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 <irahul@google.com>

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