feat: retrieve read timestamp from read-only transaction

[olavloite]

Exposes the ReadOnlyTransactionTimestamp() method on the SpannerConn interface to retrieve the read timestamp of an active read-only or batch read-only transaction. Also corrects mock server behavior to avoid returning read timestamps for read-write transactions.

Fixes #543

[gemini-code-assist]

Code Review

This pull request introduces the ReadOnlyTransactionTimestamp method to retrieve the read timestamp of active read-only transactions in Cloud Spanner. The changes span across the connection interface, transaction implementations, in-memory test server, and include a new unit test. Feedback on the changes highlights two improvement opportunities in transaction.go: first, optimizing delegatingTransaction.ReadOnlyTransactionTimestamp to avoid prematurely activating the transaction when it is not yet initialized; second, removing a redundant check for tx.boTx in readOnlyTransaction.ReadOnlyTransactionTimestamp as tx.roTx is guaranteed to be initialized in that scenario.

[gemini-code-assist]

Calling d.ensureActivated() here is inefficient because it forces the activation/creation of the underlying Spanner transaction even if no query has been executed yet. Since a transaction cannot have a read timestamp before any query is run, we can avoid this overhead by checking if d.contextTransaction is nil and returning the appropriate error directly based on the transaction's read-only/read-write configuration.

func (d *delegatingTransaction) ReadOnlyTransactionTimestamp() (time.Time, error) {
	if d.contextTransaction == nil {
		if !propertyTransactionReadOnly.GetValueOrDefault(d.conn.state) {
			return time.Time{}, spanner.ToSpannerError(status.Error(codes.FailedPrecondition, "cannot retrieve read timestamp on a read-write transaction"))
		}
		return time.Time{}, spanner.ToSpannerError(status.Error(codes.FailedPrecondition, "underlying read-only transaction is not initialized"))
	}
	return d.contextTransaction.ReadOnlyTransactionTimestamp()
}

[gemini-code-assist]

The check if tx.boTx != nil is redundant and represents dead code. When tx.boTx (the batch read-only transaction) is non-nil, tx.roTx is also always initialized to &bo.ReadOnlyTransaction during activation. Therefore, tx.roTx will never be nil in that case, and calling tx.roTx.Timestamp() is sufficient and equivalent to calling tx.boTx.Timestamp().

func (tx *readOnlyTransaction) ReadOnlyTransactionTimestamp() (time.Time, error) {
	if tx.roTx != nil {
		return tx.roTx.Timestamp()
	}
	return time.Time{}, spanner.ToSpannerError(status.Error(codes.FailedPrecondition, "underlying read-only transaction is not initialized"))
}

[olavloite]

LGTM. This exposes ReadOnlyTransactionTimestamp() on the SpannerConn interface, allowing clients to query Spanner's chosen read timestamp. Correcting the mock server to not return read timestamps for read-write transactions is a great fix for accuracy. Good test coverage.