dscache: Only log unusual memcache errors.

filter/dscache/ds.go

Index: filter/dscache/ds.go
diff --git a/filter/dscache/ds.go b/filter/dscache/ds.go
index 542ada6b111cf2335a76602acdf94e10addec238..b466d46e3c2782e17ff4ab5bf3022ba8e9904808 100644
--- a/filter/dscache/ds.go
+++ b/filter/dscache/ds.go
@@ -9,6 +9,7 @@ import (
 
 	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"
 )
@@ -44,7 +45,7 @@ func (d *dsCache) GetMulti(keys []*ds.Key, metas ds.MultiMetaGetter, cb ds.GetMu
 		// (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")
 	}
@@ -139,3 +140,14 @@ func (d *dsCache) RunInTransaction(f func(context.Context) error, opts *ds.Trans
 	}
 	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)
+}