Implement a server-side config service interface.

server/config/authority.go

Index: server/config/authority.go
diff --git a/server/config/authority.go b/server/config/authority.go
new file mode 100644
index 0000000000000000000000000000000000000000..e8e7e915e69194343631092eaeb2ae7ced40971a
--- /dev/null
+++ b/server/config/authority.go
@@ -0,0 +1,102 @@
+// Copyright 2016 The LUCI Authors. All rights reserved.

[iannucci, 2017/01/07 20:12:15]

maybe 'luci-config' top level folder and maybe 'server/cfgclient' package name?

+// 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 (
+	"encoding/json"
+	"fmt"
+
+	"github.com/luci/luci-go/common/errors"
+	"github.com/luci/luci-go/server/auth"
+)
+
+// 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
+)
+
+// String implements fmt.Stringer.
+func (a Authority) String() string {

[iannucci, 2017/01/07 20:12:15]

use stringer tool? https://godoc.org/golang.org/x/tools/cmd/stringer

[dnj, 2017/01/10 03:25:57]

Done.

+	switch a {
+	case AsAnonymous:
+		return "AsAnonymous"
+	case AsService:
+		return "AsService"
+	case AsUser:
+		return "AsUser"
+	default:
+		return fmt.Sprintf("Unknown(%d)", a)
+	}
+}
+
+// rpcAuthorityKind returns the RPC authority associated with this authority
+// level.
+func (a Authority) rpcAuthorityKind() auth.RPCAuthorityKind {
+	switch a {
+	case AsAnonymous:
+		return auth.NoAuth
+	case AsService:
+		return auth.AsSelf
+	case AsUser:
+		return auth.AsUser
+	default:
+		panic(fmt.Errorf("unknown config Authority (%d)", a))
+	}
+}
+
+// MarshalJSON implements encoding/json.Marshaler.
+func (a Authority) MarshalJSON() ([]byte, error) {
+	switch a {
+	case AsAnonymous:
+		return json.Marshal("")
+	case AsService:
+		return json.Marshal("S")
+	case AsUser:
+		return json.Marshal("U")

[iannucci, 2017/01/07 20:12:15]

not sure it's worth calling json.Marshal here, constants (e.g. `[]byte("U")`)
would work fine.

[dnj, 2017/01/10 03:25:57]

It would actually be []byte(`"U"`), which is a mistake that I've made before and
the reason I like calling Marshal :) But yeah that's fair, I'll just not make
the mistake. But you see my motivation!

+	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 {

[iannucci, 2017/01/07 20:12:15]

with constants mentioned above this becomes 

switch {
  case bytes.Equal(d, anonymousJSON):
  case bytes.Equal(d, userJSON):
  case bytes.Equal(d, serviceJSON):
  default:
}

[dnj, 2017/01/10 03:25:57]

Done.

+	var v string
+	if err := json.Unmarshal(d, &v); err != nil {
+		return errors.Annotate(err).Reason("failed to unmarshal JSON: %(value)v").D("value", d).Err()
+	}
+
+	switch v {
+	case "":
+		*a = AsAnonymous
+		return nil
+	case "S":
+		*a = AsService
+		return nil
+	case "U":
+		*a = AsUser
+		return nil
+	default:
+		return errors.Reason("unknown authority JSON value: %(auth)v").D("auth", v).Err()
+	}
+}