[validation] Crash if disconnecting a block fails

If we're unable to disconnect a block during normal operation, then that is a
failure of our local system (such as disk failure) or the chain that we are on
(eg CVE-2018-17144), but cannot be due to failure of the (more work) chain that
we're trying to validate.

We should abort rather than stay on a less work chain.

Author: sdaftuar

src/validation.cpp

@@ -2295,7 +2295,7 @@ bool CChainState::DisconnectTip(CValidationState& state, const CChainParams& cha
     std::shared_ptr<CBlock> pblock = std::make_shared<CBlock>();
     CBlock& block = *pblock;
     if (!ReadBlockFromDisk(block, pindexDelete, chainparams.GetConsensus()))
-        return AbortNode(state, "Failed to read block");
+        return error("DisconnectTip(): Failed to read block");
     // Apply the block atomically to the chain state.
     int64_t nStart = GetTimeMicros();
     {
@@ -2551,6 +2551,11 @@ bool CChainState::ActivateBestChainStep(CValidationState& state, const CChainPar
             // This is likely a fatal error, but keep the mempool consistent,
             // just in case. Only remove from the mempool in this case.
             UpdateMempoolForReorg(disconnectpool, false);
+
+            // If we're unable to disconnect a block during normal operation,
+            // then that is a failure of our local system -- we should abort
+            // rather than stay on a less work chain.
+            AbortNode(state, "Failed to disconnect block; see debug.log for details");
             return false;
         }
         fBlocksDisconnected = true;