From 79c31a7680e360c45b15f37074d813c446365985 Mon Sep 17 00:00:00 2001
From: "duncan.macgregor" <duncan.macgregor@servicenow.com>
Date: Sat, 30 Nov 2024 19:22:10 +0000
Subject: [PATCH] Add an empty slot map for cases where no slots exist.

---
 .../mozilla/javascript/SlotMapContainer.java  | 50 ++++++++++++++++++-
 .../ThreadSafeSlotMapContainer.java           |  1 +
 2 files changed, 49 insertions(+), 2 deletions(-)

diff --git a/rhino/src/main/java/org/mozilla/javascript/SlotMapContainer.java b/rhino/src/main/java/org/mozilla/javascript/SlotMapContainer.java
index ef57146426..40b3d032d1 100644
--- a/rhino/src/main/java/org/mozilla/javascript/SlotMapContainer.java
+++ b/rhino/src/main/java/org/mozilla/javascript/SlotMapContainer.java
@@ -6,6 +6,7 @@
 
 package org.mozilla.javascript;
 
+import java.util.Collections;
 import java.util.Iterator;
 
 /**
@@ -23,6 +24,46 @@ class SlotMapContainer implements SlotMap {
 
     private static final int DEFAULT_SIZE = 10;
 
+    private static class EmptySlotMap implements SlotMap {
+
+        @Override
+        public Iterator<Slot> iterator() {
+            return Collections.emptyIterator();
+        }
+
+        @Override
+        public int size() {
+            return 0;
+        }
+
+        @Override
+        public boolean isEmpty() {
+            return true;
+        }
+
+        @Override
+        public Slot modify(Object key, int index, int attributes) {
+            return null;
+        }
+
+        @Override
+        public Slot query(Object key, int index) {
+            return null;
+        }
+
+        @Override
+        public void add(Slot newSlot) {
+            throw new IllegalStateException();
+        }
+
+        @Override
+        public <S extends Slot> S compute(Object key, int index, SlotComputer<S> compute) {
+            throw new IllegalStateException();
+        }
+    }
+
+    private static EmptySlotMap EMPTY_SLOT_MAP = new EmptySlotMap();
+
     protected SlotMap map;
 
     SlotMapContainer() {
@@ -30,7 +71,9 @@ class SlotMapContainer implements SlotMap {
     }
 
     SlotMapContainer(int initialSize) {
-        if (initialSize > LARGE_HASH_SIZE) {
+        if (initialSize == 0) {
+            map = EMPTY_SLOT_MAP;
+        } else if (initialSize > LARGE_HASH_SIZE) {
             map = new HashSlotMap();
         } else {
             map = new EmbeddedSlotMap();
@@ -59,6 +102,7 @@ public Slot modify(Object key, int index, int attributes) {
 
     @Override
     public <S extends Slot> S compute(Object key, int index, SlotComputer<S> c) {
+        checkMapSize();
         return map.compute(key, index, c);
     }
 
@@ -92,7 +136,9 @@ public void unlockRead(long stamp) {
      * map to a HashMap that is more robust against large numbers of hash collisions.
      */
     protected void checkMapSize() {
-        if ((map instanceof EmbeddedSlotMap) && map.size() >= LARGE_HASH_SIZE) {
+        if (map == EMPTY_SLOT_MAP) {
+            map = new EmbeddedSlotMap();
+        } else if ((map instanceof EmbeddedSlotMap) && map.size() >= LARGE_HASH_SIZE) {
             SlotMap newMap = new HashSlotMap(map);
             map = newMap;
         }
diff --git a/rhino/src/main/java/org/mozilla/javascript/ThreadSafeSlotMapContainer.java b/rhino/src/main/java/org/mozilla/javascript/ThreadSafeSlotMapContainer.java
index 8a95301ecc..c2c8d7c3ae 100644
--- a/rhino/src/main/java/org/mozilla/javascript/ThreadSafeSlotMapContainer.java
+++ b/rhino/src/main/java/org/mozilla/javascript/ThreadSafeSlotMapContainer.java
@@ -77,6 +77,7 @@ public Slot modify(Object key, int index, int attributes) {
     public <S extends Slot> S compute(Object key, int index, SlotComputer<S> c) {
         final long stamp = lock.writeLock();
         try {
+            checkMapSize();
             return map.compute(key, index, c);
         } finally {
             lock.unlockWrite(stamp);