Merge bitcoin/bitcoin#33745: mining: check witness commitment in submitBlock

6eaa00fe20 test: clarify submitBlock() mutates the template (Sjors Provoost)
862bd43283 mining: ensure witness commitment check in submitBlock (Sjors Provoost)
00d1b6ef4b doc: clarify UpdateUncommittedBlockStructures (Sjors Provoost)

Pull request description:

  When an IPC client requests a new block template via the Mining interface, we hold on to its `CBlock`. That way when they call `submitSolution()` we can modify it in place, rather than having to reconstruct the full block like the `submitblock` RPC does.

  Before this commit however we forgot to invalidate `m_checked_witness_commitment`, which we should since the client brings a new coinbase.

  This would cause us to accept an invalid chaintip.

  Fix this and add a test to confirm that we now reject such a block. As a sanity check, we add a second node to the test and confirm that will accept our mined block.

  As first noticed in #33374 the IPC code takes the coinbase as provided, unlike the `submitblock` RPC which calls `UpdateUncommittedBlockStructures()` and adds witness commitment to the coinbase if it was missing.

  Although that could have been an alternative fix, we instead document that IPC clients are expected to provide the full coinbase including witness commitment.

  Patch to produce the original issue:

  ```diff
  diff --git a/src/node/miner.cpp b/src/node/miner.cpp
  index b988e28a3f..28e9048a4d 100644
  --- a/src/node/miner.cpp
  +++ b/src/node/miner.cpp
  @@ -450,15 +450,10 @@ void AddMerkleRootAndCoinbase(CBlock& block, CTransactionRef coinbase, uint32_t
       }
       block.nVersion = version;
       block.nTime = timestamp;
       block.nNonce = nonce;
       block.hashMerkleRoot = BlockMerkleRoot(block);
  -
  -    // Reset cached checks
  -    block.m_checked_witness_commitment = false;
  -    block.m_checked_merkle_root = false;
  -    block.fChecked = false;
   }

   std::unique_ptr<CBlockTemplate> WaitAndCreateNewBlock(ChainstateManager& chainman,
                                                         KernelNotifications& kernel_notifications,
                                                         CTxMemPool* mempool,
  diff --git a/test/functional/interface_ipc.py b/test/functional/interface_ipc.py
  index cce56e3294..bf1b7048ab 100755
  --- a/test/functional/interface_ipc.py
  +++ b/test/functional/interface_ipc.py
  @@ -216,22 +216,22 @@ class IPCInterfaceTest(BitcoinTestFramework):
               assert_equal(res.result, True)

               # The remote template block will be mutated, capture the original:
               remote_block_before = await self.parse_and_deserialize_block(template, ctx)

  -            self.log.debug("Submitted coinbase must include witness")
  +            self.log.debug("Submitted coinbase with missing witness is accepted")
               assert_not_equal(coinbase.serialize_without_witness().hex(), coinbase.serialize().hex())
               res = await template.result.submitSolution(ctx, block.nVersion, block.nTime, block.nNonce, coinbase.serialize_without_witness())
  -            assert_equal(res.result, False)
  +            assert_equal(res.result, True)

               self.log.debug("Even a rejected submitBlock() mutates the template's block")
               # Can be used by clients to download and inspect the (rejected)
               # reconstructed block.
               remote_block_after = await self.parse_and_deserialize_block(template, ctx)
               assert_not_equal(remote_block_before.serialize().hex(), remote_block_after.serialize().hex())

  -            self.log.debug("Submit again, with the witness")
  +            self.log.debug("Submit again, with the witness - does not replace the invalid block")
               res = await template.result.submitSolution(ctx, block.nVersion, block.nTime, block.nNonce, coinbase.serialize())
               assert_equal(res.result, True)

               self.log.debug("Block should propagate")
               assert_equal(self.nodes[1].getchaintips()[0]["height"], current_block_height + 1)
  ```

ACKs for top commit:
  ryanofsky:
    Code review ACK 6eaa00fe20. Just documentation updates and test clarifications since last review, also splitting up a commit.
  TheCharlatan:
    Re-ACK 6eaa00fe20
  ismaelsadeeq:
    Code review and tested ACK   6eaa00fe20

Tree-SHA512: 3a6280345b0290fe8300ebc63c13ad4058d24ceb35b7d7a784b974d5f04f420860ac03a9bf2fc6a799ef3fc55552ce033e879fa369298f976b9a01d72bd55d9e

src/interfaces/mining.h

@@ -54,9 +54,22 @@ public:
     virtual std::vector<uint256> getCoinbaseMerklePath() = 0;
 
     /**
-     * Construct and broadcast the block.
+     * Construct and broadcast the block. Modifies the template in place,
+     * updating the fields listed below as well as the merkle root.
      *
-     * @returns if the block was processed, independent of block validity
+     * @param[in] version version block header field
+     * @param[in] timestamp time block header field (unix timestamp)
+     * @param[in] nonce nonce block header field
+     * @param[in] coinbase complete coinbase transaction (including witness)
+     *
+     * @note unlike the submitblock RPC, this method does NOT add the
+     *       coinbase witness automatically.
+     *
+     * @returns if the block was processed, does not necessarily indicate validity.
+     *
+     * @note Returns true if the block is already known, which can happen if
+     *       the solved block is constructed and broadcast by multiple nodes
+     *       (e.g. both the miner who constructed the template and the pool).
      */
     virtual bool submitSolution(uint32_t version, uint32_t timestamp, uint32_t nonce, CTransactionRef coinbase) = 0;