fix(spanner): prevent possible panic for Session not found errors (#10386)

The Spanner client library could panic if specifically the following
situation would occur:
1. Create a StmtBasedReadWriteTransaction
2. Only execute mutations, so that an explicit BeginTransaction
   RPC is required.
3. The BeginTransaction RPC returns "Session not found"
4. The client library tries to get a new session from the pool.

Step 4 above would panic, because the client library did not set the
readOnlyTransaction.sp field for StmtBasedReadWriteTransactions.

Fixes #10385

Co-authored-by: rahul2393 <[email protected]>

Author: olavloite

spanner/client_test.go

@@ -3474,6 +3474,21 @@ func TestReadWriteTransaction_WrapSessionNotFoundError(t *testing.T) {
 	}
 }
 
+func TestStmtBasedReadWriteTransaction_SessionNotFoundError_shouldNotPanic(t *testing.T) {
+	t.Parallel()
+	server, client, teardown := setupMockedTestServer(t)
+	defer teardown()
+	server.TestSpanner.PutExecutionTime(MethodBeginTransaction,
+		SimulatedExecutionTime{
+			Errors: []error{newSessionNotFoundError("projects/p/instances/i/databases/d/sessions/s")},
+		})
+	ctx := context.Background()
+	tx, _ := NewReadWriteStmtBasedTransaction(ctx, client)
+	_ = tx.BufferWrite([]*Mutation{Update("my_table", []string{"key", "value"}, []interface{}{int64(1), "my-value"})})
+	// This would panic, as it could not refresh the session.
+	_, _ = tx.Commit(ctx)
+}
+
 func TestClient_WriteStructWithPointers(t *testing.T) {
 	t.Parallel()
 	server, client, teardown := setupMockedTestServer(t)

spanner/transaction.go

@@ -1756,6 +1756,7 @@ func NewReadWriteStmtBasedTransactionWithOptions(ctx context.Context, c *Client,
 			txReadyOrClosed: make(chan struct{}),
 		},
 	}
+	t.txReadOnly.sp = c.idleSessions
 	t.txReadOnly.sh = sh
 	t.txReadOnly.txReadEnv = t
 	t.txReadOnly.qo = c.qo