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Hide a discarded container's content from the parser callback (#5706)

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When a parser callback rejects an object's or array's start event,
json_sax_dom_callback_parser kept calling it for everything inside
that container anyway: nested keys, values, and the start/end events
of containers below it. This contradicts parser_callback_t's own
documentation, which promises that discarding a container at its
start event also hides its content from the callback.

The same code path also kept a full copy of every key inside such a
discarded container in key_stack until the whole parse finished,
because the early return for values that are not stored skipped the
matching pop. Filtering out a large subtree is the main reason to use
a callback, so this made peak memory during the parse scale with the
size of the very subtree the callback was trying to skip.

Fix start_object(), start_array(), and key() so that a container
whose own start event was discarded, or that is nested inside one, is
never handed to the callback, and no longer pushes onto the key
stacks. A container whose start event was accepted but whose key was
rejected still gets its content reported, as documented ("the
callback is still called for the associated value, but its return
value has no further effect"); only its own bookkeeping is skipped
since it will not be stored.

Fixes #5643.

Signed-off-by: Niels Lohmann <mail@nlohmann.me>
This commit is contained in:
Niels Lohmann
2026-09-30 20:07:48 +02:00
committed by GitHub
parent 6ae17630a4
commit 7fd6895788
4 changed files with 129 additions and 6 deletions
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18
include/nlohmann/detail/input/json_sax.hpp
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@@ -582,8 +582,8 @@ class json_sax_dom_callback_parser
bool start_object(std::size_t len)
{
// check callback for object start
const bool keep = callback(static_cast<int>(ref_stack.size()), parse_event_t::object_start, discarded);
// check callback for object start; not called inside a discarded container
const bool keep = keep_stack.back() && callback(static_cast<int>(ref_stack.size()), parse_event_t::object_start, discarded);
keep_stack.push_back(keep);
// the key this object will be stored under, read before handle_value()
@@ -619,6 +619,18 @@ class json_sax_dom_callback_parser
bool key(string_t& val)
{
if (!keep_stack.back() || !ref_stack.back())
{
// the object is not stored: the value of this key is dropped in
// handle_value() without touching the key stacks
if (keep_stack.back())
{
BasicJsonType k = BasicJsonType(val);
static_cast<void>(callback(static_cast<int>(ref_stack.size()), parse_event_t::key, k));
}
return true;
}
BasicJsonType k = BasicJsonType(val);
// check callback for the key
@@ -704,7 +716,7 @@ class json_sax_dom_callback_parser
bool start_array(std::size_t len)
{
const bool keep = callback(static_cast<int>(ref_stack.size()), parse_event_t::array_start, discarded);
const bool keep = keep_stack.back() && callback(static_cast<int>(ref_stack.size()), parse_event_t::array_start, discarded);
keep_stack.push_back(keep);
// see start_object()