cache: move goroutine closure to separate function to save memory (#3353)

The goroutine closure was causing objects to be heap allocated.  Moving
it to a separate function fixes that.

```benchmark                                old ns/op     new ns/op     delta
BenchmarkCacheResponse/NoPrefetch-12     773           713           -7.76%
BenchmarkCacheResponse/Prefetch-12       878           837           -4.67%
BenchmarkHash-12                         9.17          9.18          +0.11%

benchmark                                old allocs     new allocs     delta
BenchmarkCacheResponse/NoPrefetch-12     9              8              -11.11%
BenchmarkCacheResponse/Prefetch-12       9              8              -11.11%
BenchmarkHash-12                         0              0              +0.00%

benchmark                                old bytes     new bytes     delta
BenchmarkCacheResponse/NoPrefetch-12     471           327           -30.57%
BenchmarkCacheResponse/Prefetch-12       471           327           -30.57%
BenchmarkHash-12                         0             0             +0.00%
```
Signed-off-by: Charlie Vieth <charlie.vieth@gmail.com>
Signed-off-by: Miek Gieben <miek@miek.nl>

plugin/cache/handler.go

@@ -29,28 +29,10 @@ func (c *Cache) ServeDNS(ctx context.Context, w dns.ResponseWriter, r *dns.Msg)
 	i, found := c.get(now, state, server)
 	if i != nil && found {
 		resp := i.toMsg(r, now)
-
 		w.WriteMsg(resp)
 
-		if c.prefetch > 0 {
-			ttl := i.ttl(now)
-			i.Freq.Update(c.duration, now)
-
-			threshold := int(math.Ceil(float64(c.percentage) / 100 * float64(i.origTTL)))
-			if i.Freq.Hits() >= c.prefetch && ttl <= threshold {
-				cw := newPrefetchResponseWriter(server, state, c)
-				go func(w dns.ResponseWriter) {
-					cachePrefetches.WithLabelValues(server).Inc()
-					plugin.NextOrFailure(c.Name(), c.Next, ctx, w, r)
-
-					// When prefetching we loose the item i, and with it the frequency
-					// that we've gathered sofar. See we copy the frequencies info back
-					// into the new item that was stored in the cache.
-					if i1 := c.exists(state); i1 != nil {
-						i1.Freq.Reset(now, i.Freq.Hits())
-					}
-				}(cw)
-			}
+		if c.shouldPrefetch(i, now) {
+			go c.doPrefetch(ctx, state, server, i, now)
 		}
 		return dns.RcodeSuccess, nil
 	}
@@ -59,6 +41,29 @@ func (c *Cache) ServeDNS(ctx context.Context, w dns.ResponseWriter, r *dns.Msg)
 	return plugin.NextOrFailure(c.Name(), c.Next, ctx, crr, r)
 }
 
+func (c *Cache) doPrefetch(ctx context.Context, state request.Request, server string, i *item, now time.Time) {
+	cw := newPrefetchResponseWriter(server, state, c)
+
+	cachePrefetches.WithLabelValues(server).Inc()
+	plugin.NextOrFailure(c.Name(), c.Next, ctx, cw, state.Req)
+
+	// When prefetching we loose the item i, and with it the frequency
+	// that we've gathered sofar. See we copy the frequencies info back
+	// into the new item that was stored in the cache.
+	if i1 := c.exists(state); i1 != nil {
+		i1.Freq.Reset(now, i.Freq.Hits())
+	}
+}
+
+func (c *Cache) shouldPrefetch(i *item, now time.Time) bool {
+	if c.prefetch <= 0 {
+		return false
+	}
+	i.Freq.Update(c.duration, now)
+	threshold := int(math.Ceil(float64(c.percentage) / 100 * float64(i.origTTL)))
+	return i.Freq.Hits() >= c.prefetch && i.ttl(now) <= threshold
+}
+
 // Name implements the Handler interface.
 func (c *Cache) Name() string { return "cache" }