Extend LUCI_CONTEXT["local_auth"] protocol to understand accounts.

lucictx/local_auth.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 lucictx
 
 import (
         "fmt"
 
         "golang.org/x/net/context"
 )
 
 // LocalAuth is a struct that may be used with the "local_auth" section of
 // LUCI_CONTEXT.
 type LocalAuth struct {
+        // RPCPort and Secret define how to connect to the local auth server.
         RPCPort uint32 `json:"rpc_port"`
         Secret  []byte `json:"secret"`
+
+        // Accounts and DefaultAccountID defines what access tokens are available.
+        Accounts         []LocalAuthAccount `json:"accounts"`
+        DefaultAccountID string             `json:"default_account_id"`
+}
+
+// LocalAuthAccount contains information about a service account available
+// through a local auth server.
+type LocalAuthAccount struct {
+        // ID is logical identifier of the account, e.g. "system" or "task".
+        ID string `json:"id"`
+}
+
+// CanUseByDefault returns true if the authentication context can be picked up
+// by default.
+//
+// TODO(vadimsh): Remove this method once all servers provide 'accounts'.
+func (la *LocalAuth) CanUseByDefault() bool {
+        // Old API servers don't provide list of accounts. Instead there's single
+        // account that is always used by default.
+        if len(la.Accounts) == 0 {
+                return true
+        }
+        // New API servers give a list of available account and an optional default
+        // account. Auth should be used only if default account is given.
+        return la.DefaultAccountID != ""
 }
 
 // GetLocalAuth calls Lookup and returns the current LocalAuth from LUCI_CONTEXT
 // if it was present. If no LocalAuth is in the context, this returns nil.
 func GetLocalAuth(ctx context.Context) *LocalAuth {
         ret := LocalAuth{}
         ok, err := Lookup(ctx, "local_auth", &ret)
         if err != nil {
                 panic(err)
         }
         if !ok {
                 return nil
         }
         return &ret
 }
 
 // SetLocalAuth Sets the LocalAuth in the LUCI_CONTEXT.
 func SetLocalAuth(ctx context.Context, la *LocalAuth) context.Context {
         ctx, err := Set(ctx, "local_auth", la)
         if err != nil {
                 panic(fmt.Errorf("impossible: %s", err))
         }
         return ctx
 }