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Niels Lohmann
d2309300e9
The BSON specification defines the payload of the "binary (old)" subtype 0x02 as an int32 followed by that many bytes. to_bson wrote the plain payload instead, so decoders such as pymongo and libbson-based readers rejected the bytes, and from_bson returned the inner length as part of the payload (documented as lenient behavior in gh-5333). The writer now emits the outer length (payload + 4), the subtype, the inner int32 holding the payload size, and the payload. The reader requires an outer length of at least 4 and an inner length equal to the outer length minus 4 (the cases the bson-corpus lists as decode errors), reports parse error 112 otherwise, and returns only the payload. Both directions change together so that the round trip keeps working. The single-reservation fast path for scalar entries declines subtype 0x02 because it has no room for the inner length. This is a deliberate exception to the opt-in rule for behavior changes: the previous output was invalid BSON. Signed-off-by: Niels Lohmann <mail@nlohmann.me>