If the last key's value in the query string is empty, then the trailing semicolon appears to be its value. For example,

mw.eventLog.schemas.GettingStarted.defaults = {};
mw.eventLog.logEvent( 'GettingStarted', {anon: false, action: "x", token: ""} );

validates OK but logs as /event.gif?_db=my_wiki&_id=GettingStarted&_rv=4910146&_ok=true&anon=false&action=x&token=;

and DevServer.php interprets this final parameter as { token: ";" }. The server log2json.py code strips the semicolon (using parse_qsl(q.strip('?;')) ) so that endpoint doesn't experience this bug.

One fix might be to always pad the query string with &;, in logEvent()

beacon.src = baseUri + '?' + queryString + '&;';

This bug will go away when and if we encode a JSON structure in the query string, as its key values are delimited by JSON punctuation.

Version: unspecified
Severity: minor