json/tests/src/unit-large_json.cpp

//     __ _____ _____ _____
//  __|  |   __|     |   | |  JSON for Modern C++ (supporting code)
// |  |  |__   |  |  | | | |  version 3.12.0
// |_____|_____|_____|_|___|  https://github.com/nlohmann/json
//
// SPDX-FileCopyrightText: 2013-2026 Niels Lohmann <https://nlohmann.me>
// SPDX-License-Identifier: MIT

#include "doctest_compatibility.h"

#include <nlohmann/json.hpp>
using nlohmann::json;

#include <algorithm>
#include <string>
#include <vector>

TEST_CASE("tests on very large JSONs")
{
    SECTION("issue #1419 - Segmentation fault (stack overflow) due to unbounded recursion")
    {
        const auto depth = 500000;

        std::string s(static_cast<std::size_t>(2 * depth), '[');
        std::fill(s.begin() + depth, s.end(), ']');

        json _;
        CHECK_NOTHROW(_ = nlohmann::json::parse(s));
    }
}

namespace
{

// Descend a chain of single-element containers and return the value at its end,
// reporting the number of levels traversed in @a depth.
//
// The values in the test case below are nested far deeper than the call stack
// can follow, so they must not be inspected with operator== or dump(): both are
// still recursive and would overflow the stack themselves.
const json* innermost_value(const json& j, std::size_t& depth)
{
    const json* current = &j;
    depth = 0;

    while ((current->is_array() || current->is_object()) && !current->empty())
    {
        current = current->is_array()
                  ? &current->front()
                  : &current->begin().value();
        ++depth;
    }

    return current;
}

// The text of a value nested depth levels deep around the number 0. Level i is
// an array if pattern[i % pattern.size()] is '[', and otherwise an object with
// the single member "a", which every object type enumerates in the same order.
std::string nested_text(std::size_t depth, const std::string& pattern)
{
    std::string text;
    std::string closing;
    for (std::size_t i = 0; i < depth; ++i)
    {
        const bool array = pattern[i % pattern.size()] == '[';
        text += array ? "[" : "{\"a\":";
        closing += array ? ']' : '}';
    }
    text += '0';
    text.append(closing.rbegin(), closing.rend());
    return text;
}

} // namespace

TEST_CASE("tests on deeply nested JSONs")
{
    // deep enough to exhaust the call stack, but small enough to stay cheap:
    // parsing is iterative, so building the values below costs little
    const std::size_t depth = 100000;

    SECTION("issue #5387 - stack overflow in the copy constructor")
    {
        SECTION("array")
        {
            const json j = json::parse(std::string(depth, '[') + '0' + std::string(depth, ']'));

            const json copy(j); // NOLINT(performance-unnecessary-copy-initialization): the copy is what is tested

            std::size_t copy_depth = 0;
            CHECK(*innermost_value(copy, copy_depth) == 0);
            CHECK(copy_depth == depth);
        }

        SECTION("object")
        {
            std::string s;
            s.reserve((6 * depth) + 1);
            for (std::size_t i = 0; i < depth; ++i)
            {
                s += "{\"a\":";
            }
            s += '1';
            s.append(depth, '}');

            const json j = json::parse(s);

            const json copy(j); // NOLINT(performance-unnecessary-copy-initialization): the copy is what is tested

            std::size_t copy_depth = 0;
            CHECK(*innermost_value(copy, copy_depth) == 1);
            CHECK(copy_depth == depth);
        }

        SECTION("copy assignment")
        {
            // operator=(basic_json) takes its argument by value, so the deep
            // copy happens in the copy constructor
            const json j = json::parse(std::string(depth, '[') + '0' + std::string(depth, ']'));

            json target;
            target = j;

            std::size_t target_depth = 0;
            CHECK(*innermost_value(target, target_depth) == 0);
            CHECK(target_depth == depth);
        }

        SECTION("depths around the bound of the recursive descent")
        {
            // The copy constructor descends into a bounded number of levels and
            // completes whatever is below that without the call stack. Cover
            // every depth around that bound, so that the two ways of copying
            // are known to meet cleanly - wherever the bound is set.
            for (std::size_t d = 1; d <= 300; ++d)
            {
                CAPTURE(d);

                const json array = json::parse(std::string(d, '[') + '0' + std::string(d, ']'));
                const json array_copy(array); // NOLINT(performance-unnecessary-copy-initialization): the copy is what is tested
                std::size_t array_depth = 0;
                CHECK(*innermost_value(array_copy, array_depth) == 0);
                CHECK(array_depth == d);

                std::string object_text;
                for (std::size_t i = 0; i < d; ++i)
                {
                    object_text += "{\"a\":";
                }
                object_text += '1';
                object_text.append(d, '}');

                const json object = json::parse(object_text);
                const json object_copy(object); // NOLINT(performance-unnecessary-copy-initialization): the copy is what is tested
                std::size_t object_depth = 0;
                CHECK(*innermost_value(object_copy, object_depth) == 1);
                CHECK(object_depth == d);
            }
        }

        SECTION("a value that is deep in one place only")
        {
            json j = json::object();
            j["shallow"] = 1;
            j["deep"] = json::parse(std::string(depth, '[') + '0' + std::string(depth, ']'));
            j["also_shallow"] = json::array({1, 2, 3});

            const json copy(j);

            CHECK(copy["shallow"] == 1);
            CHECK(copy["also_shallow"] == json::array({1, 2, 3}));

            std::size_t deep_depth = 0;
            CHECK(*innermost_value(copy["deep"], deep_depth) == 0);
            CHECK(deep_depth == depth);
        }

