gofeed v1.4.1 roughly doubles ParseRSS cost (+192% allocs) — parser held at v1.3.0 #212

Summary

mmcdole/gofeed v1.3.0 → v1.4.1 roughly doubles the cost of the feed parse
path. #210 holds the parser at v1.3.0 so the rest of that dependency group
could land; this issue tracks actually taking the upgrade.

Measurement

Measured on top of main with only gofeed bumped (go get github.com/mmcdole/gofeed@v1.4.1), leaving the go directive at 1.25.9 so the
toolchain change that rode along in the group PR isn't a confounder.
BenchmarkParseRSS, -benchmem -count=6, benchstat:

ParseRSS-16  sec/op       707.7µ ± 4%  →  1330.0µ ± 13%   +87.94% (p=0.002 n=6)
ParseRSS-16  B/op        878.9Ki ± 0%  →  1572.7Ki ± 0%   +78.95% (p=0.002 n=6)
ParseRSS-16  allocs/op    8.498k ± 0%  →   24.801k ± 0%  +191.85% (p=0.002 n=6)

These numbers match the group PR's gate failure exactly, and pinning gofeed
back returns ParseRSS to baseline (~, p=0.310), so the parser bump accounts
for the entire regression.

The allocation count is the tell: +192% allocs/op for the same input is a
per-item or per-element allocation that used to be avoided, not a constant-factor
slowdown. sec/op tracking it at ~+88% is consistent with allocator/GC pressure
being the mechanism rather than added parsing work.

Why it matters here

Feed parsing is the service's hot path — every poll cycle, every feed. At the
default poll intervals a 2x parse cost is mostly GC pressure rather than
wall-clock risk, but it scales with feed count, and the catalog ships 179+ feeds.

What to do

• Diff gofeed v1.3.0...v1.4.1 for the allocation change (the goxpp
  dependency also moved, and it's the XML pull-parser underneath — suspect
  it first).
• Decide: absorb the cost, work around it at our call site, or raise it
  upstream with this benchmark attached.
• If it's staying pinned for a while, add a dependabot.yml ignore entry so
  the group PR doesn't keep re-proposing it.
• Un-pin in go.mod once resolved.

Notes

scripts/bench-compare.sh caught this on the dependency PR before it merged,
which is exactly the job it was added for.