Bug Found in bitcoin/block.c

[Schnema1]

Hi,

During investigation of #8973 Claude discovered an error introduced with commit #40dd780 Link to commit

The switch from pull_bitcoin_tx to pull_bitcoin_tx_only removed an implicit null check, causing a segfault if pull_bitcoin_tx_only returns NULL:
/lightning/bitcoin/block.c

c
// line 212-213 - missing null check
b->tx[i] = pull_bitcoin_tx_only(b->tx, &p, &len);
b->tx[i]->chainparams = chainparams;  // segfaults if above returns NULL

Fix:

cb->tx[i] = pull_bitcoin_tx_only(b->tx, &p, &len);
if (!b->tx[i])
        return tal_free(b);
b->tx[i]->chainparams = chainparams;

This null check is consistent with the existing pattern already used elsewhere in the same file and in pull_bitcoin_tx itself. Without this fix, a corrupted block response from bitcoind causes an immediate segfault rather than a graceful error. This patch is recommended regardless of whether you have a corrupted block file, as it converts a segfault into a handled error condition.

Diagnostic Steps for Others
If you see this crash, check in this order:

1. Verify bitcoind is healthy and fully synced:
   bashbitcoin-cli getblockchaininfo

2. Find which block CLN is failing on from the logs:
   bashgrep -B 50 "FATAL SIGNAL" ~/.lightning/cln.log | grep "Adding block" | tail -5

3. Test that specific block at different verbosity levels:

bashbitcoin-cli getblock <blockhash> 0  # should return hex
bitcoin-cli getblock <blockhash> 1  # will fail if corrupted
bitcoin-cli getblockstats <height>  # will fail if corrupted

Check bitcoind logs for corruption:
bashtail -100 ~/.bitcoin/debug.log | grep -E "ERROR|error|corrupt"
Environment

CLN version: v25.12.1
Bitcoin Core: v29 / v30.2
Downgrade to v25.09 not possible due to database migration introduced in v25.12

FYI @cdecker as you marked the pull request #40dd780 in Jan. 2024 as unverified

[Schnema1]

tried it myself with #9003