mutex asyncpg / aiomysql connection state changes

Added an ``asyncio.Lock()`` within SQLAlchemy's emulated DBAPI cursor,
local to the connection, for the asyncpg dialect, so that the space between
the call to ``prepare()`` and ``fetch()`` is prevented from allowing
concurrent executions on the connection from causing interface error
exceptions, as well as preventing race conditions when starting a new
transaction. Other PostgreSQL DBAPIs are threadsafe at the connection level
so this intends to provide a similar behavior, outside the realm of server
side cursors.

Apply the same idea to the aiomysql dialect which also would
otherwise be subject to corruption if the connection were used
concurrently.

While this is an issue which can also occur with the threaded
connection libraries, we anticipate asyncio users are more likely
to attempt using the same connection in multiple awaitables
at a time, even though this won't achieve concurrency for that
use case, as the asyncio programming style is very encouraging
of this.  As the failure modes are also more complicated under
asyncio, we'd rather not have this being reported.

Fixes: #5967
Change-Id: I3670ba0c8f0b593c587c5aa7c6c61f9e8c5eb93a

1 files changed, 24 insertions, 14 deletions

diff --git a/lib/sqlalchemy/dialects/mysql/aiomysql.py b/lib/sqlalchemy/dialects/mysql/aiomysql.py
index 6c968a1e7..cab6df499 100644
--- a/lib/sqlalchemy/dialects/mysql/aiomysql.py
+++ b/lib/sqlalchemy/dialects/mysql/aiomysql.py
@@ -35,6 +35,7 @@ handling.
 from .pymysql import MySQLDialect_pymysql
 from ... import pool
 from ... import util
+from ...util.concurrency import asyncio
 from ...util.concurrency import await_fallback
 from ...util.concurrency import await_only
 
@@ -84,24 +85,32 @@ class AsyncAdapt_aiomysql_cursor:
         self._rows[:] = []
 
     def execute(self, operation, parameters=None):
-        if parameters is None:
-            result = self.await_(self._cursor.execute(operation))
-        else:
-            result = self.await_(self._cursor.execute(operation, parameters))
-
-        if not self.server_side:
-            # aiomysql has a "fake" async result, so we have to pull it out
-            # of that here since our default result is not async.
-            # we could just as easily grab "_rows" here and be done with it
-            # but this is safer.
-            self._rows = list(self.await_(self._cursor.fetchall()))
-        return result
+        return self.await_(self._execute_async(operation, parameters))
 
     def executemany(self, operation, seq_of_parameters):
         return self.await_(
-            self._cursor.executemany(operation, seq_of_parameters)
+            self._executemany_async(operation, seq_of_parameters)
         )
 
+    async def _execute_async(self, operation, parameters):
+        async with self._adapt_connection._execute_mutex:
+            if parameters is None:
+                result = await self._cursor.execute(operation)
+            else:
+                result = await self._cursor.execute(operation, parameters)
+
+            if not self.server_side:
+                # aiomysql has a "fake" async result, so we have to pull it out
+                # of that here since our default result is not async.
+                # we could just as easily grab "_rows" here and be done with it
+                # but this is safer.
+                self._rows = list(await self._cursor.fetchall())
+            return result
+
+    async def _executemany_async(self, operation, seq_of_parameters):
+        async with self._adapt_connection._execute_mutex:
+            return await self._cursor.executemany(operation, seq_of_parameters)
+
     def setinputsizes(self, *inputsizes):
         pass
 
@@ -161,11 +170,12 @@ class AsyncAdapt_aiomysql_ss_cursor(AsyncAdapt_aiomysql_cursor):
 
 class AsyncAdapt_aiomysql_connection:
     await_ = staticmethod(await_only)
-    __slots__ = ("dbapi", "_connection")
+    __slots__ = ("dbapi", "_connection", "_execute_mutex")
 
     def __init__(self, dbapi, connection):
         self.dbapi = dbapi
         self._connection = connection
+        self._execute_mutex = asyncio.Lock()
 
     def ping(self, reconnect):
         return self.await_(self._connection.ping(reconnect))