token-server: Warmup some local caches in /_ah/warmup.

tokenserver/appengine/impl/certchecker/warmup.go

 // Copyright 2017 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 certchecker
 
 import (
         "golang.org/x/net/context"
 
+        "github.com/luci/luci-go/common/errors"
+        "github.com/luci/luci-go/common/logging"
         "github.com/luci/luci-go/server/warmup"
+        "github.com/luci/luci-go/tokenserver/appengine/impl/certconfig"
 )
 
 func init() {
         warmup.Register("tokenserver/appengine/impl/certchecker", func(c context.Context) error {
-                // TODO
-                return nil
+                names, err := certconfig.ListCAs(c)
+                if err != nil {
+                        return err
+                }
+                var merr errors.MultiError
+                for _, cn := range names {
+                        logging.Infof(c, "Warming up %q", cn)
+                        checker, err := GetCertChecker(c, cn)
+                        if err == nil {
+                                _, err = checker.GetCA(c)
+                        }
+                        if err != nil {
+                                logging.WithError(err).Warningf(c, "Failed to warm up %q", cn)
+                                merr = append(merr, err)
+                        }
+                }
+                if len(merr) == 0 {
+                        return nil
+                }
+                return merr
         })
 }