Avoid an exception on sign out when the principal is populated from an incomplete external login

src/Umbraco.Infrastructure/Security/BackOfficeUserStore.cs

@@ -1,4 +1,4 @@
 using System.Data;
 using System.Globalization;
 using Microsoft.AspNetCore.Identity;
 using Microsoft.Extensions.DependencyInjection;
@@ -281,7 +281,20 @@
         cancellationToken.ThrowIfCancellationRequested();
         ThrowIfDisposed();
 
-        IUser? user = _userService.GetUserById(UserIdToInt(userId));
+        // In the external login flow - see BackOfficeController.ExternalSignInAsync - we can have a situation where an
+        // error has occured but the user is signed in. For that reason, at the end of the process, if errors are
+        // recorded, the user is signed out.
+        // Before signing out, we request the user in order to update the security stamp - see UmbracoSignInManager.SignOutAsync.
+        // But we can have a situation where the signed in principal has the ID from the external provider, which may not be something
+        // we can parse to an integer.
+        // If that's the case, return null rather than throwing an exception. Without an Umbraco user, we can't update the security stamp,
+        // so no need to fail here.
+        if (!TryUserIdToInt(userId, out var userIdAsInt))
+        {
+            return Task.FromResult((BackOfficeIdentityUser?)null)!;
+        }
+
+        IUser? user = _userService.GetUserById(userIdAsInt);
         if (user == null)
         {
             return Task.FromResult((BackOfficeIdentityUser?)null)!;

src/Umbraco.Infrastructure/Security/UmbracoUserStore.cs

@@ -33,18 +33,29 @@ protected UmbracoUserStore(IdentityErrorDescriber describer)
 
     protected static int UserIdToInt(string? userId)
     {
-        if (int.TryParse(userId, NumberStyles.Integer, CultureInfo.InvariantCulture, out var result))
+        if (TryUserIdToInt(userId, out int result))
         {
             return result;
         }
 
+        throw new InvalidOperationException($"Unable to convert user ID ({userId})to int using InvariantCulture");
+    }
+
+    protected static bool TryUserIdToInt(string? userId, out int result)
+    {
+        if (int.TryParse(userId, NumberStyles.Integer, CultureInfo.InvariantCulture, out result))
+        {
+            return true;
+        }
+
         if (Guid.TryParse(userId, out Guid key))
         {
             // Reverse the IntExtensions.ToGuid
-            return BitConverter.ToInt32(key.ToByteArray(), 0);
+            result = BitConverter.ToInt32(key.ToByteArray(), 0);
+            return true;
         }
 
-        throw new InvalidOperationException($"Unable to convert user ID ({userId})to int using InvariantCulture");
+        return false;
     }
 
     protected static string UserIdToString(int userId) => string.Intern(userId.ToString(CultureInfo.InvariantCulture));