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message decompresion can be slow

Version: e004ca4f

Artificial DNS message full of compression pointers can cause named to spend lots of time in decompression loop. Flame chart then looks like this:

512b_prereqs.svg

Reproducer:

  1. get DNSPerf version with https://github.com/DNS-OARC/dnsperf/pull/201 (new option -B)
  2. Fire at named with crafted DNS message. Here is binary in format for dnsperf: tcpnotify.bin. Raw example DNS message is here: 512notify.bin.
  3. Observe throughput going down, compared to 512-byte message without lots of decompression.
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