fix concurrency issues of parallel transactions (#327)

databases/core.py

@@ -15,8 +15,10 @@
 
 if sys.version_info >= (3, 7):  # pragma: no cover
     from contextvars import ContextVar
+    import contextvars as contextvars
 else:  # pragma: no cover
     from aiocontextvars import ContextVar
+    import aiocontextvars as contextvars
 
 try:  # pragma: no cover
     import click
@@ -173,21 +175,34 @@ async def iterate(
             async for record in connection.iterate(query, values):
                 yield record
 
+    def _new_connection(self) -> "Connection":
+        connection = Connection(self._backend)
+        self._connection_context.set(connection)
+        return connection
+
     def connection(self) -> "Connection":
         if self._global_connection is not None:
             return self._global_connection
 
         try:
             return self._connection_context.get()
         except LookupError:
-            connection = Connection(self._backend)
-            self._connection_context.set(connection)
-            return connection
+            return self._new_connection()
 
     def transaction(
         self, *, force_rollback: bool = False, **kwargs: typing.Any
     ) -> "Transaction":
-        return Transaction(self.connection, force_rollback=force_rollback, **kwargs)
+        try: 
+            connection = self._connection_context.get()
+            if not connection._transaction_stack:
+                newcontext = contextvars.copy_context()
+                get_conn  = lambda: newcontext.run(self._new_connection)
+            else:
+                get_conn = self.connection
+        except LookupError:
+            get_conn = self.connection
+
+        return Transaction(get_conn, force_rollback=force_rollback, **kwargs)
 
     @contextlib.contextmanager
     def force_rollback(self) -> typing.Iterator[None]:
@@ -357,6 +372,7 @@ async def wrapper(*args: typing.Any, **kwargs: typing.Any) -> typing.Any:
     async def start(self) -> "Transaction":
         self._connection = self._connection_callable()
         self._transaction = self._connection._connection.transaction()
+        logger.warning(self._connection)
 
         async with self._connection._transaction_lock:
             is_root = not self._connection._transaction_stack

tests/test_databases.py

@@ -800,7 +800,7 @@ async def test_queries_with_expose_backend_connection(database_url):
     """
     async with Database(database_url) as database:
         async with database.connection() as connection:
-            async with database.transaction(force_rollback=True):
+            async with connection.transaction(force_rollback=True):
                 # Get the raw connection
                 raw_connection = connection.raw_connection
 
@@ -996,3 +996,20 @@ async def test_column_names(database_url, select_query):
             assert sorted(results[0].keys()) == ["completed", "id", "text"]
             assert results[0]["text"] == "example1"
             assert results[0]["completed"] == True
+
+@pytest.mark.parametrize("database_url", DATABASE_URLS)
+@async_adapter
+async def test_parallel_transactions(database_url):
+    """
+    Test parallel transaction execution. 
+    """
+    async def test_task(db): 
+        async with db.transaction():
+            await db.fetch_one("SELECT 1")
+
+    async with Database(database_url) as database:
+        await database.fetch_one("SELECT 1")
+
+        tasks = [test_task(database) for i in range(4)]
+        await asyncio.gather(*tasks)
+