mining: ensure witness commitment check in submitBlock

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.

Note that 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.

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;
 

src/node/miner.cpp

@@ -452,6 +452,11 @@ void AddMerkleRootAndCoinbase(CBlock& block, CTransactionRef coinbase, uint32_t
     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,

test/functional/interface_ipc.py

@@ -8,7 +8,11 @@ from io import BytesIO
 from pathlib import Path
 import shutil
 from test_framework.messages import (CBlock, CTransaction, ser_uint256, COIN)
-from test_framework.test_framework import (BitcoinTestFramework, assert_equal)
+from test_framework.test_framework import BitcoinTestFramework
+from test_framework.util import (
+    assert_equal,
+    assert_not_equal
+)
 from test_framework.wallet import MiniWallet
 
 # Test may be skipped and not have capnp installed
@@ -49,10 +53,10 @@ class IPCInterfaceTest(BitcoinTestFramework):
         }
 
     def set_test_params(self):
-        self.num_nodes = 1
+        self.num_nodes = 2
 
     def setup_nodes(self):
-        self.extra_init = [{"ipcbind": True}]
+        self.extra_init = [{"ipcbind": True}, {}]
         super().setup_nodes()
         # Use this function to also load the capnp modules (we cannot use set_test_params for this,
         # as it is being called before knowing whether capnp is available).
@@ -206,11 +210,26 @@ class IPCInterfaceTest(BitcoinTestFramework):
             self.log.debug("Submit a valid block")
             block.nVersion = original_version
             block.solve()
+
+            self.log.debug("First call checkBlock()")
             res = await mining.result.checkBlock(block.serialize(), check_opts)
             assert_equal(res.result, True)
+
+            self.log.debug("Submitted coinbase must include witness")
+            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)
+
+            self.log.debug("Submit again, with the witness")
             res = await template.result.submitSolution(ctx, block.nVersion, block.nTime, block.nNonce, coinbase.serialize())
             assert_equal(res.result, True)
-            assert_equal(self.nodes[0].getchaintips()[0]["height"], current_block_height + 1)
+
+            self.log.debug("Block should propagate")
+            assert_equal(self.nodes[1].getchaintips()[0]["height"], current_block_height + 1)
+            # Stalls if a regression causes submitBlock() to accept an invalid block:
+            self.sync_all()
+            assert_equal(self.nodes[0].getchaintips()[0], self.nodes[1].getchaintips()[0])
+
             miniwallet.rescan_utxos()
             assert_equal(miniwallet.get_balance(), balance + 1)
             self.log.debug("Check block should fail now, since it is a duplicate")