RUBY-3786 Retry inside txns on overload errors

[comandeo-mongo]

Implements DRIVERS-3411 (RUBY-3786): retry reads and writes inside transactions
on overload errors (RetryableError + SystemOverloadedError labels).

Key changes:

• Allow in-transaction reads/writes to fall through to overload retry logic
• Preserve startTransaction: true on retries of the first command via revert_to_starting_transaction!
• Skip w: majority write concern upgrade on commitTransaction when all failures were overload-only

[Copilot]

Pull request overview

Adds support for retrying reads/writes inside transactions when the server returns overload-related labels (RetryableError + SystemOverloadedError), and updates unified transaction spec tests to cover the new behavior.

Changes:

• Allow overload retry logic to run for in-transaction reads/writes (instead of immediately raising).
• Preserve startTransaction: true on retries of the first transactional write by reverting session state before retry.
• Track “overload-only” retry sequences to avoid upgrading commitTransaction write concern to w: majority when all failures were overload-only.

Reviewed changes

Copilot reviewed 8 out of 8 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
spec/spec_tests/data/transactions_unified/backpressure-retryable-writes.yml	New unified spec coverage for retrying writes in txns on overload labels.
spec/spec_tests/data/transactions_unified/backpressure-retryable-reads.yml	New unified spec coverage for retrying reads in txns on overload labels.
spec/spec_tests/data/transactions_unified/backpressure-retryable-commit.yml	New unified spec coverage for overload retries on commitTransaction.
spec/spec_tests/data/transactions_unified/backpressure-retryable-abort.yml	New unified spec coverage for overload retries on abortTransaction.
lib/mongo/session.rb	Skip w: majority upgrade on commit retries when retries were overload-only; add revert_to_starting_transaction!.
lib/mongo/retryable/write_worker.rb	Plumb overload-only retry flag and revert session state to preserve startTransaction: true for first-op write retries.
lib/mongo/retryable/read_worker.rb	Allow overload retries in transactions for reads.
lib/mongo/operation/context.rb	Add overload_only_retry? flag accessor on operation context.

Comments suppressed due to low confidence (1)

lib/mongo/retryable/read_worker.rb:217

• Overload retries for reads inside transactions now proceed past this guard, but the overload retry path does not restore STARTING_TRANSACTION_STATE when the failing read is the first command in a transaction. Because Session#update_state! runs during message build, the session becomes TRANSACTION_IN_PROGRESS_STATE after the first attempt, and a retry may omit startTransaction: true, breaking the transaction. Capture whether the session was starting before the first attempt and call session.revert_to_starting_transaction! before performing an overload retry (similar to WriteWorker). Adding a unified test where the first operation is a find that fails with overload labels would prevent regressions.

      def modern_read_with_retry(session, server_selector, context, &block)
        server = select_server(
          cluster,
          server_selector,
          session,
          timeout: context&.remaining_timeout_sec
        )
        result = yield server
        retry_policy.record_success(is_retry: false)
        result
      rescue *retryable_exceptions, Error::OperationFailure::Family, Auth::Unauthorized, Error::PoolError => e
        e.add_notes('modern retry', 'attempt 1')
        raise e if session.in_transaction? && !retryable_overload_error?(e)

        if retryable_overload_error?(e)
          overload_read_retry(e, session, server_selector, context, server, error_count: 1, &block)
        else
          raise e if !is_retryable_exception?(e) && !e.write_retryable?
          retry_read(e, session, server_selector, context: context, failed_server: server, &block)
        end

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