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[C++] Reject a wire blockLength shorter than the fixed fields of the acting version in the OTF decoder - #1179
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…acting version in the OTF decoder. decode() and decodeGroups() reject the block before any field or onBeginGroup is dispatched. Fields newer than the acting version do not count, so older versions with shorter blocks still decode, and empty groups are not checked.
This was referenced Nov 14, 2026
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OtfMessageDecoder checks the buffer only against the blockLength from the wire, then hands listeners pointers at the schema field offsets. A frame whose message or group blockLength is shorter than the fixed fields gives listeners pointers past the buffer, and a group with blockLength 0 lets numInGroup entries be decoded from the same bytes, so the work no longer depends on the message size.
decode() and decodeGroups() now throw before dispatching any field or onBeginGroup when the blockLength is below the extent of the fields present at the acting version. Fields, and composite members, newer than the acting version do not count, and groups with numInGroup 0 are not checked. Listeners still receive offsets for fields newer than the acting version, as before, so they must check the version before reading them.
Part of #969.
Verified: OtfBlockLengthTest fails on master (2 of the 4 decode tests, plus a heap-buffer-overflow under ASan) and passes here, including a composite-member case; sbe-tool checkstyle and test, cppbuild and cppbuild --debug-build --sanitise-build pass.