From 6476260e8fc8759e96a9c4cbff499436e421b2cc Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Mattias=20Sch=C3=A4ffersmann?=
 <117294155+MattiasTF@users.noreply.github.com>
Date: Mon, 30 Jun 2025 12:04:11 +0200
Subject: [PATCH] fix(esp32): Fix appending to Strings longer than 64k (#11523)

If oldLen is truncated to uint16_t, appending to a String
that is longer than 65535 bytes will create a broken string.
---
 cores/esp32/WString.cpp | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/cores/esp32/WString.cpp b/cores/esp32/WString.cpp
index 18e647675..6632aa5f8 100644
--- a/cores/esp32/WString.cpp
+++ b/cores/esp32/WString.cpp
@@ -180,7 +180,7 @@ bool String::changeBuffer(unsigned int maxStrLen) {
   if (maxStrLen < sizeof(sso.buff) - 1) {
     if (isSSO() || !buffer()) {
       // Already using SSO, nothing to do
-      uint16_t oldLen = len();
+      size_t oldLen = len();
       setSSO(true);
       setLen(oldLen);
     } else {  // if bufptr && !isSSO()
@@ -188,7 +188,7 @@ bool String::changeBuffer(unsigned int maxStrLen) {
       char temp[sizeof(sso.buff)];
       memcpy(temp, buffer(), maxStrLen);
       free(wbuffer());
-      uint16_t oldLen = len();
+      size_t oldLen = len();
       setSSO(true);
       memcpy(wbuffer(), temp, maxStrLen);
       setLen(oldLen);
@@ -201,7 +201,7 @@ bool String::changeBuffer(unsigned int maxStrLen) {
   if (newSize > CAPACITY_MAX) {
     return false;
   }
-  uint16_t oldLen = len();
+  size_t oldLen = len();
   char *newbuffer = (char *)realloc(isSSO() ? nullptr : wbuffer(), newSize);
   if (newbuffer) {
     size_t oldSize = capacity() + 1;  // include NULL.