// Copyright 2014 The oauth2 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 oauth2 provides support for making
// OAuth2 authorized and authenticated HTTP requests.
// It can additionally grant authorization with Bearer JWT.
package oauth2

import (
	"bytes"
	"errors"
	"net/http"
	"net/url"
	"strings"
	"sync"

	"golang.org/x/net/context"
	"golang.org/x/oauth2/internal"
)

// NoContext is the default context you should supply if not using
// your own context.Context (see https://golang.org/x/net/context).
var NoContext = context.TODO()

// Config describes a typical 3-legged OAuth2 flow, with both the
// client application information and the server's endpoint URLs.
type Config struct {
	// ClientID is the application's ID.
	ClientID string

	// ClientSecret is the application's secret.
	ClientSecret string

	// Endpoint contains the resource server's token endpoint
	// URLs. These are constants specific to each server and are
	// often available via site-specific packages, such as
	// google.Endpoint or github.Endpoint.
	Endpoint Endpoint

	// RedirectURL is the URL to redirect users going through
	// the OAuth flow, after the resource owner's URLs.
	RedirectURL string

	// Scope specifies optional requested permissions.
	Scopes []string
}

// A TokenSource is anything that can return a token.
type TokenSource interface {
	// Token returns a token or an error.
	// Token must be safe for concurrent use by multiple goroutines.
	// The returned Token must not be modified.
	Token() (*Token, error)
}

// Endpoint contains the OAuth 2.0 provider's authorization and token
// endpoint URLs.
type Endpoint struct {
	AuthURL  string
	TokenURL string
}

var (
	// AccessTypeOnline and AccessTypeOffline are options passed
	// to the Options.AuthCodeURL method. They modify the
	// "access_type" field that gets sent in the URL returned by
	// AuthCodeURL.
	//
	// Online is the default if neither is specified. If your
	// application needs to refresh access tokens when the user
	// is not present at the browser, then use offline. This will
	// result in your application obtaining a refresh token the
	// first time your application exchanges an authorization
	// code for a user.
	AccessTypeOnline  AuthCodeOption = SetAuthURLParam("access_type", "online")
	AccessTypeOffline AuthCodeOption = SetAuthURLParam("access_type", "offline")

	// ApprovalForce forces the users to view the consent dialog
	// and confirm the permissions request at the URL returned
	// from AuthCodeURL, even if they've already done so.
	ApprovalForce AuthCodeOption = SetAuthURLParam("approval_prompt", "force")
)

// An AuthCodeOption is passed to Config.AuthCodeURL.
type AuthCodeOption interface {
	setValue(url.Values)
}

type setParam struct{ k, v string }

func (p setParam) setValue(m url.Values) { m.Set(p.k, p.v) }

// SetAuthURLParam builds an AuthCodeOption which passes key/value parameters
// to a provider's authorization endpoint.
func SetAuthURLParam(key, value string) AuthCodeOption {
	return setParam{key, value}
}

// AuthCodeURL returns a URL to OAuth 2.0 provider's consent page
// that asks for permissions for the required scopes explicitly.
//
// State is a token to protect the user from CSRF attacks. You must
// always provide a non-zero string and validate that it matches the
// the state query parameter on your redirect callback.
// See http://tools.ietf.org/html/rfc6749#section-10.12 for more info.
//
// Opts may include AccessTypeOnline or AccessTypeOffline, as well
// as ApprovalForce.
func (c *Config) AuthCodeURL(state string, opts ...AuthCodeOption) string {
	var buf bytes.Buffer
	buf.WriteString(c.Endpoint.AuthURL)
	v := url.Values{
		"response_type": {"code"},
		"client_id":     {c.ClientID},
		"redirect_uri":  internal.CondVal(c.RedirectURL),
		"scope":         internal.CondVal(strings.Join(c.Scopes, " ")),
		"state":         internal.CondVal(state),
	}
	for _, opt := range opts {
		opt.setValue(v)
	}
	if strings.Contains(c.Endpoint.AuthURL, "?") {
		buf.WriteByte('&')
	} else {
		buf.WriteByte('?')
	}
	buf.WriteString(v.Encode())
	return buf.String()
}

// PasswordCredentialsToken converts a resource owner username and password
// pair into a token.
//
// Per the RFC, this grant type should only be used "when there is a high
// degree of trust between the resource owner and the client (e.g., the client
// is part of the device operating system or a highly privileged application),
// and when other authorization grant types are not available."
// See https://tools.ietf.org/html/rfc6749#section-4.3 for more info.
//
// The HTTP client to use is derived from the context.
// If nil, http.DefaultClient is used.
func (c *Config) PasswordCredentialsToken(ctx context.Context, username, password string) (*Token, error) {
	return retrieveToken(ctx, c, url.Values{
		"grant_type": {"password"},
		"username":   {username},
		"password":   {password},
		"scope":      internal.CondVal(strings.Join(c.Scopes, " ")),
	})
}

