feat(client): OAuth2 Client credential support (#259)

* Initial implementation

* Added OAuth2 support to `client` config

* Revert "Initial implementation"

This reverts commit 7f2f3a603ae018b1cd1c6a282104f44cd9a1a1d1.

* Restore vendored clientcredentials

* configureOAuth2 is now a func (including tests)

* README update

* Use the same OAuth2Config in all related tests

* Cleanup & comments

vendor/golang.org/x/oauth2/clientcredentials/clientcredentials.go

@@ -0,0 +1,120 @@
+// Copyright 2014 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 clientcredentials implements the OAuth2.0 "client credentials" token flow,
+// also known as the "two-legged OAuth 2.0".
+//
+// This should be used when the client is acting on its own behalf or when the client
+// is the resource owner. It may also be used when requesting access to protected
+// resources based on an authorization previously arranged with the authorization
+// server.
+//
+// See https://tools.ietf.org/html/rfc6749#section-4.4
+package clientcredentials // import "golang.org/x/oauth2/clientcredentials"
+
+import (
+	"context"
+	"fmt"
+	"net/http"
+	"net/url"
+	"strings"
+
+	"golang.org/x/oauth2"
+	"golang.org/x/oauth2/internal"
+)
+
+// Config describes a 2-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
+
+	// TokenURL is the resource server's token endpoint
+	// URL. This is a constant specific to each server.
+	TokenURL string
+
+	// Scope specifies optional requested permissions.
+	Scopes []string
+
+	// EndpointParams specifies additional parameters for requests to the token endpoint.
+	EndpointParams url.Values
+
+	// AuthStyle optionally specifies how the endpoint wants the
+	// client ID & client secret sent. The zero value means to
+	// auto-detect.
+	AuthStyle oauth2.AuthStyle
+}
+
+// Token uses client credentials to retrieve a token.
+//
+// The provided context optionally controls which HTTP client is used. See the oauth2.HTTPClient variable.
+func (c *Config) Token(ctx context.Context) (*oauth2.Token, error) {
+	return c.TokenSource(ctx).Token()
+}
+
+// Client returns an HTTP client using the provided token.
+// The token will auto-refresh as necessary.
+//
+// The provided context optionally controls which HTTP client
+// is returned. See the oauth2.HTTPClient variable.
+//
+// The returned Client and its Transport should not be modified.
+func (c *Config) Client(ctx context.Context) *http.Client {
+	return oauth2.NewClient(ctx, c.TokenSource(ctx))
+}
+
+// TokenSource returns a TokenSource that returns t until t expires,
+// automatically refreshing it as necessary using the provided context and the
+// client ID and client secret.
+//
+// Most users will use Config.Client instead.
+func (c *Config) TokenSource(ctx context.Context) oauth2.TokenSource {
+	source := &tokenSource{
+		ctx:  ctx,
+		conf: c,
+	}
+	return oauth2.ReuseTokenSource(nil, source)
+}
+
+type tokenSource struct {
+	ctx  context.Context
+	conf *Config
+}
+
+// Token refreshes the token by using a new client credentials request.
+// tokens received this way do not include a refresh token
+func (c *tokenSource) Token() (*oauth2.Token, error) {
+	v := url.Values{
+		"grant_type": {"client_credentials"},
+	}
+	if len(c.conf.Scopes) > 0 {
+		v.Set("scope", strings.Join(c.conf.Scopes, " "))
+	}
+	for k, p := range c.conf.EndpointParams {
+		// Allow grant_type to be overridden to allow interoperability with
+		// non-compliant implementations.
+		if _, ok := v[k]; ok && k != "grant_type" {
+			return nil, fmt.Errorf("oauth2: cannot overwrite parameter %q", k)
+		}
+		v[k] = p
+	}
+
+	tk, err := internal.RetrieveToken(c.ctx, c.conf.ClientID, c.conf.ClientSecret, c.conf.TokenURL, v, internal.AuthStyle(c.conf.AuthStyle))
+	if err != nil {
+		if rErr, ok := err.(*internal.RetrieveError); ok {
+			return nil, (*oauth2.RetrieveError)(rErr)
+		}
+		return nil, err
+	}
+	t := &oauth2.Token{
+		AccessToken:  tk.AccessToken,
+		TokenType:    tk.TokenType,
+		RefreshToken: tk.RefreshToken,
+		Expiry:       tk.Expiry,
+	}
+	return t.WithExtra(tk.Raw), nil
+}

vendor/modules.txt

@@ -116,6 +116,7 @@ golang.org/x/net/ipv6
 # golang.org/x/oauth2 v0.0.0-20210514164344-f6687ab2804c
 ## explicit; go 1.11
 golang.org/x/oauth2
+golang.org/x/oauth2/clientcredentials
 golang.org/x/oauth2/internal
 # golang.org/x/sync v0.0.0-20210220032951-036812b2e83c
 ## explicit