sql: session tracing inside a txn is painful

[andreimatei]

If one uses, for example, crdb.ExecuteTx() and has code like the following:

                if _, err := tx.Exec(`set tracing=on`); err != nil {
                    return err
                }
                defer func() {
                    _, _ = tx.Exec(`set tracing=off`)
                }()

that's no good cause the final set tracing=off is rejected if the txn is in an error state. Moreover, starting tracing on that conn again fails. We should allow set tracing=off in all states. Or something.

Hacks like the following help, although tracing leaks on conns where it isn't stopped properly.

            var tracing string
            if err := tx.QueryRow(`show tracing`).Scan(&tracing); err != nil {
                return err
            }
            if tracing != "off" {
                if _, err := tx.Exec(`set tracing=off; set tracing=on;`); err != nil {
                    return err
                }
            } else {
                if _, err := tx.Exec(`set tracing=on`); err != nil {
                    return err
                }
            }

cc @petermattis

[benesch]

This is awesome. I ran straight into the issue you described while attempting to debug some TPCC-C performance issues and it was not fun.