Niels Lohmann c41152e620 Fall back to plain-object encoding when to_bjdata()'s _ArrayType_ annotation is not a string (#5494) ...

* Fall back to plain-object encoding when _ArrayType_ is not a string

write_bjdata_ndarray() looked up _ArrayType_ by calling get<string_t>()
directly, which throws type_error.302 when the annotation is not a
string (e.g. a number, null, boolean, array, or object). Per the
documented BJData ndarray contract, an object only qualifies for the
compact ndarray encoding if _ArrayType_ names a known type; anything
else must fall back to plain-object encoding, the same way an unknown
type-name string already does.

Add an is_string() check before the get<string_t>() call so a
non-string _ArrayType_ takes the existing "unrecognized type name"
fallback path instead of throwing.

Fixes #5398.

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

* Relax the BJData fuzzer's round-trip check from byte-exact to value-exact

Fixing #5398 lets to_bjdata() proceed past the object it used to reject,
which exposed a pre-existing, unrelated round-trip quirk to the fuzzer:
a binary_t value serialized through the non-optimized ("$U#"-less)
array encoding is parsed back as a plain array of numbers, since
from_bjdata() has no way to tell "array of uint8 numbers" apart from
"array of bytes" without that optimized header. Re-serializing that
plain array then goes through the generic smallest-type writer, which
- unrelated to this PR, and long predating it - prefers the 'i' (int8)
marker over 'U' (uint8) for values that fit both, so the re-encoded
bytes can differ from the original even though both decode to the same
value.

This is not introduced by the #5398 fix; the same divergence reproduces
from a bare json::binary_t value with no _ArrayType_ annotation
involved at all, on the commit immediately preceding it. A general fix
would mean changing the shared UBJSON/BJData smallest-type selection
that hundreds of existing tests pin to 'i' for small positive
integers, which is out of scope and too risky for this PR.

Update fuzzer-parse_bjdata.cpp's round-trip assertions to check that
re-serializing is value-stable (from_bjdata(to_bjdata(j)) == j) rather
than byte-exact, matching the guarantee BJData actually provides, and
add a regression test in unit-bjdata.cpp using the exact OSS-Fuzz input
that documents the behavior.

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

* Compare dump()s instead of json values in the BJData fuzzer's round-trip check

The value-stability assertion added to fix the earlier OSS-Fuzz crash
(json::from_bjdata(to_bjdata(j2)) == j2) itself broke on a NaN payload:
IEEE 754 NaN is never equal to itself, so operator== reports two
structurally-identical trees containing a non-finite double as
different -- not a round-trip bug, just NaN's ordinary
non-reflexivity. dump() serializes any non-finite double the same
deterministic way (as JSON null, since JSON cannot represent NaN or
Infinity), so comparing dumps is stable under exactly the values that
break operator==.

Verified against both the original OSS-Fuzz crash input and the new
one (0x68 0x68 0x7c, which decodes to a NaN), plus a local 2.5M-case
random-input sweep with no failures.

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

---------

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

2026-09-16 20:20:22 +02:00

..

json_fwd.hpp

📄 adjust year (#5044)

2026-01-01 20:00:39 +01:00

json.hpp

Fall back to plain-object encoding when to_bjdata()'s _ArrayType_ annotation is not a string (#5494)

2026-09-16 20:20:22 +02:00