LogDog: Use datastore config cache when available.

logdog/server/service/config/opts.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 config
 
 import (
         "time"
 
+        "github.com/luci/luci-go/appengine/datastorecache"
+        log "github.com/luci/luci-go/common/logging"
+        "github.com/luci/luci-go/common/sync/mutexpool"
+        "github.com/luci/luci-go/luci_config/appengine/backend/datastore"
+        "github.com/luci/luci-go/luci_config/appengine/gaeconfig"
         "github.com/luci/luci-go/luci_config/server/cfgclient/backend"
         "github.com/luci/luci-go/luci_config/server/cfgclient/backend/caching"
 
         "golang.org/x/net/context"
 )
 
 // CacheOptions is the set of configuration options.
 type CacheOptions struct {
         // CacheExpiration is the amount of time that a config entry should be cached.
         //
         // If this value is <= 0, no configuration caching will be enabled. This
         // should not be set for production systems.
         CacheExpiration time.Duration
+
+        // DatastoreCacheAvailable, if true, indicates that requisite services for
+        // datastore cache access are installed in the Context. This requires:
+        // - A luci/gae datastore service instnace
+        // - A settings service instance, presumably gaesettings.
+        DatastoreCacheAvailable bool
 }
 
 // WrapBackend wraps the supplied base backend in caching layers and returns a
 // Conext with the resulting backend installed.
 func (o *CacheOptions) WrapBackend(c context.Context, base backend.B) context.Context {
         return backend.WithFactory(c, func(c context.Context) backend.B {
                 // Start with our base Backend.
                 be := base
 
+                // If our datastore cache is available, fetch settings to see if it's
+                // enabled.
+                if o.DatastoreCacheAvailable {
+                        switch s, err := gaeconfig.FetchCachedSettings(c); err {
+                        case nil:
+                                // Successfully fetched settings, is our datastore cache enabled?
+                                switch s.DatastoreCacheMode {
+                                case gaeconfig.DSCacheEnabled, gaeconfig.DSCacheStrict:
+                                        // Datastore cache is enabled, do we have an expiration configured?
+                                        exp := time.Duration(s.CacheExpirationSec) * time.Second
+                                        if exp > 0 {
+                                                // The cache enabled enable. Install it into our backend.
+                                                locker := &dsCacheLocker{}
+                                                dsCfg := datastore.Config{
+                                                        RefreshInterval: exp,
+                                                        FailOpen:        s.DatastoreCacheMode == gaeconfig.DSCacheEnabled,
+                                                        LockerFunc:      func(context.Context) datastorecache.Locker { return locker },
+                                                }
+                                                be = dsCfg.Backend(be)
+                                        }
+                                }
+
+                        default:
+                                log.WithError(err).Warningf(c, "Failed to fetch datastore cache settings.")
+                        }
+                }
+
                 // Add a proccache-based config cache.
                 if o.CacheExpiration > 0 {
                         be = caching.ProcCache(be, o.CacheExpiration)
                 }
 
                 return be
         })
 }
+
+type dsCacheLocker struct {
+        p mutexpool.P
+}
+
+func (l *dsCacheLocker) TryWithLock(c context.Context, key string, fn func(context.Context) error) (err error) {
+        l.p.WithMutex(key, func() {
+                err = fn(c)
+        })
+        return
+}