Do not attempt to apply internal migrations if already up to date

This removes unnecessary calls to backend.lock() which were causing concurrency
related tests to fail.

2 files changed, 12 insertions, 8 deletions

diff --git a/yoyo/backends.py b/yoyo/backends.py
index 0548412..c0a92e7 100644
--- a/yoyo/backends.py
+++ b/yoyo/backends.py
@@ -360,11 +360,12 @@ class DatabaseBackend(object):
         """
         if self._internal_schema_updated:
             return
-        assert not self._in_transaction
-        with self.lock():
-            internalmigrations.upgrade(self)
-            self.connection.commit()
-            self._internal_schema_updated = True
+        if internalmigrations.needs_upgrading(self):
+            assert not self._in_transaction
+            with self.lock():
+                internalmigrations.upgrade(self)
+                self.connection.commit()
+                self._internal_schema_updated = True
 
     def is_applied(self, migration):
         return migration.hash in self.get_applied_migration_hashes()
diff --git a/yoyo/internalmigrations/__init__.py b/yoyo/internalmigrations/__init__.py
index 30c495f..fee2c49 100644
--- a/yoyo/internalmigrations/__init__.py
+++ b/yoyo/internalmigrations/__init__.py
@@ -10,6 +10,7 @@ from . import v2
 
 #: Mapping of {schema version number: module}
 schema_versions = {
+    0: None,
     1: v1,
     2: v2,
 }
@@ -19,6 +20,10 @@ schema_versions = {
 USE_VERSION_TABLE_FROM = 2
 
 
+def needs_upgrading(backend):
+    return get_current_version(backend) < max(schema_versions)
+
+
 def upgrade(backend, version=None):
     """
     Check the currently installed yoyo migrations version and update the
@@ -29,8 +34,6 @@ def upgrade(backend, version=None):
     else:
         desired_version = version
     current_version = get_current_version(backend)
-    if current_version is None:
-        current_version = 0
     with backend.transaction():
         while current_version < desired_version:
             next_version = current_version + 1
@@ -46,7 +49,7 @@ def get_current_version(backend):
     tables = set(backend.list_tables())
     version_table = backend.version_table
     if backend.migration_table not in tables:
-        return None
+        return 0
     if version_table not in tables:
         return 1
     with backend.transaction():