LogDog: Use datastore config cache when available.

logdog/server/service/config/opts.go

Index: logdog/server/service/config/opts.go
diff --git a/logdog/server/service/config/opts.go b/logdog/server/service/config/opts.go
index d26518b02a089326698795498fb683e1da2b8184..b62de5134dd1f339136f1c1c98c045a015d0c4fb 100644
--- a/logdog/server/service/config/opts.go
+++ b/logdog/server/service/config/opts.go
@@ -7,6 +7,11 @@ 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"
 
@@ -20,6 +25,12 @@ type CacheOptions struct {
 	// 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
@@ -29,6 +40,33 @@ func (o *CacheOptions) WrapBackend(c context.Context, base backend.B) context.Co
 		// 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)
@@ -37,3 +75,14 @@ func (o *CacheOptions) WrapBackend(c context.Context, base backend.B) context.Co
 		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
+}