Implement DEFAULT_ISOLATION_LEVEL setting

[apstndb]

This PR implements DEFAULT_ISOLATION_LEVEL system variable/property.

Reference

• feat: add default_isolation_level connection property googleapis/java-spanner#3702
  • It is not yet merged so it can be changed. If it is changed, spanner-mycli will do a breaking change. (We don't guarantee stability!)

[gemini-code-assist]

Hello @apstndb, 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!

Summary of Changes

This pull request implements the DEFAULT_ISOLATION_LEVEL system variable/property. This allows users to configure the default transaction isolation level for read-write transactions. The changes include adding the new variable to the systemVariables struct, updating the README.md file to document the new variable, and adding a test case to verify that the isolation level is correctly applied when beginning a read-write transaction.

Highlights

• New Feature: Implements the DEFAULT_ISOLATION_LEVEL system variable/property.
• Configuration: Adds the DefautIsolationLevel field to the systemVariables struct in system_variables.go.
• Documentation: Updates README.md to include documentation for the new DEFAULT_ISOLATION_LEVEL variable.
• Testing: Adds a new test case TestIsolationLevel in session_slow_test.go to verify the functionality.

Changelog

Click here to see the changelog

• README.md
  • Added DEFAULT_ISOLATION_LEVEL to the table of system variables with a default value of REPEATABLE_READ.
• session.go
  • Modified BeginReadWriteTransaction to include the IsolationLevel from systemVariables when creating a new read-write transaction.
• session_slow_test.go
  • Added TestIsolationLevel to verify that the default isolation level is correctly applied to read-write transactions.
  • Uses spanemuboost to setup an emulator for testing.
  • Records gRPC requests to verify the transaction options.
• system_variables.go
  • Added DefautIsolationLevel field to the systemVariables struct to store the default isolation level.
  • Added a new entry for DEFAULT_ISOLATION_LEVEL in systemVariableDefMap to define its description, setter, and getter.

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A new setting takes its place,
Isolation level, set with grace.
Repeatable read or serial,
The database knows what's real.

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[gemini-code-assist]

Code Review

This PR introduces the DEFAULT_ISOLATION_LEVEL system variable, enhancing the configurability of transaction isolation levels. The changes seem well-structured and include a test case to verify the functionality. However, there are a few areas that could benefit from further attention.

Summary of Findings

• Missing Error Handling: The Setter function for DEFAULT_ISOLATION_LEVEL in system_variables.go does not handle the case where the provided value does not match any of the valid sppb.TransactionOptions_IsolationLevel values. This could lead to unexpected behavior if an invalid value is set.
• Inconsistent Naming: The variable name DefautIsolationLevel in session.go and system_variables.go has a typo. It should be DefaultIsolationLevel for consistency and readability.

Merge Readiness

While the changes are a valuable addition, addressing the error handling and naming inconsistency would improve the robustness and maintainability of the code. I recommend addressing these issues before merging. I am unable to directly approve this pull request, and other reviewers should also review this code before merging.

[gemini-code-assist]

This setter doesn't handle the case where the value string doesn't match any of the enum values in sppb.TransactionOptions_IsolationLevel_value. Consider adding a default case or returning an error to handle invalid input.

				if ok {
					this.DefaultIsolationLevel = sppb.TransactionOptions_IsolationLevel(isolation)
				} else {
					return fmt.Errorf("invalid isolation level: %s", value)
				}

[gemini-code-assist]

The variable name DefautIsolationLevel has a typo. Consider renaming it to DefaultIsolationLevel for consistency and readability.

Suggested change

	IsolationLevel: s.systemVariables.DefautIsolationLevel,
	IsolationLevel: s.systemVariables.DefaultIsolationLevel,

[gemini-code-assist]

The variable name DefautIsolationLevel has a typo. Consider renaming it to DefaultIsolationLevel for consistency and readability.

Suggested change

	DefautIsolationLevel sppb.TransactionOptions_IsolationLevel // DEFAULT_ISOLATION_LEVEL
	DefaultIsolationLevel sppb.TransactionOptions_IsolationLevel // DEFAULT_ISOLATION_LEVEL