Implement a server-side config service interface.

luci_config/server/cfgclient/format.go

Index: luci_config/server/cfgclient/format.go
diff --git a/luci_config/server/cfgclient/format.go b/luci_config/server/cfgclient/format.go
new file mode 100644
index 0000000000000000000000000000000000000000..ef1620c148f7348735f4fff0d6beae521267e4a2
--- /dev/null
+++ b/luci_config/server/cfgclient/format.go
@@ -0,0 +1,54 @@
+// 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 cfgclient
+
+import (
+	"fmt"
+	"sync"
+)
+
+// Formatter applies a transformation to the data's cached representation.
+//
+// The Formatter is supplied with the content to format along with the
+// formatter-specific metadata, fd.
+//
+// Formatter operates on a backend.Item's Content via the "format" Backend.
+type Formatter interface {
+	FormatItem(c, fd string) (string, error)
+}
+
+// FormatterRegistry is a registry of Resolver key mapped to the Formatter to
+// use to format that key.
+type FormatterRegistry struct {
+	registryMu sync.RWMutex
+	registry   map[string]Formatter
+}
+
+// Register registers a Formatter with the FormatBackend.
+//
+// If the supplied key is already registered, Register will panic.
+func (r *FormatterRegistry) Register(rk string, f Formatter) {
+	if rk == "" {
+		panic("cannot register empty key")
+	}
+
+	r.registryMu.Lock()
+	defer r.registryMu.Unlock()
+
+	if _, ok := r.registry[rk]; ok {
+		panic(fmt.Errorf("key %q is already registered", rk))
+	}
+	if r.registry == nil {
+		r.registry = map[string]Formatter{}
+	}
+	r.registry[rk] = f
+}
+
+// Get returns the Formatter associated with the provided Format.
+func (r *FormatterRegistry) Get(f string) Formatter {
+	r.registryMu.RLock()
+	defer r.registryMu.RUnlock()
+	return r.registry[f]
+}