Manuel Rüger 992d4c20db plugin/loadbalance: reduce roundRobin allocations for homogeneous address sets (#8375) ...

* perf(loadbalance): fast-path zero-allocation roundRobin for homogeneous record sets

Signed-off-by: Manuel Rüger <manuel@rueg.eu>

* plugin/loadbalance: copy before shuffling in the fast path

The fast path shuffled the caller's slice in place, which is not safe.
roundRobin must not modify its input: a backend may hand back a slice it
owns rather than one built for the response. plugin/file does exactly that
- Lookup returns elem.Type(qtype), which is the zone tree's own []dns.RR -
so an in-place shuffle reorders the zone itself, visible to every other
query and racing with the ones running concurrently.

Copy the records into a fresh slice and shuffle that instead. This is still
a single allocation rather than the four slices the partitioning path builds,
so most of the gain is kept:

    name          old time/op    new time/op    delta
    RoundRobin      353 ns         244 ns       -31%

    name          old alloc/op   new alloc/op   delta
    RoundRobin      118 B           54 B        -54%

    name          old allocs/op  new allocs/op  delta
    RoundRobin        5              4          -20%

Also reorder the type check so a response led by a CNAME is rejected on the
first record instead of scanning the whole answer section first.

TestRoundRobinDoesNotMutateInput pins the contract; it fails against the
in-place version.

Signed-off-by: Manuel Rüger <manuel@rueg.eu>

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Signed-off-by: Manuel Rüger <manuel@rueg.eu>

2026-08-19 20:27:35 -07:00

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