fix: allow setting connection.read_only to same value (#1247)

Setting the read_only value of a connection to the same value as the
current value should be allowed during a transaction, as it does not
change anything. SQLAlchemy regularly does this if engine options have
been specified.

Fixes googleapis/python-spanner-sqlalchemy#493

Author: olavloite

google/cloud/spanner_dbapi/connection.py

@@ -239,7 +239,7 @@ def read_only(self, value):
         Args:
             value (bool): True for ReadOnly mode, False for ReadWrite.
         """
-        if self._spanner_transaction_started:
+        if self._read_only != value and self._spanner_transaction_started:
             raise ValueError(
                 "Connection read/write mode can't be changed while a transaction is in progress. "
                 "Commit or rollback the current transaction and try again."

google/cloud/spanner_v1/database.py

@@ -47,6 +47,7 @@
 from google.cloud.spanner_v1 import TransactionSelector
 from google.cloud.spanner_v1 import TransactionOptions
 from google.cloud.spanner_v1 import RequestOptions
+from google.cloud.spanner_v1 import SpannerAsyncClient
 from google.cloud.spanner_v1 import SpannerClient
 from google.cloud.spanner_v1._helpers import _merge_query_options
 from google.cloud.spanner_v1._helpers import (
@@ -143,6 +144,7 @@ class Database(object):
     """
 
     _spanner_api = None
+    _spanner_async_api: SpannerAsyncClient = None
 
     def __init__(
         self,
@@ -438,6 +440,28 @@ def spanner_api(self):
             )
         return self._spanner_api
 
+    @property
+    def spanner_async_api(self):
+        if self._spanner_async_api is None:
+            client_info = self._instance._client._client_info
+            client_options = self._instance._client._client_options
+            if self._instance.emulator_host is not None:
+                channel = grpc.aio.insecure_channel(target=self._instance.emulator_host)
+                transport = SpannerGrpcTransport(channel=channel)
+                self._spanner_async_api = SpannerAsyncClient(
+                    client_info=client_info, transport=transport
+                )
+                return self._spanner_async_api
+            credentials = self._instance._client.credentials
+            if isinstance(credentials, google.auth.credentials.Scoped):
+                credentials = credentials.with_scopes((SPANNER_DATA_SCOPE,))
+            self._spanner_async_api = SpannerAsyncClient(
+                credentials=credentials,
+                client_info=client_info,
+                client_options=client_options,
+            )
+        return self._spanner_async_api
+
     def __eq__(self, other):
         if not isinstance(other, self.__class__):
             return NotImplemented

tests/unit/spanner_dbapi/test_connection.py

@@ -138,6 +138,10 @@ def test_read_only_connection(self):
         ):
             connection.read_only = False
 
+        # Verify that we can set the value to the same value as it already has.
+        connection.read_only = True
+        self.assertTrue(connection.read_only)
+
         connection._spanner_transaction_started = False
         connection.read_only = False
         self.assertFalse(connection.read_only)