dnsimple / dns_erlang / 30157417737

92%

Build Type

push

github

Committed by NelsonVides

Commit Message

Honour an OPT RR anywhere in the additional section (RFC 6891 §6.1.1)

RFC 6891 §6.1.1: the OPT RR "MAY be placed anywhere within the additional
data section". Four places in the encoder matched it only at the head of
that list -- get_max_size/2, preserve_optrr_size/1, ensure_optrr/2 and
append_optrr/2 -- so an OPT in any other position was invisible to all of
them, with two consequences:

  - Truncated responses dropped the OPT entirely, contra the same
    section's requirement that a responder include an OPT whenever the
    request carried one. Reserving space for it was skipped too, since
    preserve_optrr_size/1 returned 0.
  - The advertised payload size was ignored, falling back to the 512
    default, so a client offering a 4096-byte buffer got needlessly
    truncated answers.

Neither shows up in ordinary traffic, because clients usually put the OPT
first -- but nothing requires it, and our own decoder preserves wire
order, so [A, OPT] is exactly what a caller is handed for a legal query.
It also cannot be last when TSIG or SIG(0) is present, since those must
be the final RR.

Each of the four keeps its head match as a fast path and falls back to a
scan only when the OPT is elsewhere, so the common case costs exactly what
it did before: 500k encodes of a small response measured 496ms/543ms/684ms
for additional = []/[OPT]/[A,OPT] against 523ms/544ms/711ms before, i.e.
at parity within the ~5% noise this corpus carries. Two rejected
alternatives, both measured: threading one lookup through the default path
instead of three head matches cost ~5% more, because it traded three
allocation-free matches for a scan plus a tuple; and lists:keyfind/3,
though a BIF, ran ~2.3x slower than the open-coded loop on [] and [OPT],
only overtaking it past roughly ten records -- far longer than any real
additional section.

Coverage Stats

23 of 24 new or added lines in 1 file covered. (95.83%)

1 existing line in 1 file now uncovered.

2928 of 3218 relevant lines covered (90.99%)

10766.07 hits per line