FIX: An infinite loop in InputSystemUIInputModule was causing an editor / player hang (#2102)

Author: Secticide

Packages/com.unity.inputsystem/CHANGELOG.md

@@ -38,6 +38,7 @@ however, it has to be formatted properly to pass verification tests.
 - Fixed an issue where batch jobs would fail with "Error: Error building Player because scripts are compiling" if a source generated .inputactions asset is out of sync with its generated source code (ISXB-1300).
 - Fixed multiple `OnScreenStick` Components that does not work together when using them simultaneously in isolation mode. [ISXB-813](https://issuetracker.unity3d.com/product/unity/issues/guid/ISXB-813)
 - Fixed an issue in input actions editor window that caused certain fields in custom input composite bindings to require multiple clicks to action / focus. [ISXB-1171](https://issuetracker.unity3d.com/product/unity/issues/guid/ISXB-1171)
+- Fixed an editor/player hang in `InputSystemUIInputModule` due to an infinite loop. This was caused by the assumption that `RemovePointerAtIndex` would _always_ successfully remove the pointer, which is not the case with touch based pointers. [ISXB-1258](https://issuetracker.unity3d.com/product/unity/issues/guid/ISXB-1258)
 
 ### Changed
 - Changed location of the link xml file (code stripping rules), from a temporary directory to the project Library folder (ISX-2140).

Packages/com.unity.inputsystem/InputSystem/Plugins/UI/InputSystemUIInputModule.cs

@@ -1662,9 +1662,11 @@ protected override void OnDisable()
 
         private void ResetPointers()
         {
-            var numPointers = m_PointerStates.length;
-            for (var i = 0; i < numPointers; ++i)
-                SendPointerExitEventsAndRemovePointer(0);
+            for (var i = 0; i < m_PointerStates.length; ++i)
+            {
+                if (SendPointerExitEventsAndRemovePointer(i))
+                    --i;
+            }
 
             m_CurrentPointerId = -1;
             m_CurrentPointerIndex = -1;
@@ -2021,24 +2023,26 @@ private int AllocatePointer(int pointerId, int displayIndex, int touchId, UIPoin
             return m_PointerStates.AppendWithCapacity(new PointerModel(eventData));
         }
 
-        private void SendPointerExitEventsAndRemovePointer(int index)
+        // Returns true if the pointer was successfully removed (ISXB-1258)
+        private bool SendPointerExitEventsAndRemovePointer(int index)
         {
             var eventData = m_PointerStates[index].eventData;
             if (eventData.pointerEnter != null)
                 ProcessPointerMovement(eventData, null);
 
-            RemovePointerAtIndex(index);
+            return RemovePointerAtIndex(index);
         }
 
-        private void RemovePointerAtIndex(int index)
+        private bool RemovePointerAtIndex(int index)
         {
             Debug.Assert(m_PointerStates[index].eventData.pointerEnter == null, "Pointer should have exited all objects before being removed");
 
             // We don't want to release touch pointers on the same frame they are released (unpressed). They get cleaned up one frame later in Process()
             ref var state = ref GetPointerStateForIndex(index);
             if (state.pointerType == UIPointerType.Touch && (state.leftButton.isPressed || state.leftButton.wasReleasedThisFrame))
             {
-                return;
+                // The pointer was not removed
+                return false;
             }
 
             // Retain event data so that we can reuse the event the next time we allocate a PointerModel record.
@@ -2086,6 +2090,8 @@ private void RemovePointerAtIndex(int index)
                 m_PointerStates.firstValue.eventData = eventData;
             else
                 m_PointerStates.additionalValues[m_PointerStates.length - 1].eventData = eventData;
+
+            return true;
         }
 
         // Remove any pointer that no longer has the ability to point.
@@ -2100,8 +2106,9 @@ private void PurgeStalePointers()
                      !HaveControlForDevice(device, trackedDevicePosition) &&
                      !HaveControlForDevice(device, trackedDeviceOrientation)))
                 {
-                    SendPointerExitEventsAndRemovePointer(i);
-                    --i;
+                    // Only decrement 'i' if the pointer was successfully removed
+                    if (SendPointerExitEventsAndRemovePointer(i))
+                        --i;
                 }
             }
 
@@ -2310,8 +2317,8 @@ private void FilterPointerStatesByType()
                         }
                         if (m_PointerStates[i].pointerType != UIPointerType.MouseOrPen && m_PointerStates[i].pointerType != UIPointerType.Touch || (m_PointerStates[i].pointerType == UIPointerType.Touch && !state.leftButton.isPressed && !state.leftButton.wasReleasedThisFrame))
                         {
-                            SendPointerExitEventsAndRemovePointer(i);
-                            --i;
+                            if (SendPointerExitEventsAndRemovePointer(i))
+                                --i;
                         }
                     }
                 }
@@ -2322,8 +2329,8 @@ private void FilterPointerStatesByType()
                     {
                         if (m_PointerStates[i].pointerType == UIPointerType.MouseOrPen)
                         {
-                            SendPointerExitEventsAndRemovePointer(i);
-                            --i;
+                            if (SendPointerExitEventsAndRemovePointer(i))
+                                --i;
                         }
                     }
                 }