head_maintainer = HeadMaintainer(self, heads)

[edigaryev]

any interest on working on a PR ?

Yes, but it seems that you've already nailed it :)

I've opened a PR (#695) and added a simple test for this behavior.

[sqla-tester]

Nikolay Edigaryev has proposed a fix for this issue in the master branch:

Ensure "alembic current" won't unnecessarily mutate the database https://gerrit.sqlalchemy.org/c/sqlalchemy/alembic/+/1970

[dogweather]

I'm getting this error in CI (GitHub) with v. 1.12.1 inside a docker container. Anyone know if it was resolved? My workaround:

docker compose exec web alembic current
docker compose exec web alembic upgrade head
docker compose exec web alembic current

[CaselIT]

Can you open a new issue. Your workaround will create the table so I'm not sure it's the same issue