use single instance of typed error and use errors.Is() for comparison (…

…thanos-io#6719)

Signed-off-by: Jake Keeys <[email protected]>

pkg/cacheutil/async_op.go

@@ -5,8 +5,6 @@ package cacheutil
 
 import (
 	"sync"
-
-	"github.com/pkg/errors"
 )
 
 type asyncOperationProcessor struct {
@@ -54,13 +52,11 @@ func (p *asyncOperationProcessor) asyncQueueProcessLoop() {
 	}
 }
 
-var errAsyncBufferFull = errors.New("the async buffer is full")
-
 func (p *asyncOperationProcessor) enqueueAsync(op func()) error {
 	select {
 	case p.asyncQueue <- op:
 		return nil
 	default:
-		return errAsyncBufferFull
+		return errMemcachedAsyncBufferFull
 	}
 }

pkg/cacheutil/memcached_client.go

@@ -400,7 +400,7 @@ func (c *memcachedClient) SetAsync(key string, value []byte, ttl time.Duration)
 		c.duration.WithLabelValues(opSet).Observe(time.Since(start).Seconds())
 	})
 
-	if err == errMemcachedAsyncBufferFull {
+	if errors.Is(err, errMemcachedAsyncBufferFull) {
 		c.skipped.WithLabelValues(opSet, reasonAsyncBufferFull).Inc()
 		level.Debug(c.logger).Log("msg", "failed to store item to memcached because the async buffer is full", "err", err, "size", len(c.p.asyncQueue))
 		return nil