From 9ac37cd62629ef732161a1cbbb0c6148292ce5e0 Mon Sep 17 00:00:00 2001
From: Jean Boussier <jean.boussier@gmail.com>
Date: Mon, 8 Jun 2026 09:11:20 +0200
Subject: [PATCH] [ruby/json] generator.c: don't start with a stack buffer in
 IO case

It was never intended to be initialized with a stack buffer
in that case, as when dumping into an IO we want larger chunks.

https://github.com/ruby/json/commit/996bac686d
---
 ext/json/fbuffer/fbuffer.h     | 43 +++++++++++++++++-----------------
 ext/json/generator/generator.c | 12 ++++------
 2 files changed, 25 insertions(+), 30 deletions(-)

diff --git a/ext/json/fbuffer/fbuffer.h b/ext/json/fbuffer/fbuffer.h
index b4f5266ca5..cc44ea0308 100644
--- a/ext/json/fbuffer/fbuffer.h
+++ b/ext/json/fbuffer/fbuffer.h
@@ -37,13 +37,17 @@ static void fbuffer_append_long(FBuffer *fb, long number);
 static inline void fbuffer_append_char(FBuffer *fb, char newchr);
 static VALUE fbuffer_finalize(FBuffer *fb);
 
-static void fbuffer_stack_init(FBuffer *fb, size_t initial_length, char *stack_buffer, size_t stack_buffer_size)
+static void fbuffer_init(FBuffer *fb, size_t initial_length, VALUE io, char *stack_buffer, size_t stack_buffer_size)
 {
-    fb->initial_length = (initial_length > 0) ? initial_length : FBUFFER_INITIAL_LENGTH_DEFAULT;
-    if (stack_buffer) {
+    if (RTEST(io)) {
+        JSON_ASSERT(fb->type == FBUFFER_HEAP_ALLOCATED);
+        fb->io = io;
+        fb->initial_length = (initial_length > 0) ? initial_length : FBUFFER_IO_BUFFER_SIZE;
+    } else {
         fb->type = FBUFFER_STACK_ALLOCATED;
         fb->ptr = stack_buffer;
         fb->capa = stack_buffer_size;
+        fb->initial_length = (initial_length > 0) ? initial_length : FBUFFER_INITIAL_LENGTH_DEFAULT;
     }
 #if JSON_DEBUG
     fb->requested = 0;
@@ -79,45 +83,40 @@ static void fbuffer_flush(FBuffer *fb)
     fbuffer_clear(fb);
 }
 
-static void fbuffer_realloc(FBuffer *fb, size_t required)
+static void fbuffer_realloc(FBuffer *fb, size_t new_capa)
 {
-    if (required > fb->capa) {
+    if (new_capa > fb->capa) {
         if (fb->type == FBUFFER_STACK_ALLOCATED) {
             const char *old_buffer = fb->ptr;
-            fb->ptr = ALLOC_N(char, required);
+            fb->ptr = ALLOC_N(char, new_capa);
             fb->type = FBUFFER_HEAP_ALLOCATED;
             MEMCPY(fb->ptr, old_buffer, char, fb->len);
         } else {
-            JSON_SIZED_REALLOC_N(fb->ptr, char, required, fb->capa);
+            JSON_SIZED_REALLOC_N(fb->ptr, char, new_capa, fb->capa);
         }
-        fb->capa = required;
+        fb->capa = new_capa;
     }
 }
 
 static void fbuffer_do_inc_capa(FBuffer *fb, size_t requested)
 {
     if (RB_UNLIKELY(fb->io)) {
-        if (fb->capa < FBUFFER_IO_BUFFER_SIZE) {
-            fbuffer_realloc(fb, FBUFFER_IO_BUFFER_SIZE);
-        } else {
+        if (fb->capa != 0) {
             fbuffer_flush(fb);
-        }
-
-        if (RB_LIKELY(requested < fb->capa)) {
-            return;
+            if (RB_LIKELY(requested < fb->capa)) {
+                return;
+            }
         }
     }
 
-    size_t required;
+    size_t new_capa = fb->capa ? fb->capa : fb->initial_length;
+    size_t needed_capa = requested + fb->len;
 
-    if (RB_UNLIKELY(!fb->ptr)) {
-        fb->ptr = ALLOC_N(char, fb->initial_length);
-        fb->capa = fb->initial_length;
+    while (new_capa < needed_capa) {
+        new_capa *= 2;
     }
 
-    for (required = fb->capa; requested > required - fb->len; required <<= 1);
-
-    fbuffer_realloc(fb, required);
+    fbuffer_realloc(fb, new_capa);
 }
 
 static inline void fbuffer_inc_capa(FBuffer *fb, size_t requested)
diff --git a/ext/json/generator/generator.c b/ext/json/generator/generator.c
index 82853633ba..08c1fd0db6 100644
--- a/ext/json/generator/generator.c
+++ b/ext/json/generator/generator.c
@@ -1308,10 +1308,8 @@ static inline VALUE cState_partial_generate(VALUE self, VALUE obj, generator_fun
     GET_STATE(self);
 
     char stack_buffer[FBUFFER_STACK_SIZE];
-    FBuffer buffer = {
-        .io = RTEST(io) ? io : Qfalse,
-    };
-    fbuffer_stack_init(&buffer, state->buffer_initial_length, stack_buffer, FBUFFER_STACK_SIZE);
+    FBuffer buffer = { 0 };
+    fbuffer_init(&buffer, state->buffer_initial_length, io, stack_buffer, FBUFFER_STACK_SIZE);
 
     struct generate_json_data data = {
         .buffer = &buffer,
@@ -1870,10 +1868,8 @@ static VALUE cState_m_do_generate(VALUE klass, VALUE obj, VALUE opts, VALUE io,
     configure_state(&state, Qfalse, opts);
 
     char stack_buffer[FBUFFER_STACK_SIZE];
-    FBuffer buffer = {
-        .io = RTEST(io) ? io : Qfalse,
-    };
-    fbuffer_stack_init(&buffer, state.buffer_initial_length, stack_buffer, FBUFFER_STACK_SIZE);
+    FBuffer buffer = { 0 };
+    fbuffer_init(&buffer, state.buffer_initial_length, io, stack_buffer, FBUFFER_STACK_SIZE);
 
     struct generate_json_data data = {
         .buffer = &buffer,