LogDog: Use luci_config/server packages for config

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/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.
+        CacheExpiration time.Duration

[iannucci, 2017/01/21 00:23:13]

a negative (or 0?) value has no effect

[dnj, 2017/01/21 01:37:31]

Done.

+}
+
+// WrapBackend wraps the supplied backend with caching as appropriately
+// configured.

[iannucci, 2017/01/21 00:23:13]

Maybe:

  WrapBackend updates the context with a new backend that applies these
CacheOptions.

[dnj, 2017/01/21 01:37:31]

Done.

+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
+
+                // Add a proccache-based config cache.
+                if o.CacheExpiration > 0 {

[iannucci, 2017/01/21 00:23:13]

== 0 could be problematic... I'd skip proccache in that case as well (especially
since go has this whole zero-value initialization thing)

[dnj, 2017/01/21 01:37:31]

Done.

+                        be = caching.ProcCache(be, o.CacheExpiration)
+                }
+
+                return be
+        })
+}