Implement a server-side config service interface.

server/config/authority.go

+// 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()
+        }
+}