// Exchange converts an authorization code into a token.
//
// It is used after a resource provider redirects the user back
// to the Redirect URI (the URL obtained from AuthCodeURL).
//
// The HTTP client to use is derived from the context.
// If a client is not provided via the context, http.DefaultClient is used.
//
// The code will be in the *http.Request.FormValue("code"). Before
// calling Exchange, be sure to validate FormValue("state").
func (c *Config) Exchange(ctx context.Context, code string) (*Token, error) {
	return retrieveToken(ctx, c, url.Values{
		"grant_type":   {"authorization_code"},
		"code":         {code},
		"redirect_uri": internal.CondVal(c.RedirectURL),
		"scope":        internal.CondVal(strings.Join(c.Scopes, " ")),
	})
}

// Client returns an HTTP client using the provided token.
// The token will auto-refresh as necessary. The underlying
// HTTP transport will be obtained using the provided context.
// The returned client and its Transport should not be modified.
func (c *Config) Client(ctx context.Context, t *Token) *http.Client {
	return NewClient(ctx, c.TokenSource(ctx, t))
}

// TokenSource returns a TokenSource that returns t until t expires,
// automatically refreshing it as necessary using the provided context.
//
// Most users will use Config.Client instead.
func (c *Config) TokenSource(ctx context.Context, t *Token) TokenSource {
	tkr := &tokenRefresher{
		ctx:  ctx,
		conf: c,
	}
	if t != nil {
		tkr.refreshToken = t.RefreshToken
	}
	return &reuseTokenSource{
		t:   t,
		new: tkr,
	}
}

// tokenRefresher is a TokenSource that makes "grant_type"=="refresh_token"
// HTTP requests to renew a token using a RefreshToken.
type tokenRefresher struct {
	ctx          context.Context // used to get HTTP requests
	conf         *Config
	refreshToken string
}

// WARNING: Token is not safe for concurrent access, as it
// updates the tokenRefresher's refreshToken field.
// Within this package, it is used by reuseTokenSource which
// synchronizes calls to this method with its own mutex.
func (tf *tokenRefresher) Token() (*Token, error) {
	if tf.refreshToken == "" {
		return nil, errors.New("oauth2: token expired and refresh token is not set")
	}

	tk, err := retrieveToken(tf.ctx, tf.conf, url.Values{
		"grant_type":    {"refresh_token"},
		"refresh_token": {tf.refreshToken},
	})

	if err != nil {
		return nil, err
	}
	if tf.refreshToken != tk.RefreshToken {
		tf.refreshToken = tk.RefreshToken
	}
	return tk, err
}

// reuseTokenSource is a TokenSource that holds a single token in memory
// and validates its expiry before each call to retrieve it with
// Token. If it's expired, it will be auto-refreshed using the
// new TokenSource.
type reuseTokenSource struct {
	new TokenSource // called when t is expired.

	mu sync.Mutex // guards t
	t  *Token
}

// Token returns the current token if it's still valid, else will
// refresh the current token (using r.Context for HTTP client
// information) and return the new one.
func (s *reuseTokenSource) Token() (*Token, error) {
	s.mu.Lock()
	defer s.mu.Unlock()
	if s.t.Valid() {
		return s.t, nil
	}
	t, err := s.new.Token()
	if err != nil {
		return nil, err
	}
	s.t = t
	return t, nil
}

// StaticTokenSource returns a TokenSource that always returns the same token.
// Because the provided token t is never refreshed, StaticTokenSource is only
// useful for tokens that never expire.
func StaticTokenSource(t *Token) TokenSource {
	return staticTokenSource{t}
}

// staticTokenSource is a TokenSource that always returns the same Token.
type staticTokenSource struct {
	t *Token
}

func (s staticTokenSource) Token() (*Token, error) {
	return s.t, nil
}

// HTTPClient is the context key to use with golang.org/x/net/context's
// WithValue function to associate an *http.Client value with a context.
var HTTPClient internal.ContextKey

// NewClient creates an *http.Client from a Context and TokenSource.
// The returned client is not valid beyond the lifetime of the context.
//
// As a special case, if src is nil, a non-OAuth2 client is returned
// using the provided context. This exists to support related OAuth2
// packages.
func NewClient(ctx context.Context, src TokenSource) *http.Client {
	if src == nil {
		c, err := internal.ContextClient(ctx)
		if err != nil {
			return &http.Client{Transport: internal.ErrorTransport{err}}
		}
		return c
	}
	return &http.Client{
		Transport: &Transport{
			Base:   internal.ContextTransport(ctx),
			Source: ReuseTokenSource(nil, src),
		},
	}
}

// ReuseTokenSource returns a TokenSource which repeatedly returns the
// same token as long as it's valid, starting with t.
// When its cached token is invalid, a new token is obtained from src.
//
// ReuseTokenSource is typically used to reuse tokens from a cache
// (such as a file on disk) between runs of a program, rather than
// obtaining new tokens unnecessarily.
//
// The initial token t may be nil, in which case the TokenSource is
// wrapped in a caching version if it isn't one already. This also
// means it's always safe to wrap ReuseTokenSource around any other
// TokenSource without adverse effects.
func ReuseTokenSource(t *Token, src TokenSource) TokenSource {
	// Don't wrap a reuseTokenSource in itself. That would work,
	// but cause an unnecessary number of mutex operations.
	// Just build the equivalent one.
	if rt, ok := src.(*reuseTokenSource); ok {
		if t == nil {
			// Just use it directly.
			return rt
		}
		src = rt.new
	}
	return &reuseTokenSource{
		t:   t,
		new: src,
	}
}
