chore(spanner): fix mutation only case in rw mux with aborted error

[harshachinta]

This PR fixes the following scenario.

In a mutation-only scenario for a read-write transaction using multiplexed sessions, an explicit BeginTransaction RPC is triggered after the user operation completes.

However, if an ABORT occurs during this BeginTransaction RPC (this can happen if backend is unable to refresh the schema when a mutation key is passed), the client retry mechanism performs an explicit BeginTransaction before the user operation begins. This behavior causes issues in multiplexed sessions because, in mutation-only cases, the BeginTransaction requires a mutation key to generate a precommit token. This precommit token is essential for committing the transaction.

Currently, since the BeginTransaction during a retry is executed before the user operation, no mutation key is available, and consequently, no precommit token is included in the transaction response. This leads to commit failures with an error as shown below

com.google.cloud.spanner.SpannerException: INVALID_ARGUMENT: io.grpc.StatusRuntimeException: INVALID_ARGUMENT: A Commit request for a read-write transaction on a multiplexed session must specify a precommit token.

Fix: The fix involves retrying the BeginTransaction RPC for mutation-only cases with multiplexed sessions when it fails with ABORTED error.

[olavloite]

However, if an ABORT occurs during this BeginTransaction RPC

Is this something that can actually happen?

[harshachinta]

this can happen if backend is unable to refresh the schema when a mutation key is passed

Yes, this can happen if backend is unable to refresh the schema when a mutation key is passed with a BeginTransaction RPC.

[harshachinta]

Thanks @olavloite. Updated the approach to solve this problem by following the commit
cafefb6