From 945394bb97d7e1aed639c3c978b7474b61e472ab Mon Sep 17 00:00:00 2001
From: Jean Boussier <jean.boussier@gmail.com>
Date: Wed, 17 Jun 2026 08:26:47 +0200
Subject: [PATCH] ResumableParser#<<: call rb_str_modify before shrinking the
 buffer

Fix: https://github.com/ruby/json/issues/1013

The string may be shared.
---
 ext/json/ext/parser/parser.c       |  9 ++++++---
 test/json/resumable_parser_test.rb | 15 +++++++++++++++
 2 files changed, 21 insertions(+), 3 deletions(-)

diff --git a/ext/json/ext/parser/parser.c b/ext/json/ext/parser/parser.c
index c216faf4..136aab6a 100644
--- a/ext/json/ext/parser/parser.c
+++ b/ext/json/ext/parser/parser.c
@@ -2300,16 +2300,19 @@ static VALUE cResumableParser_feed(VALUE self, VALUE str)
 
         const size_t size = parser->state.end - parser->state.start;
         const size_t consumed = size - remaining;
-        char *old_ptr = RSTRING_PTR(parser->buffer);
 
         if (RB_OBJ_FROZEN_RAW(parser->buffer)) {
             VALUE new_buffer = rb_obj_hide(rb_str_buf_new(remaining + RSTRING_LEN(str)));
-            MEMCPY(RSTRING_PTR(new_buffer), old_ptr + consumed, char, remaining);
+
+            char *old_ptr = RSTRING_PTR(parser->buffer);
+            memcpy(RSTRING_PTR(new_buffer), old_ptr + consumed, remaining);
             rb_str_set_len(new_buffer, remaining);
             offset = 0;
             parser->buffer = new_buffer;
         } else if (consumed > (size / 2) && size >= 512) {
-            MEMMOVE(old_ptr, old_ptr + consumed, char, remaining);
+            rb_str_modify(parser->buffer);
+            char *old_ptr = RSTRING_PTR(parser->buffer);
+            memmove(old_ptr, old_ptr + consumed, remaining);
             rb_str_set_len(parser->buffer, remaining);
             offset = 0;
         }
diff --git a/test/json/resumable_parser_test.rb b/test/json/resumable_parser_test.rb
index 600cd33e..52f1356a 100644
--- a/test/json/resumable_parser_test.rb
+++ b/test/json/resumable_parser_test.rb
@@ -244,6 +244,21 @@ def test_spill_frames_stack
     assert_equal expected, @parser.value
   end
 
+  def test_buffer_shrink
+    doc1 = '{"a":"' + ("x" * 800) + '"} {'   # >= 512 bytes
+    doc2 = '"b":1} '
+
+    parser = JSON::ResumableParser.new({})
+
+    parser << doc1 # internal buffer becomes a *shared* string here
+    parser.parse # consume doc1 -> >50% of a >=512B buffer is now consumed
+    parser.value
+
+    parser << doc2 # buffer is shrinked
+    parser.parse
+    parser.value
+  end
+
   private
 
   def assert_partial_value(expected, json)