Keep the server error when the connection closes mid-operation

asyncpg/protocol/protocol.pyx

@@ -716,9 +716,19 @@ cdef class BaseProtocol(CoreProtocol):
 
     cdef _handle_waiter_on_connection_lost(self, cause):
         if self.waiter is not None and not self.waiter.done():
-            exc = apg_exc.ConnectionDoesNotExistError(
-                'connection was closed in the middle of '
-                'operation')
+            msg = 'connection was closed in the middle of operation'
+            if (self.result_type == RESULT_FAILED and
+                    isinstance(self.result, dict)):
+                # The server sent an ErrorResponse and then closed the
+                # connection without a ReadyForQuery (a FATAL error, or
+                # pgbouncer's query_wait_timeout).  Do not lose it.
+                server_exc = apg_exc_base.PostgresError.new(
+                    self.result, query=self.last_query)
+                if cause is not None:
+                    server_exc.__cause__ = cause
+                cause = server_exc
+                msg = '{}: {}'.format(msg, server_exc.args[0])
+            exc = apg_exc.ConnectionDoesNotExistError(msg)
             if cause is not None:
                 exc.__cause__ = cause
             self.waiter.set_exception(exc)

tests/test_execute.py

@@ -98,6 +98,32 @@ async def test_execute_script_interrupted_terminate(self):
 
         self.con.terminate()
 
+    async def test_execute_script_interrupted_by_server(self):
+        # The server reports why it is closing the connection with an
+        # ErrorResponse and then closes it without a ReadyForQuery (this
+        # is also what pgbouncer does on query_wait_timeout).  The reported
+        # error must not be lost.
+        pid = await self.con.fetchval('SELECT pg_backend_pid()')
+        fut = self.loop.create_task(
+            self.con.execute('''SELECT pg_sleep(10)'''))
+
+        await asyncio.sleep(0.2)
+
+        other = await self.connect()
+        try:
+            await other.execute('SELECT pg_terminate_backend($1)', pid)
+        finally:
+            await other.close()
+
+        with self.assertRaisesRegex(
+                asyncpg.ConnectionDoesNotExistError,
+                'closed in the middle of operation: '
+                'terminating connection'):
+            await fut
+
+        exc = fut.exception()
+        self.assertIsInstance(exc.__cause__, exceptions.AdminShutdownError)
+
 
 class TestExecuteMany(tb.ConnectedTestCase):
     def setUp(self):