MessagePack reader: read runs of scalar array elements in one loop (#5980)

* MessagePack reader: read runs of scalar array elements in one loop

As for CBOR: when an array opens and the input is contiguous, read the
integers, floats, booleans, and nils that follow in a tight loop instead
of going through parse_msgpack_internal and parse_msgpack_value for every
element. The loop stops at any other kind of value and when fewer than 9
bytes are left; those values are read by the general path as before.
Non-contiguous inputs are not affected.

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

* MessagePack reader: enter the scalar run only when a scalar follows

As for CBOR: enter_msgpack_array tests the first byte and the remaining length
inline and calls the out-of-line run only for a scalar, so arrays of maps or
strings no longer pay for a call that reads nothing; the run's loop condition
moves to the end (do-while).

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

* MessagePack reader: mark the initial accepted in the scalar run as a false-positive dead store

ci_infer reports DEAD_STORE for 'bool accepted = true;' in the MessagePack
scalar run, as in the CBOR one, because every case assigns accepted before it
is read.

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

* MessagePack tests: assign the parsed value instead of casting it to void

GCC (ci_test_gcc) reports -Werror=unused-result for
static_cast<void>(json::from_msgpack(...)): a cast to void does not silence
warn_unused_result on GCC. Assign to a json, as the other tests in the file do.

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

* MessagePack reader: drop the scalar pre-check before the run

enter_msgpack_array now calls the run directly, and parse_msgpack_scalar_run
tests the remaining count and length at the top of a while loop instead of
relying on msgpack_scalar_follows(); its own switch already leaves on any
value that is not a scalar.

Instructions per repetition, new/old: SAX canada 0.946 (clang) and 0.973
(GCC 13), DOM read canada 0.991 / 0.992; no row more than 0.3 % higher.

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

* MessagePack reader: say why the initial accepted in the scalar run stays

As for CBOR: Infer's DEAD_STORE report is right, not a false positive. The
initializer is kept for clang-tidy's cppcoreguidelines-init-variables.

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

* MessagePack reader: give the scalar run the bulk tag instead of a wrapper

As for CBOR: parse_msgpack_scalar_run takes the bulk-scan tag itself and is
the function kept out of line, so parse_msgpack_array_start goes.

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

* MessagePack reader: start the scalar run's accepted at false

As for CBOR: Infer ignores dead stores of 0, so the suppression goes.

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

* MessagePack reader: let the scalar run find its array itself

As for CBOR: parse_msgpack_scalar_run reads container_stack.back() in its
bulk version, so the caller no longer evaluates it for input that is not
contiguous.

