simplify configuration of oauth clients

README.md

@@ -43,11 +43,40 @@ import (
 func main() {
 	client := &tailscale.Client{
 		Tailnet: os.Getenv("TAILSCALE_TAILNET"),
-		HTTP:    tailscale.OAuthConfig{
+		Auth: &tailscale.OAuth{
 			ClientID:     os.Getenv("TAILSCALE_OAUTH_CLIENT_ID"),
 			ClientSecret: os.Getenv("TAILSCALE_OAUTH_CLIENT_SECRET"),
 			Scopes:       []string{"all:write"},
-		}.HTTPClient(),
+		},
+	}
+
+	devices, err := client.Devices().List(context.Background())
+}
+```
+
+## Example (Using Your Own Authentication Mechanism)
+
+```go
+package main
+
+import (
+	"context"
+	"os"
+
+	"tailscale.com/client/tailscale/v2"
+)
+
+type MyAuth struct {...}
+
+func (a *MyAuth) HTTPClient(orig *http.Client, baseURL string) *http.Client {
+	// build an HTTP client that adds authentication to outgoing requests
+	// see tailscale.OAuth for an example.
+}
+
+func main() {
+	client := &tailscale.Client{
+		Tailnet: os.Getenv("TAILSCALE_TAILNET"),
+		Auth: &MyAuth{...},
 	}
 
 	devices, err := client.Devices().List(context.Background())

client.go

@@ -21,15 +21,26 @@ import (
 	"github.com/tailscale/hujson"
 )
 
+// Auth is a pluggable mechanism for authenticating requests.
+type Auth interface {
+	// HTTPClient builds an http.Client that uses orig as a starting point and
+	// adds its own authentication to outgoing requests. baseURL is the base URL
+	// of the API server to which we will be authenticating.
+	HTTPClient(orig *http.Client, baseURL string) *http.Client
+}
+
 // Client is used to perform actions against the Tailscale API.
 type Client struct {
 	// BaseURL is the base URL for accessing the Tailscale API server. Defaults to https://api.tailscale.com.
 	BaseURL *url.URL
 	// UserAgent configures the User-Agent HTTP header for requests. Defaults to "tailscale-client-go".
 	UserAgent string
 	// APIKey allows specifying an APIKey to use for authentication.
-	// To use OAuth Client credentials, construct an [http.Client] using [OAuthConfig] and specify that below.
+	// To use OAuth Client credentials, specify OAuthCredentials instead.
 	APIKey string
+	// Auth specifies a mechanism for adding authentication to outgoing requests.
+	// If provided, APIKey is ignored.
+	Auth Auth
 	// Tailnet allows specifying a specific tailnet by name, to which this Client will connect by default.
 	// If Tailnet is left blank, the client will connect to default tailnet based on the client's credential,
 	// using the "-" (dash) default tailnet path.
@@ -97,6 +108,10 @@ func (c *Client) init() {
 		if c.Tailnet == "" {
 			c.Tailnet = "-"
 		}
+		if c.Auth != nil {
+			c.APIKey = ""
+			c.HTTP = c.Auth.HTTPClient(c.HTTP, c.BaseURL.String())
+		}
 		c.contacts = &ContactsResource{c}
 		c.devicePosture = &DevicePostureResource{c}
 		c.devices = &DevicesResource{c}

oauth.go

@@ -7,10 +7,50 @@ import (
 	"context"
 	"net/http"
 
+	"golang.org/x/oauth2"
 	"golang.org/x/oauth2/clientcredentials"
 )
 
+// Ensure that [OAuth] implements the [Auth] interface.
+var _ Auth = &OAuth{}
+
+// OAuth configures OAuth authentication.
+type OAuth struct {
+	// ClientID is the client ID of the OAuth client.
+	ClientID string
+	// ClientSecret is the client secret of the OAuth client.
+	ClientSecret string
+	// Scopes are the scopes to request when generating tokens for this OAuth client.
+	Scopes []string
+}
+
+// HTTPClient constructs an HTTP client based on the provided HTTP client, authenticating
+// every request using OAuth and fetching tokens from baseURL + "/api/v2/oauth/token" as
+// necessary based on when the token expires.
+func (o *OAuth) HTTPClient(orig *http.Client, baseURL string) *http.Client {
+	oauthConfig := clientcredentials.Config{
+		ClientID:     o.ClientID,
+		ClientSecret: o.ClientSecret,
+		Scopes:       o.Scopes,
+		TokenURL:     baseURL + "/api/v2/oauth/token",
+	}
+
+	// Use context.Background() here, since this is used to refresh the token in the future.
+	tokenSource := oauthConfig.TokenSource(context.Background())
+
+	return &http.Client{
+		Transport: &oauth2.Transport{
+			Base:   orig.Transport,
+			Source: oauth2.ReuseTokenSource(nil, tokenSource),
+		},
+		CheckRedirect: orig.CheckRedirect,
+		Jar:           orig.Jar,
+		Timeout:       orig.Timeout,
+	}
+}
+
 // OAuthConfig provides a mechanism for configuring OAuth authentication.
+// Deprecated: use [OAuth] instead.
 type OAuthConfig struct {
 	// ClientID is the client ID of the OAuth client.
 	ClientID string
@@ -23,6 +63,7 @@ type OAuthConfig struct {
 }
 
 // HTTPClient constructs an HTTP client that authenticates using OAuth.
+// Deprecated: use [OAuth] instead.
 func (ocfg OAuthConfig) HTTPClient() *http.Client {
 	baseURL := ocfg.BaseURL
 	if baseURL == "" {