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Restore a duplicate key's prior value when the callback rejects its new value

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json_sax_dom_callback_parser::key() unconditionally overwrote the object
slot for a key with a `discarded` placeholder as soon as the key was
accepted by the parser callback. For a duplicate key (legal JSON), this
destroyed the pre-existing value from an earlier occurrence of the same
key before the new value was even parsed. If the new value was then
rejected by the callback, remove_discarded_value() erased the member
entirely instead of leaving the original value in place, contradicting
the documented behavior that a discarded value behaves as if it was
never read.

Add a small stash of (slot pointer, previous value) pairs so that when
key() overwrites an existing member with the discarded placeholder, the
previous value can be restored later if the corresponding value (scalar,
object, or array) is rejected, instead of being erased. The stash entry
is dropped without restoring once the new value is definitively
accepted (in handle_value() for scalars, end_object()/end_array() for
containers), so a duplicate key whose new value is accepted still keeps
the last value as before. Non-duplicate keys are unaffected: rejecting
their value still removes the member entirely, since there is nothing
to restore.

Signed-off-by: Niels Lohmann <mail@nlohmann.me>
This commit is contained in:
Niels Lohmann
2026-09-05 22:01:41 +02:00
committed by GitHub
parent 450fc8dce7
commit 9bf840209d
3 changed files with 255 additions and 26 deletions
Download Patch File Download Diff File Expand all files Collapse all files
include/nlohmann/detail/input/json_sax.hpp
+92 -13
View File
@@ -8,10 +8,11 @@
#pragma once
#include <algorithm> // find_if
#include <cstddef>
#include <string> // string
#include <type_traits> // enable_if_t
#include <utility> // move
#include <utility> // move, pair
#include <vector> // vector
#include <nlohmann/detail/exceptions.hpp>
@@ -585,7 +586,17 @@ class json_sax_dom_callback_parser
// add discarded value at the given key and store the reference for later
if (keep && ref_stack.back())
{
object_element = &(ref_stack.back()->m_data.m_value.object->operator[](val) = discarded);
auto& obj = *ref_stack.back()->m_data.m_value.object;
const auto it = obj.find(val);
if (it != obj.end())
{
// this is a duplicate key (legal in JSON); remember its
// current value so it can be restored later if the new
// value is rejected by the callback, instead of being
// erased together with the discarded placeholder
duplicate_key_stash.emplace_back(&(it->second), it->second);
}
object_element = &(obj[val] = discarded);
}
return true;
@@ -597,13 +608,18 @@ class json_sax_dom_callback_parser
{
if (!callback(static_cast<int>(ref_stack.size()) - 1, parse_event_t::object_end, *ref_stack.back()))
{
// discard object
*ref_stack.back() = discarded;
// discard object, unless this slot holds a duplicate key's
// previous value pending restoration, in which case that
// value is restored instead of being discarded
if (!resolve_duplicate_key_stash(ref_stack.back(), true))
{
*ref_stack.back() = discarded;
#if JSON_DIAGNOSTIC_POSITIONS
// Set start/end positions for discarded object.
handle_diagnostic_positions_for_json_value(*ref_stack.back());
// Set start/end positions for discarded object.
handle_diagnostic_positions_for_json_value(*ref_stack.back());
#endif
}
}
else
{
@@ -617,6 +633,10 @@ class json_sax_dom_callback_parser
#endif
ref_stack.back()->set_parents();
// this object is finally, definitively kept; drop any
// pending duplicate-key stash entry for its slot since it
// can no longer be restored
resolve_duplicate_key_stash(ref_stack.back(), false);
}
}
@@ -686,16 +706,25 @@ class json_sax_dom_callback_parser
#endif
ref_stack.back()->set_parents();
// this array is finally, definitively kept; drop any
// pending duplicate-key stash entry for its slot since it
// can no longer be restored
resolve_duplicate_key_stash(ref_stack.back(), false);
}
else
{
// discard array
*ref_stack.back() = discarded;
// discard array, unless this slot holds a duplicate key's
// previous value pending restoration, in which case that
// value is restored instead of being discarded
if (!resolve_duplicate_key_stash(ref_stack.back(), true))
{
*ref_stack.back() = discarded;
#if JSON_DIAGNOSTIC_POSITIONS
// Set start/end positions for discarded array.
