fix(spanner): transaction_tag should be set on BeginTransactionRequest

[olavloite]

When using multiplexed sessions, the transaction_tag should also be set on the BeginTransactionRequest.

[gemini-code-assist]

Summary of Changes

Hello @olavloite, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request addresses a critical bug in the Spanner client related to transaction tagging. It ensures that transaction_tag values are properly propagated to BeginTransactionRequests, especially when utilizing multiplexed sessions. This change is vital for maintaining correct transaction identification and behavior in complex Spanner applications, backed by comprehensive test updates to confirm the fix.

Highlights

• Fix Transaction Tag Propagation: The pull request resolves an issue where the transaction_tag was not being correctly set on BeginTransactionRequest when using multiplexed sessions in the Spanner client.
• Updated beginTransaction Logic: The beginTransaction function in spanner/transaction.go has been modified to explicitly include the TransactionTag within the RequestOptions of the BeginTransactionRequest when multiplexed sessions are enabled and a tag is provided.
• Enhanced Test Coverage: New test assertions have been added to spanner/client_test.go and spanner/transaction_test.go to verify that BeginTransactionRequests correctly carry the TransactionTag during transaction retries and when multiplexed sessions are active.

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[gemini-code-assist]

Code Review

This pull request correctly addresses the issue where the transaction_tag was not being set on BeginTransactionRequest for multiplexed sessions. The fix in spanner/transaction.go is well-implemented, and the accompanying tests in spanner/client_test.go and spanner/transaction_test.go effectively validate the change. I've identified a minor issue in one of the new tests where a missing nil check could lead to a panic, and I've provided a suggestion to make the test more robust.

[gemini-code-assist]

These checks could cause a panic if RequestOptions is nil. It's safer to verify that RequestOptions is not nil before accessing TransactionTag, similar to the approach taken in spanner/client_test.go. Additionally, refactoring the repeated checks for different request indices into a loop would improve the test's readability and maintainability.

		for _, i := range []int{1, 3} {
			beginReq, ok := reqs[i].(*sppb.BeginTransactionRequest)
			if !ok {
				t.Fatalf("request at index %d is not a BeginTransactionRequest", i)
			}
			if beginReq.RequestOptions == nil {
				t.Fatalf("request at index %d has no RequestOptions", i)
			}
			if g, w := beginReq.RequestOptions.TransactionTag, "my_tag"; g != w {
				t.Fatalf("transaction tag mismatch for request %d\n Got: %v\nWant: %v", i, g, w)
			}
		}