From a8b24ef5ed7b6cd3a8a22fd7dd35ed013dad0def Mon Sep 17 00:00:00 2001
From: Iain Ireland <iireland@mozilla.com>
Date: Tue, 29 Apr 2025 18:44:10 +0000
Subject: [PATCH] Bug 1963431: Add JS_StringifyWithLengthHint r=tcampbell

Differential Revision: https://phabricator.services.mozilla.com/D247145
---
 js/public/JSON.h |  7 +++++++
 js/src/jsapi.cpp | 12 ++++++++++++
 2 files changed, 19 insertions(+)

diff --git a/js/public/JSON.h b/js/public/JSON.h
index feef53f755e8..2cf546e9c8da 100644
--- a/js/public/JSON.h
+++ b/js/public/JSON.h
@@ -26,12 +26,19 @@ using JSONWriteCallback = bool (*)(const char16_t* buf, uint32_t len,
  *
  * In cases where JSON.stringify would return undefined, this function calls
  * |callback| with the string "null".
+ *
+ * If a length hint is passed, space will be reserved for at least that many
+ * characters.
  */
 extern JS_PUBLIC_API bool JS_Stringify(JSContext* cx,
                                        JS::MutableHandle<JS::Value> value,
                                        JS::Handle<JSObject*> replacer,
                                        JS::Handle<JS::Value> space,
                                        JSONWriteCallback callback, void* data);
+extern JS_PUBLIC_API bool JS_StringifyWithLengthHint(
+    JSContext* cx, JS::MutableHandle<JS::Value> value,
+    JS::Handle<JSObject*> replacer, JS::Handle<JS::Value> space,
+    JSONWriteCallback callback, void* data, size_t lengthHint);
 
 namespace JS {
 
diff --git a/js/src/jsapi.cpp b/js/src/jsapi.cpp
index dc589a8d6a9a..415b611d5e69 100644
--- a/js/src/jsapi.cpp
+++ b/js/src/jsapi.cpp
@@ -3665,6 +3665,15 @@ JS_PUBLIC_API bool JS::PropertySpecNameEqualsId(JSPropertySpec::Name name,
 JS_PUBLIC_API bool JS_Stringify(JSContext* cx, MutableHandleValue vp,
                                 HandleObject replacer, HandleValue space,
                                 JSONWriteCallback callback, void* data) {
+  return JS_StringifyWithLengthHint(cx, vp, replacer, space, callback, data, 0);
+}
+
+JS_PUBLIC_API bool JS_StringifyWithLengthHint(JSContext* cx,
+                                              MutableHandleValue vp,
+                                              HandleObject replacer,
+                                              HandleValue space,
+                                              JSONWriteCallback callback,
+                                              void* data, size_t lengthHint) {
   AssertHeapIsIdle();
   CHECK_THREAD(cx);
   cx->check(replacer, space);
@@ -3672,6 +3681,9 @@ JS_PUBLIC_API bool JS_Stringify(JSContext* cx, MutableHandleValue vp,
   if (!sb.ensureTwoByteChars()) {
     return false;
   }
+  if (lengthHint && !sb.reserve(lengthHint)) {
+    return false;
+  }
   if (!Stringify(cx, vp, replacer, space, sb, StringifyBehavior::Normal)) {
     return false;
   }