Implement a server-side config service interface.

luci_config/server/cfgclient/backend/authority.go

+// 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
+}