dscache: Only log unusual memcache errors.

filter/dscache/ds.go

 // Copyright 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 package dscache
 
 import (
         "time"
 
         ds "github.com/luci/gae/service/datastore"
         "github.com/luci/gae/service/memcache"
+        "github.com/luci/luci-go/common/errors"
         log "github.com/luci/luci-go/common/logging"
         "golang.org/x/net/context"
 )
 
 type dsCache struct {
         ds.RawInterface
 
         *supportContext
 }
 
@@ skipping 15 matching lines @@
         lockItems, nonce := d.mkRandLockItems(keys, metas)
         if len(lockItems) == 0 {
                 return d.RawInterface.GetMulti(keys, metas, cb)
         }
 
         if err := d.mc.AddMulti(lockItems); err != nil {
                 // Ignore this error. Either we couldn't add them because they exist
                 // (so, not an issue), or because memcache is having sad times (in which
                 // case we'll see so in the GetMulti which immediately follows this).
         }
-        if err := d.mc.GetMulti(lockItems); err != nil {
+        if err := filterMCErr(d.mc.GetMulti(lockItems)); err != nil {
                 (log.Fields{log.ErrorKey: err}).Warningf(
                         d.c, "dscache: GetMulti: memcache.GetMulti")
         }
 
         p := makeFetchPlan(d.c, d.aid, d.ns, &facts{keys, metas, lockItems, nonce})
 
         if !p.empty() {
                 // looks like we have something to pull from datastore, and maybe some work
                 // to save stuff back to memcache.
 
@@ skipping 74 matching lines @@
                 if err == nil {
                         err = txnState.apply(d.supportContext)
                 }
                 return err
         }, opts)
         if err == nil {
                 txnState.release(d.supportContext)
         }
         return err
 }
+
+// filterMCErr filters expected memcache failure errors from the supplied err.
+// If err is composed exclusively of expected failure errors, filterMCErr will
+// return nil
+//
+// The following error types are filtered:
+//      - ErrCacheMiss: memcache is permitted to evict cache items at its
+//        discretion.
+func filterMCErr(err error) error {

[iannucci, 2016/02/13 00:16:44]

inline this sucka!

+        return errors.Filter(err, memcache.ErrCacheMiss)
+}