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

appengine/gaemiddleware/context.go

 // Copyright 2015 The LUCI Authors. All rights reserved.
 // Use of this source code is governed under the Apache License, Version 2.0
 // that can be found in the LICENSE file.
 
 package gaemiddleware
 
 import (
         "net/http"
 
         "golang.org/x/net/context"
         "google.golang.org/appengine"
 
         "github.com/luci/gae/filter/dscache"
         "github.com/luci/gae/impl/prod"
         "github.com/luci/gae/service/urlfetch"
 
         "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"
         "github.com/luci/luci-go/server/auth/authdb"
         "github.com/luci/luci-go/server/middleware"
         "github.com/luci/luci-go/server/router"
         "github.com/luci/luci-go/server/settings"
 
         "github.com/luci/luci-go/appengine/gaeauth/client"
         "github.com/luci/luci-go/appengine/gaeauth/server"
@@ skipping 24 matching lines @@
         globalAuthConfig = auth.Config{
                 DBProvider:          authdb.NewDBCache(server.GetAuthDB),
                 Signer:              gaesigner.Signer{},
                 AccessTokenProvider: client.GetAccessToken,
                 AnonymousTransport:  urlfetch.Get,
                 GlobalCache:         &server.Memcache{Namespace: "__luciauth__"},
         }
 
         // 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)
 //   * github.com/luci/luci-go/server/settings (global app settings)
 //   * github.com/luci/luci-go/appengine/gaesecrets (access to secret keys in datastore)
 //   * github.com/luci/luci-go/appengine/gaeauth/server/gaesigner (RSA signer)
 //   * github.com/luci/luci-go/appengine/gaeauth/server/auth (user groups database)
 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)
 
         // Fetch and apply configuration stored in the datastore.
         cachedSettings := fetchCachedSettings(c)
         c = logging.SetLevel(c, cachedSettings.LoggingLevel)
         if !cachedSettings.DisableDSCache {
                 c = dscache.AlwaysFilterRDS(c)
         }
 
@@ skipping 29 matching lines @@
 // BaseProd returns a list of middleware: WithProd middleware, a panic catcher if this
 // is not a devserver, and the monitoring middleware.
 func BaseProd() router.MiddlewareChain {
         if appengine.IsDevAppServer() {
                 return router.NewMiddlewareChain(ProdServices, globalTsMonState.Middleware)
         }
         return router.NewMiddlewareChain(
                 ProdServices, middleware.WithPanicCatcher, globalTsMonState.Middleware,
         )
 }