Add support for nested context manager use

This commit adds support for nested use of the UsePrimaryDB context manager. If a new instance of the context manager was created at each level we could use instance variables to store the pinning state but the class object is generally used directly. Instead, we store a stack of pinning values locally in the thread and pop each value as we unwind the stack.

multidb/pinning.py

@@ -34,7 +34,7 @@ def unpin_this_thread():
 
 
 class UsePrimaryDB(object):
-    """A contextmanager/decorator to use the master database."""
+    """A contextmanager/decorator to use the primary database."""
     def __call__(self, func):
         @wraps(func)
         def decorator(*args, **kw):
@@ -43,11 +43,12 @@ def decorator(*args, **kw):
         return decorator
 
     def __enter__(self):
-        _locals.old = this_thread_is_pinned()
+        _locals.old = getattr(_locals, 'old', [])
+        _locals.old.append(this_thread_is_pinned())
         pin_this_thread()
 
     def __exit__(self, type, value, tb):
-        if not _locals.old:
+        if not _locals.old.pop():
             unpin_this_thread()
 
 

multidb/tests.py

@@ -202,6 +202,22 @@ def check():
         check()
         assert not this_thread_is_pinned()
 
+    def test_decorator_nested(self):
+        @use_primary_db
+        def check_inner():
+            assert this_thread_is_pinned()
+
+        @use_primary_db
+        def check_outer():
+            assert this_thread_is_pinned()
+            check_inner()
+            assert this_thread_is_pinned()
+
+        unpin_this_thread()
+        assert not this_thread_is_pinned()
+        check_outer()
+        assert not this_thread_is_pinned()
+
     def test_decorator_resets(self):
         @use_primary_db
         def check():
@@ -211,20 +227,56 @@ def check():
         check()
         assert this_thread_is_pinned()
 
+    def test_decorator_resets_nested(self):
+        @use_primary_db
+        def check_inner():
+            assert this_thread_is_pinned()
+
+        @use_primary_db
+        def check_outer():
+            assert this_thread_is_pinned()
+            check_inner()
+            assert this_thread_is_pinned()
+
+        pin_this_thread()
+        assert this_thread_is_pinned()
+        check_outer()
+        assert this_thread_is_pinned()
+
     def test_context_manager(self):
         unpin_this_thread()
         assert not this_thread_is_pinned()
         with use_primary_db:
             assert this_thread_is_pinned()
         assert not this_thread_is_pinned()
 
+    def test_context_manager_nested(self):
+        unpin_this_thread()
+        assert not this_thread_is_pinned()
+        with use_primary_db:
+            assert this_thread_is_pinned()
+            with use_primary_db:
+                assert this_thread_is_pinned()
+            assert this_thread_is_pinned()
+        assert not this_thread_is_pinned()
+
     def test_context_manager_resets(self):
         pin_this_thread()
         assert this_thread_is_pinned()
         with use_primary_db:
             assert this_thread_is_pinned()
         assert this_thread_is_pinned()
 
+    def test_context_manager_resets_nested(self):
+        pin_this_thread()
+        assert this_thread_is_pinned()
+        with use_primary_db:
+            assert this_thread_is_pinned()
+            with use_primary_db:
+                assert this_thread_is_pinned()
+            assert this_thread_is_pinned()
+        assert this_thread_is_pinned()
+
     def test_context_manager_exception(self):
         unpin_this_thread()
         assert not this_thread_is_pinned()