Every 5xx became 499 once the request context was cancelled, so a panic or
a real database error on a request the browser had abandoned passed the
phase gate's 5xx scan. Now a 5xx is logged as 499 only when the client had
already gone when the status was written and the handler did not panic;
recoverJSON and recoverBare mark the access log entry. Every record of an
abandoned request carries client_gone=true. The gate self-test plants a
5xx with client_gone=true and must refuse it.
Jakub Zych
c30145ed25