From ecdbaeb6ef8180a4c06352e77f59ecadfe73453f Mon Sep 17 00:00:00 2001
From: ydah <t.yudai92@gmail.com>
Date: Mon, 5 Oct 2026 23:15:50 +0900
Subject: [PATCH] Report the attempted depth in generator nesting errors

---
 ext/json/ext/generator/generator.c    |  7 +++----
 java/src/json/ext/GeneratorState.java | 10 ++++------
 test/json/json_generator_test.rb      | 12 ++++++++++++
 3 files changed, 19 insertions(+), 10 deletions(-)

diff --git a/ext/json/ext/generator/generator.c b/ext/json/ext/generator/generator.c
index fbef1980b..abe5fd2c8 100644
--- a/ext/json/ext/generator/generator.c
+++ b/ext/json/ext/generator/generator.c
@@ -1045,11 +1045,10 @@ json_object_i(VALUE key, VALUE val, VALUE _arg)
 static inline long increase_depth(struct generate_json_data *data)
 {
     JSON_Generator_State *state = data->state;
-    long depth = ++data->depth;
-    if (RB_UNLIKELY(depth > state->max_nesting && state->max_nesting)) {
-        rb_raise(eNestingError, "nesting of %ld is too deep. Did you try to serialize objects with circular references?", --data->depth);
+    if (RB_UNLIKELY(data->depth >= state->max_nesting && state->max_nesting)) {
+        rb_raise(eNestingError, "nesting of %ld is too deep. Did you try to serialize objects with circular references?", data->depth + 1);
     }
-    return depth;
+    return ++data->depth;
 }
 
 static void generate_json_object(FBuffer *buffer, struct generate_json_data *data, VALUE obj)
diff --git a/java/src/json/ext/GeneratorState.java b/java/src/json/ext/GeneratorState.java
index 778500ef7..97809134c 100644
--- a/java/src/json/ext/GeneratorState.java
+++ b/java/src/json/ext/GeneratorState.java
@@ -649,9 +649,8 @@ public RubyHash to_h(ThreadContext context) {
     }
 
     public int increaseDepth(ThreadContext context) {
-        depth++;
         checkMaxNesting(context);
-        return depth;
+        return ++depth;
     }
 
     public int decreaseDepth() {
@@ -659,13 +658,12 @@ public int decreaseDepth() {
     }
 
     /**
-     * Checks if the current depth is allowed as per this state's options.
+     * Checks if increasing the depth is allowed as per this state's options.
      * @param context The current context
      */
     private void checkMaxNesting(ThreadContext context) {
-        if (maxNesting != 0 && depth > maxNesting) {
-            depth--;
-            throw Utils.newException(context, Utils.M_NESTING_ERROR, "nesting of " + depth + " is too deep. Did you try to serialize objects with circular references?");
+        if (maxNesting != 0 && depth >= maxNesting) {
+            throw Utils.newException(context, Utils.M_NESTING_ERROR, "nesting of " + (depth + 1) + " is too deep. Did you try to serialize objects with circular references?");
         }
     }
 }
diff --git a/test/json/json_generator_test.rb b/test/json/json_generator_test.rb
index 326780177..8f3ca4395 100755
--- a/test/json/json_generator_test.rb
+++ b/test/json/json_generator_test.rb
@@ -477,6 +477,18 @@ def test_depth_nesting_error
     assert_raise(JSON::NestingError) { JSON.pretty_generate(ary) }
   end
 
+  def test_nesting_error_reports_attempted_depth
+    [[[]], { a: {} }].each do |object|
+      [0, 1].each do |depth|
+        state = JSON.state.new(depth: depth, max_nesting: depth + 1)
+        error = assert_raise(JSON::NestingError) { state.generate(object) }
+        assert_match(/\Anesting of #{depth + 2} is too deep\./, error.message)
+        assert_equal depth, state.depth
+        assert_equal '[]', state.generate([])
+      end
+    end
+  end
+
   def test_depth_nesting_error_to_json
     ary = []; ary << ary
     s = JSON.state.new(depth: 1)