handle_diagnostic_positions_for_json_value(*ref_stack.back());
// Set start/end positions for discarded array.
handle_diagnostic_positions_for_json_value(*ref_stack.back());
#endif
}
}
}
@@ -809,14 +838,48 @@ class json_sax_dom_callback_parser
}
#endif
/// remove the discarded value the callback rejected from its parent
static void remove_discarded_value(BasicJsonType& parent)
/// if there is a pending duplicate-key stash entry for this exact slot,
/// remove it from the stash; if restore_value is true, the stashed
/// previous value is moved back into the slot first (use this when the
/// new value at that slot was rejected); otherwise the stash entry is
/// simply dropped (use this when the new value was accepted, so it
/// correctly supersedes the old one and no restore should ever happen
/// for this slot again)
/// @return whether a matching stash entry was found (and processed)
bool resolve_duplicate_key_stash(BasicJsonType* slot, bool restore_value)
{
const auto it = std::find_if(duplicate_key_stash.begin(), duplicate_key_stash.end(),
[slot](const std::pair<BasicJsonType*, BasicJsonType>& entry)
{
return entry.first == slot;
});
if (it == duplicate_key_stash.end())
{
return false;
}
if (restore_value)
{
*slot = std::move(it->second);
}
duplicate_key_stash.erase(it);
return true;
}
/// remove the discarded value the callback rejected from its parent,
/// unless it is a duplicate key's slot with a stashed previous value,
/// in which case that previous value is restored instead
void remove_discarded_value(BasicJsonType& parent)
{
for (auto it = parent.begin(); it != parent.end(); ++it)
{
if (it->is_discarded())
{
parent.erase(it);
if (!resolve_duplicate_key_stash(&(*it), true))
{
parent.erase(it);
}
break;
}
}
@@ -914,6 +977,16 @@ class json_sax_dom_callback_parser
JSON_ASSERT(object_element);
*object_element = std::move(value);
if (!skip_callback)
{
// this scalar value finally, definitively replaces whatever was
// at this slot; drop any pending duplicate-key stash entry for
// it since it can no longer be restored (a container value at
// this slot is resolved later, in end_object()/end_array(),
// since skip_callback is true for the placeholder handling that
// happens here for those)
resolve_duplicate_key_stash(object_element, false);
}
return {true, object_element};
}
@@ -927,6 +1000,12 @@ class json_sax_dom_callback_parser
std::vector<bool> key_keep_stack {}; // NOLINT(readability-redundant-member-init)
/// helper to hold the reference for the next object element
BasicJsonType* object_element = nullptr;
/// stash of (slot pointer, previous value) for object members that
/// already existed when key() was called again for the same key
/// (duplicate keys); used to restore the previous value if the new
/// value is later rejected by the callback, instead of erasing the
/// member entirely
std::vector<std::pair<BasicJsonType*, BasicJsonType>> duplicate_key_stash {};
/// whether a syntax error occurred
bool errored = false;
/// callback function
single_include/nlohmann/json.hpp
+92 -13
View File
@@ -7768,10 +7768,11 @@ NLOHMANN_JSON_NAMESPACE_END
#include <algorithm> // find_if
#include <cstddef>
#include <string> // string
#include <type_traits> // enable_if_t
#include <utility> // move
#include <utility> // move, pair
#include <vector> // vector
// #include <nlohmann/detail/exceptions.hpp>
@@ -10113,7 +10114,17 @@ class json_sax_dom_callback_parser
// add discarded value at the given key and store the reference for later
if (keep && ref_stack.back())
{
object_element = &(ref_stack.back()->m_data.m_value.object->operator[](val) = discarded);
auto& obj = *ref_stack.back()->m_data.m_value.object;
const auto it = obj.find(val);
if (it != obj.end())
{
// this is a duplicate key (legal in JSON); remember its
// current value so it can be restored later if the new
// value is rejected by the callback, instead of being
// erased together with the discarded placeholder
duplicate_key_stash.emplace_back(&(it->second), it->second);
}
object_element = &(obj[val] = discarded);
}
return true;
@@ -10125,13 +10136,18 @@ class json_sax_dom_callback_parser
{
if (!callback(static_cast<int>(ref_stack.size()) - 1, parse_event_t::object_end, *ref_stack.back()))
{
// discard object
*ref_stack.back() = discarded;
// discard object, unless this slot holds a duplicate key's
// previous value pending restoration, in which case that
// value is restored instead of being discarded
if (!resolve_duplicate_key_stash(ref_stack.back(), true))
{
*ref_stack.back() = discarded;
#if JSON_DIAGNOSTIC_POSITIONS
// Set start/end positions for discarded object.
