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Add an atomic.Bool to singleflight prefetching (#7963)

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Also updated plugin to document single-flighting

Signed-off-by: Ryan Brewster <rpb@anthropic.com>
This commit is contained in:
rpb-ant
2026-03-30 16:18:24 -04:00
committed by GitHub
parent 0ba8e3c850
commit 20626a7464
4 changed files with 113 additions and 4 deletions
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20
plugin/cache/handler.go vendored
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@@ -55,13 +55,11 @@ func (c *Cache) ServeDNS(ctx context.Context, w dns.ResponseWriter, r *dns.Msg)
// Adjust the time to get a 0 TTL in the reply built from a stale item.
now = now.Add(time.Duration(ttl) * time.Second)
if !c.verifyStale {
cw := newPrefetchResponseWriter(server, rc, do, cd, c)
go c.doPrefetch(ctx, cw, i, now)
c.tryPrefetch(ctx, i, server, rc, do, cd, now)
}
servedStale.WithLabelValues(server, c.zonesMetricLabel, c.viewMetricLabel).Inc()
} else if c.shouldPrefetch(i, now) {
cw := newPrefetchResponseWriter(server, rc, do, cd, c)
go c.doPrefetch(ctx, cw, i, now)
c.tryPrefetch(ctx, i, server, rc, do, cd, now)
}
if i.wildcard != "" {
@@ -91,6 +89,20 @@ func wildcardFunc(ctx context.Context) func() string {
}
}
// tryPrefetch dispatches a background prefetch for i if one is not already in
// flight. The CAS on i.refreshing ensures at most one prefetch goroutine per
// item, so prefetch load scales with distinct stale keys rather than QPS.
func (c *Cache) tryPrefetch(ctx context.Context, i *item, server string, req *dns.Msg, do, cd bool, now time.Time) {
if !i.refreshing.CompareAndSwap(false, true) {
return
}
cw := newPrefetchResponseWriter(server, req, do, cd, c)
go func() {
defer i.refreshing.Store(false)
c.doPrefetch(ctx, cw, i, now)
}()
}
func (c *Cache) doPrefetch(ctx context.Context, cw *ResponseWriter, i *item, now time.Time) {
// Use a fresh metadata map to avoid concurrent writes to the original request's metadata.
ctx = metadata.ContextWithMetadata(ctx)