[MM-41022] - Fix panic when AllowCookiesSubdomain setting is false (#19339)

* [MM-41022] - Cloud: Panic after log in when AllowCookiesSubdomain setting is false

* remove print

* improvement

* remove configration setting from flow

* fix tests

* feedback impl

* feedback impl

app/login.go

@@ -8,6 +8,7 @@ import (
 	"errors"
 	"fmt"
 	"net/http"
+	"net/url"
 	"os"
 	"strconv"
 	"strings"
@@ -245,21 +246,35 @@ func (a *App) AttachCloudSessionCookie(c *request.Context, w http.ResponseWriter
 	}
 
 	maxAge := *a.Config().ServiceSettings.SessionLengthWebInDays * 60 * 60 * 24
-	domain := a.GetCookieDomain()
 	subpath, _ := utils.GetSubpathFromConfig(a.Config())
 	expiresAt := time.Unix(model.GetMillis()/1000+int64(maxAge), 0)
 
-	var val string
+	domain := ""
+	if siteURL, err := url.Parse(a.GetSiteURL()); err == nil {
+		domain = siteURL.Hostname()
+	}
+
+	if domain == "" {
+		return
+	}
+
+	var workspaceName string
 	if strings.Contains(domain, "localhost") {
-		val = "localhost"
+		workspaceName = "localhost"
 	} else {
-		val = strings.SplitN(domain, ".", 2)[0]
+
+		// ensure we have a format for a cloud workspace url i.e. example.cloud.mattermost.com
+		if len(strings.Split(domain, ".")) != 4 {
+			return
+		}
+		workspaceName = strings.SplitN(domain, ".", 2)[0]
 		domain = strings.SplitN(domain, ".", 3)[2]
+		domain = "." + domain
 	}
 
 	cookie := &http.Cookie{
 		Name:     model.SessionCookieCloudUrl,
-		Value:    val,
+		Value:    workspaceName,
 		Path:     subpath,
 		MaxAge:   maxAge,
 		Expires:  expiresAt,
@@ -269,6 +284,7 @@ func (a *App) AttachCloudSessionCookie(c *request.Context, w http.ResponseWriter
 	}
 
 	http.SetCookie(w, cookie)
+
 }
 
 func (a *App) AttachSessionCookies(c *request.Context, w http.ResponseWriter, r *http.Request) {