username, serverId and ip were forwarded to every configured source at
whatever length the caller sent, up to the megabyte net/http allows in a
request line. Velocity never sends more than a 16-character name, a
41-character signed SHA-1 serverId and a textual IP address, so only a
direct caller reaches those sizes, and each such request cost one
oversized upstream call per source.
Any of the three over 64 bytes is now answered 204 before a source is
asked, the same as a missing username or serverId. 64 bytes still
leaves room for a 16-character name in multi-byte UTF-8.
The subtest behind this points a source that validates anything at the
handler and sends missing and oversized fields, expecting 204 and zero
upstream requests, then a well-formed login that gets 200. It replaces
the old missing-username case, whose only source was unreachable, so
the test passed even with the guard removed. Dropping the length check
now fails it on the long username; dropping the whole guard fails it on
the first missing field.
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