Fix ResumableParser#partial_value SEGV on incomplete nested containers

ext/json/ext/parser/parser.c

@@ -2625,11 +2625,16 @@ static VALUE cResumableParser_partial_value_body(VALUE self)
         missing_object_value = 1;
     }
 
-    // Copy the value stack as we need to mutate it.
+    // Copy the value stack as we need to mutate it. The collapse loop folds each
+    // open container by popping its entries and pushing the single result, so a
+    // parent always reclaims its child's slot; head exceeds its live size by at
+    // most one, either for the missing-value placeholder pushed below or for the
+    // result of folding an empty innermost container. That one spare slot keeps
+    // rvalue_stack_push from growing (reallocating) this ALLOCV buffer.
     long capa = parser.value_stack.head;
-    parser.value_stack.capa = (capa + missing_object_value);
-    VALUE tmpbuf, *value_stack_buffer = ALLOCV_N(VALUE, tmpbuf, capa + missing_object_value);
-    MEMCPY(value_stack_buffer, parser.value_stack.ptr, VALUE, parser.value_stack.capa);
+    parser.value_stack.capa = capa + 1;
+    VALUE tmpbuf, *value_stack_buffer = ALLOCV_N(VALUE, tmpbuf, parser.value_stack.capa);
+    MEMCPY(value_stack_buffer, parser.value_stack.ptr, VALUE, capa);
     parser.value_stack.ptr = value_stack_buffer;
 
     JSON_ParserState *state = &parser.state;

test/json/resumable_parser_test.rb

@@ -247,6 +247,17 @@ def test_partial_value
     assert_partial_value([1, { "a" => 1, "b" => { "c" => nil } }], '[1, { "a": 1, "b": { "c"')
   end
 
+  def test_partial_value_collapses_nested_incomplete_containers
+    # partial_value rebuilds the open containers on a scratch value stack; folding
+    # an empty inner container pushes a value, so that stack must hold more than its
+    # live size or the push reallocates the scratch buffer.
+    assert_partial_value({ "abc" => {} }, '{"abc":{"d')
+    assert_partial_value({ "a" => { "b" => { "c" => {} } } }, '{"a":{"b":{"c":{"e')
+    assert_partial_value([1, { "a" => {} }], '[1,{"a":{"d')
+    assert_partial_value({ "a" => [1, { "b" => [2, { "c" => nil }] }] }, '{"a":[1,{"b":[2,{"c"')
+    assert_partial_value([1, [2, [3, { "x" => nil }]]], '[1,[2,[3,{"x":[')
+  end
+
   def test_partial_value_issue_1005
     data = <<~JSON
       [