make sure only traitors can see uplink button (#6083)

* make sure only traitors can see uplink button

* comments and pda.Owner -> uid

Content.Server/PDA/PDASystem.cs

@@ -97,7 +97,7 @@ namespace Content.Server.PDA
                 appearance.SetData(PDAVisuals.IDCardInserted, pda.ContainedID != null);
         }
 
-        private void UpdatePDAUserInterface(PDAComponent pda, EntityUid? user = default)
+        private void UpdatePDAUserInterface(PDAComponent pda, EntityUid? user = null)
         {
             var ownerInfo = new PDAIdInfoText
             {
@@ -106,13 +106,39 @@ namespace Content.Server.PDA
                 JobTitle = pda.ContainedID?.JobTitle
             };
 
-            var hasUplink = EntityManager.HasComponent<UplinkComponent>(pda.Owner);
-
-            if (user is not null)
-                hasUplink &= _uplinkAccounts.HasAccount(user.Value);
-
             var ui = pda.Owner.GetUIOrNull(PDAUiKey.Key);
-            ui?.SetState(new PDAUpdateState(pda.FlashlightOn, pda.PenSlot.HasItem, ownerInfo, hasUplink));
+            if (ui == null)
+                return;
+
+            ui.SetState(new PDAUpdateState(pda.FlashlightOn, pda.PenSlot.HasItem, ownerInfo, ShouldShowUplink(pda.Owner, ui, user)));
+        }
+
+        /// <summary>
+        ///     Check whether the PDA has an uplink, and ensure that the only person that can see the PDA UI has an
+        ///     uplink account.
+        /// </summary>
+        public bool ShouldShowUplink(EntityUid uid, BoundUserInterface ui, EntityUid? user = null)
+        {
+            // TODO UPLINK RINGTONES/SECRETS This is just a janky placeholder way of hiding uplinks from non syndicate
+            // players. This should really use a sort of key-code entry system that selects an account which is not directly tied to
+            // a player entity.
+
+            if (!HasComp<UplinkComponent>(uid))
+                return false;
+
+            // If a user is trying to open the UI, make sure that they have an uplink account before showing the UI.
+            if (user != null && !_uplinkAccounts.HasAccount(user.Value))
+                return false;
+
+            // If other users currently have the UI open, check that they too should be allowed to see the button..
+            foreach (var session in ui.SubscribedSessions)
+            {
+                if (session.AttachedEntity != null && !_uplinkAccounts.HasAccount(session.AttachedEntity.Value))
+                    return false;
+            }
+
+            // everyone has an uplink account, show the button.
+            return true;
         }
 
         private void OnUIMessage(PDAComponent pda, ServerBoundUserInterfaceMessage msg)