JDS needs tighter validation against `DeclareMiningJob.coinbase_tx_prefix`

[plebhash]

JDS's DeclaredCustomJob::get_coinbase_tx is a bit too naive:

sv2-apps/pool-apps/jd-server/src/lib/job_declarator/job_validation/bitcoin_core_ipc.rs

Lines 172 to 209 in fff6abc

	fn get_coinbase_tx(&self) -> Result<Transaction, ()> {
	let declared_coinbase_tx_prefix: Vec<u8> =
	self.declare_mining_job.coinbase_tx_prefix.to_owned_bytes();
	let declared_coinbase_tx_suffix: Vec<u8> =
	self.declare_mining_job.coinbase_tx_suffix.to_owned_bytes();
	// Parse scriptSig size from coinbase prefix
	// Coinbase structure: version(4) + marker+flag(2) + input_count(1) + outpoint(32) +
	// index(4) = 43 bytes Then comes scriptSig length (VarInt) followed by scriptSig
	// data
	const COINBASE_PREFIX_LEN: usize = 43;
	let script_sig_size: usize = {
	let mut cursor = &declared_coinbase_tx_prefix[COINBASE_PREFIX_LEN..];
	match bitcoin::VarInt::consensus_decode(&mut cursor) {
	Ok(varint) => varint.0 as usize,
	Err(e) => {
	tracing::error!(
	"Failed to decode scriptSig size from coinbase prefix: {}",
	e
	);
	return Err(());
	}
	}
	};
	// Calculate the size of scriptSig bytes already in the prefix.
	let varint_size = bitcoin::VarInt(script_sig_size as u64).size();
	let script_sig_offset = COINBASE_PREFIX_LEN + varint_size;
	let script_sig_bytes_in_prefix = declared_coinbase_tx_prefix.len() - script_sig_offset;
	// The full extranonce fills the remaining space in scriptSig
	let full_extranonce_size: usize = script_sig_size - script_sig_bytes_in_prefix;
	// Concatenate prefix + full extranonce (zeros) + suffix to form the complete transaction
	// bytes
	let mut declared_coinbase_tx = declared_coinbase_tx_prefix;
	declared_coinbase_tx.extend_from_slice(&vec![0; full_extranonce_size]);
	declared_coinbase_tx.extend_from_slice(&declared_coinbase_tx_suffix);

if a DeclareMiningJob comes with coinbase_tx_prefix that don't satisfy some naive assumptions, that can lead to panics

more specifically:

• if coinbase_tx_prefix is smaller than the expected 43 bytes, &declared_coinbase_tx_prefix[COINBASE_PREFIX_LEN..] panics with index-out-of-bounds
• adjacent arithmetic (prefix.len() - script_sig_offset, script_sig_size - script_sig_bytes_in_prefix) can also underflow depending on the contents of DeclareMiningJob
• if scriptSig length is declared as a sufficiently large value (which would actually break Bitcoin consensus), it could lead to unbounded memory allocation

in summary, JDS should defensively check against all these edge cases before allocating memory, and simply reject the DeclareMiningJob in case of invalid values