Tornado: Urlencoded body parsing omits max_num_fields, so one request can stall the event loop

SummaryTornado parses application/x-www-form-urlencoded bodies with urllib.parse.parseqs and does not pass maxnumfields. A body made almost entirely of separators produces tens of millions of fields, and the parse happens on the event loop before the handler runs, so a single request stalls the...