dashboard: cache per-ns manager lists

The query may take up to 100ms in some cases, while the result changes
on quite rare occasions.

Let's use a cached version of the data when rendering UI pages.

We don't need extra tests because it's already excercised in existing
tests that trigger web endpoints.

3 files changed, 33 insertions, 4 deletions

diff --git a/dashboard/app/api.go b/dashboard/app/api.go
index a51a2328e..2dce92d55 100644
--- a/dashboard/app/api.go
+++ b/dashboard/app/api.go
@@ -642,6 +642,7 @@ func bugNeedsCommitUpdate(c context.Context, bug *Bug, manager string, fixCommit
 	return true
 }
 
+// Note: if you do not need the latest data, prefer CachedManagersList().
 func managerList(c context.Context, ns string) ([]string, error) {
 	var builds []*Build
 	_, err := db.NewQuery("Build").
diff --git a/dashboard/app/cache.go b/dashboard/app/cache.go
index 4e77d55cf..f52cf5d11 100644
--- a/dashboard/app/cache.go
+++ b/dashboard/app/cache.go
@@ -206,3 +206,31 @@ func minuteCacheNsUpdate(c context.Context, ns string) error {
 	}
 	return nil
 }
+
+func CachedManagerList(c context.Context, ns string) ([]string, error) {
+	key := fmt.Sprintf("%s-managers-list", ns)
+
+	// Check if the object is in cache.
+	list := []string{}
+	_, err := memcache.Gob.Get(c, key, &list)
+	if err == nil {
+		return list, nil
+	} else if err != memcache.ErrCacheMiss {
+		return nil, err
+	}
+
+	// Update cache.
+	list, err = managerList(c, ns)
+	if err != nil {
+		return nil, err
+	}
+	item := &memcache.Item{
+		Key:        key,
+		Object:     list,
+		Expiration: time.Minute,
+	}
+	if err := memcache.Gob.Set(c, item); err != nil {
+		return nil, err
+	}
+	return list, nil
+}
diff --git a/dashboard/app/main.go b/dashboard/app/main.go
index 18e43a897..4b525b694 100644
--- a/dashboard/app/main.go
+++ b/dashboard/app/main.go
@@ -958,7 +958,7 @@ func handleBug(c context.Context, w http.ResponseWriter, r *http.Request) error
 	if err != nil {
 		return err
 	}
-	managers, err := managerList(c, bug.Namespace)
+	managers, err := CachedManagerList(c, bug.Namespace)
 	if err != nil {
 		return err
 	}
@@ -1517,7 +1517,7 @@ func fetchNamespaceBugs(c context.Context, accessLevel AccessLevel, ns string,
 	if err != nil {
 		return nil, err
 	}
-	managers, err := managerList(c, ns)
+	managers, err := CachedManagerList(c, ns)
 	if err != nil {
 		return nil, err
 	}
@@ -1652,7 +1652,7 @@ func fetchTerminalBugs(c context.Context, accessLevel AccessLevel,
 	if err != nil {
 		return nil, nil, err
 	}
-	managers, err := managerList(c, ns)
+	managers, err := CachedManagerList(c, ns)
 	if err != nil {
 		return nil, nil, err
 	}
@@ -1744,7 +1744,7 @@ func loadSimilarBugsUI(c context.Context, r *http.Request, bug *Bug, state *Repo
 			continue
 		}
 		if managers[similar.Namespace] == nil {
-			mgrs, err := managerList(c, similar.Namespace)
+			mgrs, err := CachedManagerList(c, similar.Namespace)
 			if err != nil {
 				return nil, err
 			}