Allow for arbitrary OAuth hosts

Not all OAuth hosts use the same routes as GitHub, for example:
- Microsoft use /oauth2/v2.0/devicecode
- Google use /device/code
- Auth0 use /oauth/device/code

Similar differences are present for the authorise and access token
routes too.

This commit introduces a concept of a Server, which is a container for
the endpoints that the library uses. This is a replacement for Flow's
Hostname and as such is a breaking change.

examples_test.go

@@ -10,7 +10,7 @@ import (
 // flow support is globally available, but enables logging in to hosted GitHub instances as well.
 func Example() {
 	flow := &Flow{
-		Hostname:     "github.com",
+		Server:       ServerGitHub("github.com"),
 		ClientID:     os.Getenv("OAUTH_CLIENT_ID"),
 		ClientSecret: os.Getenv("OAUTH_CLIENT_SECRET"), // only applicable to web app flow
 		CallbackURI:  "http://127.0.0.1/callback",      // only applicable to web app flow

go.sum

@@ -1,3 +1,4 @@
 github.com/cli/browser v1.0.0 h1:RIleZgXrhdiCVgFBSjtWwkLPUCWyhhhN5k5HGSBt1js=
 github.com/cli/browser v1.0.0/go.mod h1:IEWkHYbLjkhtjwwWlwTHW2lGxeS5gezEQBMLTwDHf5Q=
+github.com/cli/safeexec v1.0.0 h1:0VngyaIyqACHdcMNWfo6+KdUYnqEr2Sg+bSP1pdF+dI=
 github.com/cli/safeexec v1.0.0/go.mod h1:Z/D4tTN8Vs5gXYHDCbaM1S/anmEDnJb1iW0+EJ5zx3Q=

oauth.go

@@ -4,7 +4,6 @@ package oauth
 
 import (
 	"errors"
-	"fmt"
 	"io"
 	"net/http"
 	"net/url"
@@ -19,8 +18,8 @@ type httpClient interface {
 
 // Flow facilitates a single OAuth authorization flow.
 type Flow struct {
-	// The host to authorize the app with.
-	Hostname string
+	// The server to authorize the app with.
+	Server Server
 	// OAuth scopes to request from the user.
 	Scopes []string
 	// OAuth application ID.
@@ -47,18 +46,6 @@ type Flow struct {
 	Stdout io.Writer
 }
 
-func deviceInitURL(host string) string {
-	return fmt.Sprintf("https://%s/login/device/code", host)
-}
-
-func webappInitURL(host string) string {
-	return fmt.Sprintf("https://%s/login/oauth/authorize", host)
-}
-
-func tokenURL(host string) string {
-	return fmt.Sprintf("https://%s/login/oauth/access_token", host)
-}
-
 // DetectFlow tries to perform Device flow first and falls back to Web application flow.
 func (oa *Flow) DetectFlow() (*api.AccessToken, error) {
 	accessToken, err := oa.DeviceFlow()

oauth_device.go

@@ -29,7 +29,7 @@ func (oa *Flow) DeviceFlow() (*api.AccessToken, error) {
 		stdout = os.Stdout
 	}
 
-	code, err := device.RequestCode(httpClient, deviceInitURL(oa.Hostname), oa.ClientID, oa.Scopes)
+	code, err := device.RequestCode(httpClient, oa.Server.DeviceCodeURL, oa.ClientID, oa.Scopes)
 	if err != nil {
 		return nil, err
 	}
@@ -54,7 +54,7 @@ func (oa *Flow) DeviceFlow() (*api.AccessToken, error) {
 		return nil, fmt.Errorf("error opening the web browser: %w", err)
 	}
 
-	return device.PollToken(httpClient, tokenURL(oa.Hostname), oa.ClientID, code)
+	return device.PollToken(httpClient, oa.Server.TokenURL, oa.ClientID, code)
 }
 
 func waitForEnter(r io.Reader) error {

oauth_webapp.go

@@ -23,7 +23,7 @@ func (oa *Flow) WebAppFlow() (*api.AccessToken, error) {
 		Scopes:      oa.Scopes,
 		AllowSignup: true,
 	}
-	browserURL, err := flow.BrowserURL(webappInitURL(oa.Hostname), params)
+	browserURL, err := flow.BrowserURL(oa.Server.AuthoriseURL, params)
 	if err != nil {
 		return nil, err
 	}
@@ -47,5 +47,5 @@ func (oa *Flow) WebAppFlow() (*api.AccessToken, error) {
 		httpClient = http.DefaultClient
 	}
 
-	return flow.AccessToken(httpClient, tokenURL(oa.Hostname), oa.ClientSecret)
+	return flow.AccessToken(httpClient, oa.Server.TokenURL, oa.ClientSecret)
 }

server.go

@@ -0,0 +1,24 @@
+package oauth
+
+import "fmt"
+
+// Server represents the endpoints used to authorise against an OAuth server.
+type Server struct {
+	DeviceCodeURL string
+	AuthoriseURL  string
+	TokenURL      string
+}
+
+// ServerGitHub gets a Server for the given GitHub (Enterprise) host. If host is empty,
+// it will default to "github.com".
+func ServerGitHub(host string) Server {
+	if host == "" {
+		host = "github.com"
+	}
+
+	return Server{
+		DeviceCodeURL: fmt.Sprintf("https://%s/login/device/code", host),
+		AuthoriseURL:  fmt.Sprintf("https://%s/login/oauth/authorize", host),
+		TokenURL:      fmt.Sprintf("https://%s/login/oauth/access_token", host),
+	}
+}