Niels Lohmann 5b26411166 Read MessagePack containers without recursing per nesting level ...

get_msgpack_array() and get_msgpack_object() read their elements by calling
back into parse_msgpack_internal(), which calls them again for a nested
container. The native call stack therefore grew with the nesting depth of the
input, and each level costs only one byte to encode: 0x91 is a one-element
array, so a few hundred thousand of them crash the process before any of the
input is rejected (#5104).

Keep the open containers on a heap stack instead, the way
parser::sax_parse_internal() has always done for JSON text. A frame records
how many elements are left and whether to close with end_object() or
end_array(); parse_msgpack_value() reads a single value and, for a container,
only opens it; and parse_msgpack_internal() loops, resuming the innermost
container after each element and closing it when its count runs out. Whether
the value that was begun is complete is answered by the stack being empty, so
no separate bookkeeping is needed.

The switch that decodes a value is untouched apart from the six container
cases, which now call enter_container() rather than a reader that loops. That
keeps this diff to the control flow and leaves the decoding of every other
type byte-identical.

enter_container() is the only place a binary reader emits start_object() or
start_array(), so a check that rejects a container can be added there once and
is guaranteed to run before the start event. The frame type and the stack are
shared, ready for the other three formats.

Verified against develop over empty, nested, counted (array 16/32, map 16/32)
and truncated inputs: identical values, error codes, messages and byte
offsets. 300,000 levels now report parse_error.110 instead of crashing, and a
well-formed 300,000-level value is read to completion through the SAX
interface, where develop crashes.

Reading such a value into a basic_json needs the return-by-move change as
well, without which the recursive copy constructor overflows on the way out;
that is the parent commit, and the test for the value path covers the two
together. Timing is unchanged: parsing 60,000 small objects and one array of
a million integers is within run-to-run noise of develop either way.

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

2026-09-06 18:12:42 +02:00

..

nlohmann

Read MessagePack containers without recursing per nesting level

2026-09-06 18:12:42 +02:00