handle_diagnostic_positions_for_json_value(*ref_stack.back());
// Set start/end positions for discarded object.
handle_diagnostic_positions_for_json_value(*ref_stack.back());
#endif
}
}
else
{
@@ -10145,6 +10161,10 @@ class json_sax_dom_callback_parser
#endif
ref_stack.back()->set_parents();
// this object is finally, definitively kept; drop any
// pending duplicate-key stash entry for its slot since it
// can no longer be restored
resolve_duplicate_key_stash(ref_stack.back(), false);
}
}
@@ -10214,16 +10234,25 @@ class json_sax_dom_callback_parser
#endif
ref_stack.back()->set_parents();
// this array is finally, definitively kept; drop any
// pending duplicate-key stash entry for its slot since it
// can no longer be restored
resolve_duplicate_key_stash(ref_stack.back(), false);
}
else
{
// discard array
*ref_stack.back() = discarded;
// discard array, unless this slot holds a duplicate key's
// previous value pending restoration, in which case that
// value is restored instead of being discarded
if (!resolve_duplicate_key_stash(ref_stack.back(), true))
{
*ref_stack.back() = discarded;
#if JSON_DIAGNOSTIC_POSITIONS
// Set start/end positions for discarded array.
handle_diagnostic_positions_for_json_value(*ref_stack.back());
// Set start/end positions for discarded array.
handle_diagnostic_positions_for_json_value(*ref_stack.back());
#endif
}
}
}
@@ -10337,14 +10366,48 @@ class json_sax_dom_callback_parser
}
#endif
/// remove the discarded value the callback rejected from its parent
static void remove_discarded_value(BasicJsonType& parent)
/// if there is a pending duplicate-key stash entry for this exact slot,
/// remove it from the stash; if restore_value is true, the stashed
/// previous value is moved back into the slot first (use this when the
/// new value at that slot was rejected); otherwise the stash entry is
/// simply dropped (use this when the new value was accepted, so it
/// correctly supersedes the old one and no restore should ever happen
/// for this slot again)
/// @return whether a matching stash entry was found (and processed)
bool resolve_duplicate_key_stash(BasicJsonType* slot, bool restore_value)
{
const auto it = std::find_if(duplicate_key_stash.begin(), duplicate_key_stash.end(),
[slot](const std::pair<BasicJsonType*, BasicJsonType>& entry)
{
return entry.first == slot;
});
if (it == duplicate_key_stash.end())
{
return false;
}
if (restore_value)
{
*slot = std::move(it->second);
}
duplicate_key_stash.erase(it);
return true;
}
/// remove the discarded value the callback rejected from its parent,
/// unless it is a duplicate key's slot with a stashed previous value,
/// in which case that previous value is restored instead
void remove_discarded_value(BasicJsonType& parent)
{
for (auto it = parent.begin(); it != parent.end(); ++it)
{
if (it->is_discarded())
{
parent.erase(it);
if (!resolve_duplicate_key_stash(&(*it), true))
{
parent.erase(it);
}
break;
}
}
@@ -10442,6 +10505,16 @@ class json_sax_dom_callback_parser
JSON_ASSERT(object_element);
*object_element = std::move(value);
if (!skip_callback)
{
// this scalar value finally, definitively replaces whatever was
// at this slot; drop any pending duplicate-key stash entry for
// it since it can no longer be restored (a container value at
