From e7453f14bc972f1206144f1155ea3dc52469ea95 Mon Sep 17 00:00:00 2001
From: Nikolay Edigaryev <edigaryev@gmail.com>
Date: Fri, 15 May 2020 17:18:47 -0400
Subject: [PATCH] Ensure "alembic current" won't unnecessarily mutate the
 database

The ``alembic current`` command no longer creates an ``alembic_version``
table in the database if one does not exist already, returning no version
as the current version. This allows checking for migrations in parallel
without introducing race conditions.  Pull request courtesy Nikolay
Edigaryev.

Fixes: #694
Closes: #695
Pull-request: https://github.com/sqlalchemy/alembic/pull/695
Pull-request-sha: fd3e3b8faf7a41dd4c35daca6c7d224e983ab496

Change-Id: I500ab9ec1fe74b5e20e6aecfe598bce7e9cdef96
---
 alembic/command.py            |  4 +++-
 alembic/operations/base.py    |  1 -
 alembic/runtime/migration.py  |  4 +++-
 docs/build/unreleased/694.rst | 10 ++++++++++
 tests/test_command.py         |  9 +++++++++
 5 files changed, 25 insertions(+), 3 deletions(-)
 create mode 100644 docs/build/unreleased/694.rst

diff --git a/alembic/command.py b/alembic/command.py
index 7d19e3c6..c925b18b 100644
--- a/alembic/command.py
+++ b/alembic/command.py
@@ -511,7 +511,9 @@ 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/operations/base.py b/alembic/operations/base.py
index 30bd87fe..86b50ca5 100644
--- a/alembic/operations/base.py
+++ b/alembic/operations/base.py
@@ -127,7 +127,6 @@ class Operations(util.ModuleClsProxy):
                     "args": args,
                     "apply_kw": apply_kw,
                     "doc": fn.__doc__,
-                    "meth": fn.__name__,
                 }
             )
             globals_ = {"op_cls": op_cls}
diff --git a/alembic/runtime/migration.py b/alembic/runtime/migration.py
index 48408a4f..5c8590d6 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)
diff --git a/docs/build/unreleased/694.rst b/docs/build/unreleased/694.rst
new file mode 100644
index 00000000..0d62b28f
--- /dev/null
+++ b/docs/build/unreleased/694.rst
@@ -0,0 +1,10 @@
+.. change::
+    :tags: bug, commands
+    :tickets: 694
+
+    The ``alembic current`` command no longer creates an ``alembic_version``
+    table in the database if one does not exist already, returning no version
+    as the current version. This allows checking for migrations in parallel
+    without introducing race conditions.  Pull request courtesy Nikolay
+    Edigaryev.
+
diff --git a/tests/test_command.py b/tests/test_command.py
index 4102f29f..a616e9cc 100644
--- a/tests/test_command.py
+++ b/tests/test_command.py
@@ -17,6 +17,7 @@ from alembic.script import ScriptDirectory
 from alembic.testing import assert_raises
 from alembic.testing import assert_raises_message
 from alembic.testing import eq_
+from alembic.testing import is_false
 from alembic.testing import is_true
 from alembic.testing import mock
 from alembic.testing.env import _get_staging_directory
@@ -33,6 +34,7 @@ from alembic.testing.fixtures import capture_context_buffer
 from alembic.testing.fixtures import capture_engine_context_buffer
 from alembic.testing.fixtures import TestBase
 from alembic.util import compat
+from alembic.util.sqla_compat import _connectable_has_table
 
 
 class _BufMixin(object):
@@ -201,6 +203,7 @@ finally:
 class CurrentTest(_BufMixin, TestBase):
     @classmethod
     def setup_class(cls):
+        cls.bind = _sqlite_file_db()
         cls.env = env = staging_env()
         cls.cfg = _sqlite_testing_config()
         cls.a1 = env.generate_revision("a1", "a1")
@@ -232,6 +235,12 @@ class CurrentTest(_BufMixin, TestBase):
 
         eq_(lines, set(revs))
 
+    def test_doesnt_create_alembic_version(self):
+        command.current(self.cfg)
+        engine = self.bind
+        with engine.connect() as conn:
+            is_false(_connectable_has_table(conn, "alembic_version", None))
+
     def test_no_current(self):
         command.stamp(self.cfg, ())
         with self._assert_lines([]):
-- 
2.47.2