        SECTION("comparing")
        {
            // Comparing used to descend once per level, and an ordered
            // comparison used to compare every pair of elements twice, once in
            // each direction, which took exponentially long in the nesting
            // depth. Both are gone: these finish in milliseconds, where the
            // second used to take longer than anyone would wait even for a
            // value nested only a few dozen levels deep.
            const std::string text = std::string(depth, '[') + '0' + std::string(depth, ']');
            const json j = json::parse(text);
            const json same = json::parse(text);
            const json larger = json::parse(std::string(depth, '[') + '1' + std::string(depth, ']'));

            CHECK(j == same);
            CHECK_FALSE(j == larger);
            CHECK(j != larger);

            CHECK(j < larger);
            CHECK_FALSE(larger < j);
            CHECK(larger > j);
            CHECK(j <= same);
            CHECK(j >= same);

            // a value that ends earlier is the smaller one
            const json shorter = json::parse(std::string(depth - 1, '[') + '0' + std::string(depth - 1, ']'));
            CHECK_FALSE(j == shorter);
        }

        SECTION("comparing objects")
        {
            std::string text;
            text.reserve((6 * depth) + 1);
            for (std::size_t i = 0; i < depth; ++i)
            {
                text += "{\"a\":";
            }
            text += '1';
            text.append(depth, '}');

            const json j = json::parse(text);
            const json same = json::parse(text);

            CHECK(j == same);
            CHECK_FALSE(j != same);
            CHECK(j <= same);
            CHECK(j >= same);
        }

        SECTION("the copy is independent of the original")
        {
            const json j = json::parse(std::string(depth, '[') + '0' + std::string(depth, ']'));

            json copy(j);

            // reach the innermost value without recursing and replace it
            json* current = &copy;
            while (current->is_array() && !current->empty())
            {
                current = &current->front();
            }
            *current = 42;

            std::size_t unused = 0;
            CHECK(*innermost_value(copy, unused) == 42);
            CHECK(*innermost_value(j, unused) == 0);
        }
    }

    SECTION("issue #5650 - stack overflow converting between specializations")
    {
        const std::vector<std::string> patterns = {"[", "{", "[{"};

        SECTION("json to ordered_json")
        {
            for (const auto& pattern : patterns)
            {
                CAPTURE(pattern);
                const std::string text = nested_text(depth, pattern);
                const json j = json::parse(text);

                const nlohmann::ordered_json converted = j;
                CHECK(converted.dump() == text);
            }
        }

        SECTION("ordered_json to json")
        {
            for (const auto& pattern : patterns)
            {
                CAPTURE(pattern);
                const std::string text = nested_text(depth, pattern);
                const nlohmann::ordered_json o = nlohmann::ordered_json::parse(text);

                const json converted = o;
                CHECK(converted.dump() == text);
            }
        }

        SECTION("get<ordered_json>()")
        {
            for (const auto& pattern : patterns)
            {
                CAPTURE(pattern);
                const std::string text = nested_text(depth, pattern);
                const json j = json::parse(text);

                CHECK(j.get<nlohmann::ordered_json>().dump() == text);
            }
        }

        SECTION("depths around the bound of the recursive descent")
        {
            for (std::size_t d = 1; d <= 300; ++d)
            {
                CAPTURE(d);
                for (const auto& pattern : patterns)
                {
                    CAPTURE(pattern);
                    const std::string text = nested_text(d, pattern);
                    const json j = json::parse(text);

                    const nlohmann::ordered_json converted = j;
                    CHECK(converted.dump() == text);
                    const json back = converted;
                    CHECK(back.dump() == text);
                }
            }
        }

        SECTION("values below the bound are converted as values above it")
        {
            // Bury a value below the bound, where it is converted without the
            // call stack, and compare it with the same value converted on its
            // own by the containers' range constructors. Its objects have
            // members that the two object types enumerate in different orders.
            const auto bury = [](nlohmann::ordered_json value)
            {
                for (std::size_t i = 0; i < 200; ++i)
                {
                    value = nlohmann::ordered_json::array({std::move(value)});
                }
                return value;
            };
            const auto dig = [](const json & value)
            {
                const json* current = &value;
                for (std::size_t i = 0; i < 200; ++i)
                {
                    current = &current->at(0);
                }
                return current;
            };

            nlohmann::ordered_json value = nlohmann::ordered_json::object();
            value["z"] = {1, -2, 3U, 4.5, true, nullptr, "six", nlohmann::ordered_json::binary({7, 8}, 9),
                          nlohmann::ordered_json::binary({10}), nlohmann::ordered_json::array(), nlohmann::ordered_json::object()
                         };
            value["y"] = {{"x", {{"w", 1}, {"v", 2}}}, {"u", {3, {{"t", 4}, {"s", 5}}}}};
            value["r"] = nlohmann::ordered_json::array({nlohmann::ordered_json(nlohmann::ordered_json::value_t::discarded)});

            const json converted_above = value;
            const json buried = bury(value);
            const json& converted_below = *dig(buried);

            CHECK(converted_below.dump() == converted_above.dump());
            CHECK(converted_below.at("z").at(7).get_binary().subtype() == 9);
            CHECK_FALSE(converted_below.at("z").at(8).get_binary().has_subtype());
            CHECK(converted_below.at("r").at(0).is_discarded());

            // a discarded value is never equal to anything, so compare the rest
            value.erase("r");
            const json without_discarded_above = value;
            const json without_discarded_buried = bury(value);
            CHECK(*dig(without_discarded_buried) == without_discarded_above);
        }
    }
}