Don't allcoate a new math/rand every request.

appengine/gaemiddleware/context.go

Index: appengine/gaemiddleware/context.go
diff --git a/appengine/gaemiddleware/context.go b/appengine/gaemiddleware/context.go
index d4215448f58a997c4e2efd5add2ab7e2c58e5c42..f55e232f2e0511736bff7bb52cf100a19497c6c3 100644
--- a/appengine/gaemiddleware/context.go
+++ b/appengine/gaemiddleware/context.go
@@ -17,6 +17,7 @@ import (
 	"github.com/luci/luci-go/common/config"
 	"github.com/luci/luci-go/common/data/caching/cacheContext"
 	"github.com/luci/luci-go/common/data/caching/proccache"
+	"github.com/luci/luci-go/common/data/rand/mathrand"
 	"github.com/luci/luci-go/common/logging"
 
 	"github.com/luci/luci-go/server/auth"
@@ -61,10 +62,14 @@ var (
 
 	// globalTsMonState holds state related to time series monitoring.
 	globalTsMonState = &tsmon.State{}
+
+	// globalMathRand is a global "math/rand" instance.

[Vadim Sh., 2016/12/20 00:56:37]

actually, nit: add a comment stating that this object has locks inside

[dnj, 2016/12/20 00:58:48]

Done.

+	globalMathRand = mathrand.Get(context.Background())
 )
 
 // WithProd installs the set of standard production AppEngine services:
 //   * github.com/luci/luci-go/common/logging (set default level to debug).
+//   * github.com/luci/luci-go/common/data/rand/mathrand.
 //   * github.com/luci/gae/impl/prod (production appengine services)
 //   * github.com/luci/luci-go/appengine/gaeauth/client (appengine urlfetch transport)
 //   * github.com/luci/luci-go/server/proccache (in process memory cache)
@@ -75,6 +80,7 @@ var (
 func WithProd(c context.Context, req *http.Request) context.Context {
 	// These are needed to use fetchCachedSettings.
 	c = logging.SetLevel(c, logging.Debug)
+	c = mathrand.SetRand(c, globalMathRand)
 	c = prod.Use(c, req)
 	c = settings.Use(c, globalSettings)