Mutex the declarative scan/map process against configure_mappers()

Applied the mapper "configure mutex" against the declarative class mapping
process, to guard against the race which can occur if mappers are used
while dynamic module import schemes are still in the process of configuring
mappers for related classes.  This does not guard against all possible race
conditions, such as if the concurrent import has not yet encountered the
dependent classes as of yet, however it guards against as much as possible
within the SQLAlchemy declarative process.

Fixes: #4686
Change-Id: I0349036b8078bd42265ab40862cfbfe5bf9d5b44

1 files changed, 12 insertions, 6 deletions

diff --git a/lib/sqlalchemy/ext/declarative/base.py b/lib/sqlalchemy/ext/declarative/base.py
index 62db282d1..622a83736 100644
--- a/lib/sqlalchemy/ext/declarative/base.py
+++ b/lib/sqlalchemy/ext/declarative/base.py
@@ -17,6 +17,7 @@ from ... import util
 from ...orm import class_mapper
 from ...orm import exc as orm_exc
 from ...orm import mapper
+from ...orm import mapperlib
 from ...orm import synonym
 from ...orm.attributes import QueryableAttribute
 from ...orm.base import _is_mapped_class
@@ -155,6 +156,7 @@ class _MapperConfig(object):
             cfg_cls = _DeferredMapperConfig
         else:
             cfg_cls = _MapperConfig
+
         cfg_cls(cls_, classname, dict_)
 
     def __init__(self, cls_, classname, dict_):
@@ -177,17 +179,21 @@ class _MapperConfig(object):
 
         self._scan_attributes()
 
-        clsregistry.add_class(self.classname, self.cls)
+        mapperlib._CONFIGURE_MUTEX.acquire()
+        try:
+            clsregistry.add_class(self.classname, self.cls)
 
-        self._extract_mappable_attributes()
+            self._extract_mappable_attributes()
 
-        self._extract_declared_columns()
+            self._extract_declared_columns()
 
-        self._setup_table()
+            self._setup_table()
 
-        self._setup_inheritance()
+            self._setup_inheritance()
 
-        self._early_mapping()
+            self._early_mapping()
+        finally:
+            mapperlib._CONFIGURE_MUTEX.release()
 
     def _early_mapping(self):
         self.map()