Spanner: Exception on BeginTransaction rpc can cause session leak #6164
Description
If an exception that cannot be retried occurs while executing the BeginTransaction gRPC, the Spanner session pool could leak a session.
Consider the following example:
SpannerOptions.Builder builder =
SpannerOptions.newBuilder()
.setProjectId("[PROJECT]")
builder.setSessionPoolOption(
SessionPoolOptions.newBuilder()
.setMinSessions(0)
.setMaxSessions(2)
.setWriteSessionsFraction(0.0f)
.build());
Spanner spanner = builder.build().getService();
DatabaseClient client =
spanner.getDatabaseClient(DatabaseId.of("[PROJECT]", "[INSTANCE]", "[DATABASE]"));
TransactionRunner runner = client.readWriteTransaction();
long updateCount =
runner.run(
new TransactionCallable<Long>() {
@Override
public Long run(TransactionContext transaction) throws Exception {
return transaction.executeUpdate(UPDATE_STATEMENT);
}
});The above session pool will never contain a prepared transaction, as the writeSessionsFraction is set to 0. Creating a read/write transaction will therefore always require the session pool to prepare a new session. If the BeginTransaction rpc returns a non-retryable error, the error is returned to the client, but the session that is taken to be prepared by the session pool is never returned to the session pool.