Add server/cache support to gaeconfig.

luci_config/appengine/gaeconfig/default.go

Index: luci_config/appengine/gaeconfig/default.go
diff --git a/luci_config/appengine/gaeconfig/default.go b/luci_config/appengine/gaeconfig/default.go
new file mode 100644
index 0000000000000000000000000000000000000000..d128bad884fcf43abf6a0cf3378df8c137472525
--- /dev/null
+++ b/luci_config/appengine/gaeconfig/default.go
@@ -0,0 +1,122 @@
+// Copyright 2016 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 gaeconfig
+
+import (
+	"errors"
+	"fmt"
+	"os"
+	"path/filepath"
+	"time"
+
+	"github.com/luci/luci-go/common/config/impl/filesystem"
+	"github.com/luci/luci-go/luci_config/appengine/backend/memcache"
+	gaeformat "github.com/luci/luci-go/luci_config/appengine/format"
+	"github.com/luci/luci-go/luci_config/server/cfgclient/backend"
+	"github.com/luci/luci-go/luci_config/server/cfgclient/backend/caching"
+	"github.com/luci/luci-go/luci_config/server/cfgclient/backend/client"
+	"github.com/luci/luci-go/luci_config/server/cfgclient/backend/erroring"
+	"github.com/luci/luci-go/luci_config/server/cfgclient/backend/format"
+	"github.com/luci/luci-go/luci_config/server/cfgclient/backend/testconfig"
+
+	"github.com/luci/gae/service/info"
+
+	"golang.org/x/net/context"
+)
+
+// ErrNotConfigured is returned by cfgclient methods if the config service URL
+// is not set. Usually happens for new apps.
+var ErrNotConfigured = errors.New("config service URL is not set in settings")
+
+// devCfgDir is a name of the directory with config files when running in
+// local dev appserver model. See New for details.
+const devCfgDir = "devcfg"
+
+// Use installs the default luci-config client.
+//
+// The client is configured to use luci-config URL specified in the settings,
+// using GAE app service account for authentication.
+//
+// If running in prod, and the settings don't specify luci-config URL, produces
+// an implementation of backend.B that returns ErrNotConfigured from all
+// methods.
+//
+// If running on devserver, and the settings don't specify luci-config URL,
+// returns a filesystem-based implementation that reads configs from a directory
+// (or a symlink) named 'devcfg' located in the GAE module directory (where
+// app.yaml is) or its immediate parent directory.
+//
+// If such directory can not be located, produces an implementation of
+// cfgclient that returns errors from all methods.
+//
+// Panics if it can't load the settings (should not happen since they are in
+// the local memory cache usually).
+func Use(c context.Context) context.Context { return useImpl(c, nil) }
+
+func useImpl(c context.Context, be backend.B) context.Context {
+	return installConfigBackend(c, mustFetchCachedSettings(c), be)
+}
+
+func installConfigBackend(c context.Context, s *Settings, be backend.B) context.Context {
+	if be == nil {
+		// Non-testing, build a Backend.
+		be = getPrimaryBackend(c, s)
+	}
+
+	// Install a FormatRegistry. Register common config service protobufs with it.
+	c = gaeformat.WithRegistry(c, gaeformat.Default())
+
+	be = &format.Backend{
+		B:           be,
+		GetRegistry: gaeformat.GetRegistry,
+	}
+
+	// Apply caching configuration.
+	exp := time.Duration(s.CacheExpirationSec) * time.Second
+	if exp > 0 {
+		// Add a ProcCache, backed by memcache.
+		be = memcache.Backend(be, exp)
+		be = caching.ProcCache(be, exp)
+	}
+
+	c = backend.WithBackend(c, be)
+	return c
+}
+
+func getPrimaryBackend(c context.Context, settings *Settings) backend.B {
+	// Identify our config service backend (in testing, it will be supplied).
+	if settings.ConfigServiceURL == "" {
+		if info.IsDevAppServer(c) {
+			return devServerBackend()
+		}
+		return erroring.New(ErrNotConfigured)
+	}
+	return &client.Backend{&client.RemoteProvider{
+		BaseURL: settings.ConfigServiceURL,
+	}}
+}
+
+// devServerConfig returns backend.B to use on a devserver.
+//
+// See New for details.
+func devServerBackend() backend.B {
+	pwd := os.Getenv("PWD") // os.Getwd works funny with symlinks, use PWD
+	candidates := []string{
+		filepath.Join(pwd, devCfgDir),
+		filepath.Join(filepath.Dir(pwd), devCfgDir),
+	}
+	for _, dir := range candidates {
+		if _, err := os.Stat(dir); err == nil {
+			fs, err := filesystem.New(dir)
+			if err != nil {
+				return erroring.New(err)
+			}
+			return &client.Backend{&testconfig.Provider{
+				Base: fs,
+			}}
+		}
+	}
+	return erroring.New(fmt.Errorf("luci-config: could not find local configs in any of %s", candidates))
+}