validation: clarify final |= BLOCK_FAILED_VALID in InvalidateBlock

This has no functional affect, as the any CBlockIndex*s which
to_mark_failed is set to will already have been marked failed.

Also prevents a situation where block already marked as
BLOCK_FAILED_CHILD is again unconditionally marked as
BLOCK_FAILED_VALID in the final |= BLOCK_FAILED_VALID.

src/test/blockchain_tests.cpp

@@ -147,6 +147,11 @@ BOOST_FIXTURE_TEST_CASE(invalidate_block, TestChain100Setup)
         WITH_LOCK(::cs_main, assert(pindex->nStatus & BLOCK_FAILED_CHILD));
         pindex = pindex->pprev;
     }
+
+    // don't mark already invalidated block (orig_tip is BLOCK_FAILED_CHILD) with BLOCK_FAILED_VALID again
+    m_node.chainman->ActiveChainstate().InvalidateBlock(state, orig_tip);
+    WITH_LOCK(::cs_main, assert(orig_tip->nStatus & BLOCK_FAILED_CHILD));
+    WITH_LOCK(::cs_main, assert((orig_tip->nStatus & BLOCK_FAILED_VALID) == 0));
 }
 
 BOOST_AUTO_TEST_SUITE_END()

src/validation.cpp

@@ -3779,11 +3779,13 @@ bool Chainstate::InvalidateBlock(BlockValidationState& state, CBlockIndex* pinde
             return false;
         }
 
-        // Mark pindex (or the last disconnected block) as invalid, even when it never was in the main chain
-        to_mark_failed->nStatus |= BLOCK_FAILED_VALID;
-        m_blockman.m_dirty_blockindex.insert(to_mark_failed);
-        setBlockIndexCandidates.erase(to_mark_failed);
-        m_chainman.m_failed_blocks.insert(to_mark_failed);
+        // Mark pindex as invalid if it never was in the main chain
+        if (!pindex_was_in_chain && !(pindex->nStatus & BLOCK_FAILED_MASK)) {
+            pindex->nStatus |= BLOCK_FAILED_VALID;
+            m_blockman.m_dirty_blockindex.insert(pindex);
+            setBlockIndexCandidates.erase(pindex);
+            m_chainman.m_failed_blocks.insert(pindex);
+        }
 
         // If any new blocks somehow arrived while we were disconnecting
         // (above), then the pre-calculation of what should go into