[SPARK-32872][CORE] Prevent BytesToBytesMap at MAX_CAPACITY from exceeding growth threshold

core/src/main/java/org/apache/spark/unsafe/map/BytesToBytesMap.java

@@ -816,6 +816,12 @@ public boolean append(Object kbase, long koff, int klen, Object vbase, long voff
           } catch (SparkOutOfMemoryError oom) {
             canGrowArray = false;
           }
+        } else if (numKeys >= growthThreshold && longArray.size() / 2 >= MAX_CAPACITY) {
+          // The map has reached its growth threshold, but is already at MAX_CAPACITY and cannot
+          // grow. Instead, we prevent the map from accepting any more new elements to make sure we
+          // don't exceed the load factor. If we need to spill later, this allows
+          // UnsafeKVExternalSorter to reuse the array for sorting.
+          canGrowArray = false;
         }
       }
       return true;