[niteshbalusu11]

Summary

Bitcoin Core's submitblock adds the coinbase witness reserved value (32 zero bytes) when the block has a witness commitment but the coinbase has no witness. It only does this once segwit is active. See UpdateUncommittedBlockStructures in Core's rpc/mining.cpp.

rbitcoin did not do this. The same block was rejected with bad-witness-nonce-size. Mining software that leaves the coinbase witness out would lose the block.

 submit_received_block
   prev-blk-not-found check
+  fill_witness_reserved_value   # Core UpdateUncommittedBlockStructures
   cheap_submit_tx_reject
   accept_received_block

The fill is skipped below segwit and when the coinbase already has a witness, like Core.

Evidence

Same block bytes sent to Core v31.1 and rbitcoin on regtest:

Block	Core 31.1	rbitcoin before	rbitcoin after
commitment, coinbase witness left out	accepted	bad-witness-nonce-size	accepted
same, plus a segwit spend	accepted	bad-witness-nonce-size	accepted

Core's own p2p_segwit.py test_submit_block now passes too. The only difference left is the reject string for a wrong nonce (witness commitment mismatch vs bad-witness-merkle-match).

New test submitblock_adds_the_witness_reserved_value_once_segwit_is_active (segwit@2):

• Before: height 2 fails with bad-witness-nonce-size.
• After: height 1 stays bare, height 2 gets 32 zero bytes, height 3 keeps the miner's own value, height 4 (no commitment) stays bare, and height 6 is filled even when its parent is only a stored header (headers first).

Merge Danger

Door: two-way

Blast Radius: small. Only submitblock. P2P block handling does not change.

🤖 Generated with Claude Code

[rearden-grok]

Summary

submitblock now inserts the BIP141 coinbase witness reserved value (one 32-byte zero stack item) before the usual accept path. That matches Core UpdateUncommittedBlockStructures: only when segwit is active for this block, the coinbase has a witness-commitment output, and the coinbase has no witness. An existing reserved value is left alone, a pre-segwit commitment stays bare data, and P2P accept_received_block does not grow a second fill. The reserved value is always zeros, so a wrong parent cannot invent a different nonce. The gap is fail-closed: an unresolved parent height is treated as segwit-off, and the block this PR exists to accept is still rejected with bad-witness-nonce-size.

Issue counts by severity

• bugs: 0
• suggestions: 1
• nits: 1

[niteshbalusu11]

@rearden-grok please re-review the latest commit (f49cd66).

[rearden-grok]

Summary

submitblock fills the BIP141 coinbase witness reserved value the way Core's UpdateUncommittedBlockStructures does: 32 zero bytes only when segwit is active for this block, the coinbase has a witness commitment, and the coinbase witness stack is empty. An existing reserved value is left alone, a pre-segwit commitment stays ordinary data, and P2P accept_received_block does not grow a second fill. The headers-first case asserts inconclusive. The 10,000-header parent walk stays fail-closed past the cap, and docs/rpc.md matches that.

Prior notes on the parent walk and the held-result assert are addressed on f49cd66d. No new comment restates the code.

Issue counts by severity

• bugs: 0
• suggestions: 0
• nits: 0

Rebase is not required. This head is mergeable, and the four commits on master since the base are the BIP30 ancestor work. They do not touch these files.