// this slot is resolved later, in end_object()/end_array(),
// since skip_callback is true for the placeholder handling that
// happens here for those)
resolve_duplicate_key_stash(object_element, false);
}
return {true, object_element};
}
@@ -10455,6 +10528,12 @@ class json_sax_dom_callback_parser
std::vector<bool> key_keep_stack {}; // NOLINT(readability-redundant-member-init)
/// helper to hold the reference for the next object element
BasicJsonType* object_element = nullptr;
/// stash of (slot pointer, previous value) for object members that
/// already existed when key() was called again for the same key
/// (duplicate keys); used to restore the previous value if the new
/// value is later rejected by the callback, instead of erasing the
/// member entirely
std::vector<std::pair<BasicJsonType*, BasicJsonType>> duplicate_key_stash {};
/// whether a syntax error occurred
bool errored = false;
/// callback function
tests/src/unit-regression2.cpp
+71
View File
@@ -1566,4 +1566,75 @@ TEST_CASE("issue #5402 - update(merge_objects=true) overwrites a primitive with
CHECK(mixed == json({{"keep", {{"a", 1}, {"b", 2}}}, {"replace", {{"x", 2}}}}));
}
TEST_CASE("regression test - parser callback must not lose a duplicate key's prior value")
{
// a callback that rejects only the scalar value 2
const json::parser_callback_t drop_value_2 = [](int /*depth*/, json::parse_event_t ev, json & v)
{
return !(ev == json::parse_event_t::value && v == 2);
};
SECTION("duplicate key, second (scalar) value rejected - prior value is restored")
{
const json j = json::parse(R"({"a":1,"a":2})", drop_value_2);
CHECK(j.dump() == "{\"a\":1}");
}
SECTION("duplicate key, second value is an object rejected at object_end - prior value is restored")
{
const json j = json::parse(R"({"a":1,"a":{"x":2}})",
[](int depth, json::parse_event_t ev, json& /*parsed*/)
{
return !(ev == json::parse_event_t::object_end && depth == 1);
});
CHECK(j.dump() == "{\"a\":1}");
}
SECTION("duplicate key, second value is an array rejected at array_end - prior value is restored")
{
const json j = json::parse(R"({"a":1,"a":[9,9]})",
[](int depth, json::parse_event_t ev, json& /*parsed*/)
{
return !(ev == json::parse_event_t::array_end && depth == 1);
});
CHECK(j.dump() == "{\"a\":1}");
}
SECTION("duplicate key, second value accepted (scalar) - last value wins")
{
const json j = json::parse(R"({"a":1,"a":2})", [](int, json::parse_event_t, json&) noexcept
{
return true;
});
CHECK(j.dump() == "{\"a\":2}");
}
SECTION("duplicate key, second value accepted (object) - last value wins")
{
const json j = json::parse(R"({"a":1,"a":{"x":2}})", [](int, json::parse_event_t, json&) noexcept
{
return true;
});
CHECK(j.dump() == "{\"a\":{\"x\":2}}");
}
SECTION("brand new (non-duplicate) key, value rejected - member is fully absent")
{
const json j = json::parse(R"({"a":1,"b":2})", drop_value_2);
CHECK(j.dump() == "{\"a\":1}");
}
SECTION("duplicate key nested two levels deep")
{
const json j = json::parse(R"({"outer":{"a":1,"a":2}})", drop_value_2);
CHECK(j.dump() == "{\"outer\":{\"a\":1}}");
}
SECTION("three occurrences of the same key - middle rejected, last accepted")
{
const json j = json::parse(R"({"k":1,"k":2,"k":3})", drop_value_2);
CHECK(j.dump() == "{\"k\":3}");
}
}
DOCTEST_CLANG_SUPPRESS_WARNING_POP