Implement a server-side config service interface.

luci_config/server/cfgclient/backend/authority.go

Index: luci_config/server/cfgclient/backend/authority.go
diff --git a/luci_config/server/cfgclient/backend/authority.go b/luci_config/server/cfgclient/backend/authority.go
new file mode 100644
index 0000000000000000000000000000000000000000..819fd873f17f6da50fdaf6fdb8800783178dea7c
--- /dev/null
+++ b/luci_config/server/cfgclient/backend/authority.go
@@ -0,0 +1,72 @@
+// 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 backend
+
+import (
+	"bytes"
+
+	"github.com/luci/luci-go/common/errors"
+)
+
+// Authority is the authority that is requesting configurations. It can be
+// installed via WithAuthority.
+//
+// Authority marshals/unmarshals to/from a compact JSON representation. This is
+// used by the caching layer.
+type Authority int
+
+const (
+	// AsAnonymous requests config data as an anonymous user.
+	//
+	// Corresponds to auth.NoAuth.
+	AsAnonymous Authority = iota
+
+	// AsService requests config data as the currently-running service.
+	//
+	// Corresponds to auth.AsSelf.
+	AsService
+
+	// AsUser requests config data as the currently logged-in user.
+	//
+	// Corresponds to auth.AsUser.
+	AsUser
+)
+
+var (
+	asAnonymousJSON = []byte(`""`)
+	asServiceJSON   = []byte(`"S"`)
+	asUserJSON      = []byte(`"U"`)
+)
+
+// MarshalJSON implements encoding/json.Marshaler.
+//
+// We use a shorthand notation so that we don't waste space in JSON.
+func (a Authority) MarshalJSON() ([]byte, error) {
+	switch a {
+	case AsAnonymous:
+		return asAnonymousJSON, nil
+	case AsService:
+		return asServiceJSON, nil
+	case AsUser:
+		return asUserJSON, nil
+	default:
+		return nil, errors.Reason("unknown authority: %(auth)v").D("auth", a).Err()
+	}
+}
+
+// UnmarshalJSON implements encoding/json.Unmarshaler.
+func (a *Authority) UnmarshalJSON(d []byte) error {
+	switch {
+	case bytes.Equal(d, asAnonymousJSON):
+		*a = AsAnonymous
+	case bytes.Equal(d, asServiceJSON):
+		*a = AsService
+	case bytes.Equal(d, asUserJSON):
+		*a = AsUser
+	default:
+		return errors.Reason("unknown authority JSON value: %(auth)v").D("auth", d).Err()
+	}
+	return nil
+}