validation: add to m_failed_blocks if found invalid during AcceptBlock

If we have accepted the block header but failed to accept the block
(because ContextualCheckBlock failed) we mark the header as BLOCK_FAILED_VALID.

Now, we also add this header to m_failed_blocks, so that we now don't
accept any headers building on a chain that includes the failed block
(and therefore will never be valid).

Before, we'd only do that if we accepted the block but failed while
connecting it.

src/validation.cpp

@@ -4481,6 +4481,7 @@ bool ChainstateManager::AcceptBlock(const std::shared_ptr<const CBlock>& pblock,
         if (state.IsInvalid() && state.GetResult() != BlockValidationResult::BLOCK_MUTATED) {
             pindex->nStatus |= BLOCK_FAILED_VALID;
             m_blockman.m_dirty_blockindex.insert(pindex);
+            m_failed_blocks.insert(pindex);
         }
         LogError("%s: %s\n", __func__, state.ToString());
         return false;

src/validation.h

@@ -1046,10 +1046,11 @@ public:
 
     /**
      * In order to efficiently track invalidity of headers, we keep the set of
-     * blocks which we tried to connect and found to be invalid here (ie which
-     * were set to BLOCK_FAILED_VALID since the last restart). We can then
+     * blocks which were found to be invalid while connecting or accepting do disk
+     * (ie which were set to BLOCK_FAILED_VALID since the last restart). We can then
      * walk this set and check if a new header is a descendant of something in
-     * this set, preventing us from having to walk m_block_index when we try
+     * this set, preventing us from accepting headers building on invalid blocks into
+     * our blockindex and from having to walk m_block_index when we try
      * to connect a bad block and fail.
      *
      * While this is more complicated than marking everything which descends

test/functional/p2p_invalid_chain.py

@@ -75,7 +75,7 @@ class InvalidChainTest (BitcoinTestFramework):
         block_time += 1
         block3 = create_block(block2a.sha256, create_coinbase(start_height + 3), block_time)
         block3.solve()
-        node.submitheader(block3.serialize().hex())  # No error, we accept more headers building on a block we have marked as invalid.
+        assert_raises_rpc_error(-25, 'bad-prevblk', lambda: node.submitheader(block3.serialize().hex()))
 
         self.log.info("Test chain with an invalid block (found invalid before acceptance)")
         # Create invalid block (too large)