kernel: reject trailing bytes in object creation #36647

btck_transaction_create and btck_block_create ignore any bytes after the serialized object, so the object's serialized length does not match the buffer.

I return null when bytes remain, as for other malformed input, and say so in the header. Two new checks in test_kernel append one byte to a valid transaction and to a valid block. They fail on master and pass with the change. The other kernel tests and the unit tests pass.

This is one of several PRs from a large audit of Bitcoin Core that I did with the help of LLM tools. The audit produced its evidence all at once, which is why the PRs arrive together. I understand each change fully and will answer review questions myself.

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External sites

• Code coverage & benchmarks: https://corecheck.dev/bitcoin/bitcoin/pulls/36647
• Ralph review <img src=https://review.fish.foo/assets/bitcoin/bitcoin/36647.png alt="Ralph review status"> https://git.fish.foo/bitcoin/bitcoin/pulls/36647#author-ralph

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LLM Linter (✨ experimental)

Possible places where comparison-specific test macros should replace generic comparisons:

• src/test/kernel/test_kernel.cpp: BOOST_CHECK_THROW(Transaction{trailing_tx_data}, std::runtime_error) → Prefer BOOST_CHECK_EXCEPTION(..., HasReason{"..."}) to check the detailed error message.
• src/test/kernel/test_kernel.cpp: BOOST_CHECK_THROW(Block{trailing_block_data}, std::runtime_error) → Prefer BOOST_CHECK_EXCEPTION(..., HasReason{"..."}) to check the detailed error message.

^{2026-10-24 08:12:44}

You are observing the current behaviour correctly, but absent a reported defect, I don't think there is any reason to change this. Closing.