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

---------

Signed-off-by: Niels Lohmann <mail@nlohmann.me>
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Niels Lohmann authored and GitHub committed 2026-11-01 17:22:32 +01:00
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@@ -27,12 +27,15 @@ using nlohmann::json;
#include <sstream>
#include <iomanip>
#include <limits>
#include <list>
#include <set>
#include "make_test_data_available.hpp"
#include "round_trip_corpus.hpp"
#include "test_utils.hpp"
#include "sax_countdown.hpp"
#include "sax_event_loggers.hpp"
using utils::SaxCountdown;
using utils::SaxEventLogger;
TEST_CASE("MessagePack")
@@ -2212,3 +2215,99 @@ TEST_CASE("MessagePack large strings and binaries (chunked reader)")
}
}
}
TEST_CASE("MessagePack arrays of scalars from contiguous and non-contiguous input")
{
// contiguous input reads the integers, floats, booleans, and nils an
// array begins with in one loop, other input value by value; both must
// give the same events, value, or error for the complete document and for
// every truncation of it, and stop at every event a SAX handler rejects
const std::vector<std::uint8_t> msgpack =
{
0x83, // map(3)
0xA1, 'a', // "a"
0xDC, 0x00, 0x16, // array 16 (22)
0x00, 0x7F, 0xCC, 0xFF, 0xCD, 0x01, 0x00, // 0, 127, 255, 256
0xCE, 0x00, 0x01, 0x00, 0x00, // 65536
0xCF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, // 18446744073709551615
0xFF, 0xE0, 0xD0, 0x80, 0xD1, 0xFF, 0x00, // -1, -32, -128, -256
0xD2, 0xFF, 0xFE, 0xFF, 0xFF, // -65537
0xD3, 0x80, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, // -9223372036854775808
0xC2, 0xC3, 0xC0, // false, true, nil
0xCA, 0x3F, 0xC0, 0x00, 0x00, // 1.5 (float 32)
0xCB, 0xBF, 0xB9, 0x99, 0x99, 0x99, 0x99, 0x99, 0x9A, // -0.1
0xA1, 'x', // "x" (ends the loop)
0x01, // 1
0xCB, 0x40, 0x04, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, // 2.5
0xD9, 0x01, 'y', // "y" (str 8)
0xC0, // nil
0xA1, 'b', // "b"
0x93, // fixarray(3)
0x92, 0xCB, 0x3F, 0xF4, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, // [1.25,
0xCB, 0xC0, 0x0E, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, // -3.75]
0x92, 0xCB, 0x3F, 0xF4, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, // [1.25,
0xCB, 0xC0, 0x0E, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, // -3.75]
0x90, // []
0xA1, 'c', // "c"
0x94, 0x80, 0x01, 0xC4, 0x01, 0x05, 0x91, 0x03 // [{}, 1, bin 8, [3]]
};
const json j = json::from_msgpack(msgpack);
CHECK(j["a"].size() == 22);
CHECK(j["a"][5] == 18446744073709551615u);
CHECK(j["a"][7] == -32);
CHECK(j["a"][11] == (std::numeric_limits<std::int64_t>::min)());
CHECK(j["a"][15] == 1.5);
CHECK(j["a"][16] == -0.1);
CHECK(j["a"][17] == "x");
CHECK(j["b"] == json::parse("[[1.25, -3.75], [1.25, -3.75], []]"));
CHECK(j["c"][2].is_binary());
for (std::size_t length = 0; length <= msgpack.size(); ++length)
{
CAPTURE(length)
const std::vector<std::uint8_t> contiguous(msgpack.begin(), msgpack.begin() + static_cast<std::ptrdiff_t>(length));
const std::list<std::uint8_t> bytewise(contiguous.begin(), contiguous.end());
CHECK(json::from_msgpack(contiguous, true, false).dump() == json::from_msgpack(bytewise.begin(), bytewise.end(), true, false).dump());
#if !defined(JSON_NOEXCEPTION)
std::string contiguous_error;
std::string bytewise_error;
try
{
json _;
_ = json::from_msgpack(contiguous);
}
catch (const json::parse_error& e)
{
contiguous_error = e.what();
}
try
{
json _;
_ = json::from_msgpack(bytewise.begin(), bytewise.end());
}
catch (const json::parse_error& e)
{
bytewise_error = e.what();
}
CHECK(contiguous_error == bytewise_error);
#endif
SaxEventLogger from_contiguous;
SaxEventLogger from_bytewise;
CHECK(json::sax_parse(contiguous, &from_contiguous, json::input_format_t::msgpack)
== json::sax_parse(bytewise.begin(), bytewise.end(), &from_bytewise, json::input_format_t::msgpack));
CHECK(from_contiguous.events == from_bytewise.events);
}
for (int events = 0; events <= 60; ++events)
{
CAPTURE(events)
const std::list<std::uint8_t> bytewise(msgpack.begin(), msgpack.end());
SaxCountdown from_contiguous(events);
SaxCountdown from_bytewise(events);
CHECK(json::sax_parse(msgpack, &from_contiguous, json::input_format_t::msgpack)
== json::sax_parse(bytewise.begin(), bytewise.end(), &from_bytewise, json::input_format_t::msgpack));
}
}