authhandler: Add support for 3-legged-OAuth

Added authhandler.go, which implements a TokenSource
to support "three-legged OAuth 2.0" via a custom
AuthorizationHandler.

Added default_authhandler.go to provide a command line
implementation for AuthorizationHandler.

authhandler/authhandler.go

@@ -0,0 +1,51 @@
+// Copyright 2021 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Package authhandler implements a TokenSource to support
+// "three-legged OAuth 2.0" via a custom AuthorizationHandler.
+package authhandler
+
+import (
+	"context"
+	"errors"
+
+	"golang.org/x/oauth2"
+)
+
+// AuthorizationHandler is a 3-legged-OAuth helper that
+// prompts the user for OAuth consent at the specified Auth URL
+// and returns an auth code and state upon approval.
+type AuthorizationHandler func(string) (string, string, error)
+
+// TokenSource returns an oauth2.TokenSource that fetches access tokens
+// using 3-legged-OAuth flow.
+//
+// The provided oauth2.Config should be a full configuration containing AuthURL,
+// TokenURL, and scope. An environment-specific AuthorizationHandler is used to
+// obtain user consent.
+//
+// Per OAuth protocol, a unique "state" string should be sent and verified
+// before exchanging auth code for OAuth token to prevent CSRF attacks.
+func TokenSource(ctx context.Context, config *oauth2.Config, authHandler AuthorizationHandler, state string) oauth2.TokenSource {
+	return oauth2.ReuseTokenSource(nil, authHandlerSource{config: config, ctx: ctx, authHandler: authHandler, state: state})
+}
+
+type authHandlerSource struct {
+	ctx         context.Context
+	config      *oauth2.Config
+	authHandler AuthorizationHandler
+	state       string
+}
+
+func (source authHandlerSource) Token() (*oauth2.Token, error) {
+	url := source.config.AuthCodeURL(source.state)
+	code, state, err := source.authHandler(url)
+	if err != nil {
+		return nil, err
+	}
+	if state == source.state {
+		return source.config.Exchange(source.ctx, code)
+	}
+	return nil, errors.New("State mismatch in 3-legged-OAuth flow.")
+}