pcloud: fix oauth on European region "eapi.pcloud.com"

Pcloud appears to have opened up a new region and they are returning
the hostname in the oauth callback, thus

    GET /?code=XXX&locationid=1&hostname=api.pcloud.com&state=XXX HTTP/1.1
    GET /?code=XXX&locationid=2&hostname=eapi.pcloud.com&state=XXX HTTP/1.1

This isn't documented yet, however pCloud have confirmed that this is
the correct interpretation.

Rclone now reads the "hostname" parameter in the oauth callback and
stores it in the config file. It uses it for all subequent API calls.

backend/pcloud/pcloud.go

@@ -42,7 +42,7 @@ const (
 	minSleep                    = 10 * time.Millisecond
 	maxSleep                    = 2 * time.Second
 	decayConstant               = 2 // bigger for slower decay, exponential
-	rootURL                     = "https://api.pcloud.com"
+	defaultHostname             = "api.pcloud.com"
 )
 
 // Globals
@@ -51,8 +51,8 @@ var (
 	oauthConfig = &oauth2.Config{
 		Scopes: nil,
 		Endpoint: oauth2.Endpoint{
-			AuthURL:  "https://my.pcloud.com/oauth2/authorize",
+			AuthURL: "https://my.pcloud.com/oauth2/authorize",
-			TokenURL: "https://api.pcloud.com/oauth2_token",
+			// TokenURL: "https://api.pcloud.com/oauth2_token", set by updateTokenURL
 		},
 		ClientID:     rcloneClientID,
 		ClientSecret: obscure.MustReveal(rcloneEncryptedClientSecret),
@@ -60,17 +60,45 @@ var (
 	}
 )
 
+// Update the TokenURL with the actual hostname
+func updateTokenURL(oauthConfig *oauth2.Config, hostname string) {
+	oauthConfig.Endpoint.TokenURL = "https://" + hostname + "/oauth2_token"
+}
+
 // Register with Fs
 func init() {
+	updateTokenURL(oauthConfig, defaultHostname)
 	fs.Register(&fs.RegInfo{
 		Name:        "pcloud",
 		Description: "Pcloud",
 		NewFs:       NewFs,
 		Config: func(name string, m configmap.Mapper) {
+			optc := new(Options)
+			err := configstruct.Set(m, optc)
+			if err != nil {
+				fs.Errorf(nil, "Failed to read config: %v", err)
+			}
+			updateTokenURL(oauthConfig, optc.Hostname)
+			checkAuth := func(oauthConfig *oauth2.Config, auth *oauthutil.AuthResult) error {
+				if auth == nil || auth.Form == nil {
+					return errors.New("form not found in response")
+				}
+				hostname := auth.Form.Get("hostname")
+				if hostname == "" {
+					hostname = defaultHostname
+				}
+				// Save the hostname in the config
+				m.Set("hostname", hostname)
+				// Update the token URL
+				updateTokenURL(oauthConfig, hostname)
+				fs.Debugf(nil, "pcloud: got hostname %q", hostname)
+				return nil
+			}
 			opt := oauthutil.Options{
+				CheckAuth:    checkAuth,
 				StateBlankOK: true, // pCloud seems to drop the state parameter now - see #4210
 			}
-			err := oauthutil.Config("pcloud", name, m, oauthConfig, &opt)
+			err = oauthutil.Config("pcloud", name, m, oauthConfig, &opt)
 			if err != nil {
 				log.Fatalf("Failed to configure token: %v", err)
 			}
@@ -96,6 +124,13 @@ func init() {
 			Help:     "Fill in for rclone to use a non root folder as its starting point.",
 			Default:  "d0",
 			Advanced: true,
+		}, {
+			Name: "hostname",
+			Help: `Hostname to connect to.
+
+This is normally set when rclone initially does the oauth connection.`,
+			Default:  defaultHostname,
+			Advanced: true,
 		}},
 	})
 }
@@ -104,6 +139,7 @@ func init() {
 type Options struct {
 	Enc          encoder.MultiEncoder `config:"encoding"`
 	RootFolderID string               `config:"root_folder_id"`
+	Hostname     string               `config:"hostname"`
 }
 
 // Fs represents a remote pcloud
@@ -253,12 +289,13 @@ func NewFs(name, root string, m configmap.Mapper) (fs.Fs, error) {
 	if err != nil {
 		return nil, errors.Wrap(err, "failed to configure Pcloud")
 	}
+	updateTokenURL(oauthConfig, opt.Hostname)
 
 	f := &Fs{
 		name:  name,
 		root:  root,
 		opt:   *opt,
-		srv:   rest.NewClient(oAuthClient).SetRoot(rootURL),
+		srv:   rest.NewClient(oAuthClient).SetRoot("https://" + opt.Hostname),
 		pacer: fs.NewPacer(pacer.NewDefault(pacer.MinSleep(minSleep), pacer.MaxSleep(maxSleep), pacer.DecayConstant(decayConstant))),
 	}
 	f.features = (&fs.Features{