Add preciousblock RPC

Includes a bugfix by Luke-Jr.

src/chain.h

@@ -200,7 +200,7 @@ public:
     unsigned int nNonce;
 
     //! (memory only) Sequential id assigned to distinguish order in which blocks are received.
-    uint32_t nSequenceId;
+    int32_t nSequenceId;
 
     void SetNull()
     {

src/main.cpp

@@ -167,7 +167,11 @@ namespace {
      */
     CCriticalSection cs_nBlockSequenceId;
     /** Blocks loaded from disk are assigned id 0, so start the counter at 1. */
-    uint32_t nBlockSequenceId = 1;
+    int32_t nBlockSequenceId = 1;
+    /** Decreasing counter (used by subsequent preciousblock calls). */
+    int32_t nBlockReverseSequenceId = -1;
+    /** chainwork for the last block that preciousblock has been applied to. */
+    arith_uint256 nLastPreciousChainwork = 0;
 
     /**
      * Sources of received blocks, saved to be able to send them reject
@@ -3107,6 +3111,36 @@ bool ActivateBestChain(CValidationState &state, const CChainParams& chainparams,
     return true;
 }
 
+
+bool PreciousBlock(CValidationState& state, const CChainParams& params, CBlockIndex *pindex)
+{
+    {
+        LOCK(cs_main);
+        if (pindex->nChainWork < chainActive.Tip()->nChainWork) {
+            // Nothing to do, this block is not at the tip.
+            return true;
+        }
+        if (chainActive.Tip()->nChainWork > nLastPreciousChainwork) {
+            // The chain has been extended since the last call, reset the counter.
+            nBlockReverseSequenceId = -1;
+        }
+        nLastPreciousChainwork = chainActive.Tip()->nChainWork;
+        setBlockIndexCandidates.erase(pindex);
+        pindex->nSequenceId = nBlockReverseSequenceId;
+        if (nBlockReverseSequenceId > std::numeric_limits<int32_t>::min()) {
+            // We can't keep reducing the counter if somebody really wants to
+            // call preciousblock 2**31-1 times on the same set of tips...
+            nBlockReverseSequenceId--;
+        }
+        if (pindex->IsValid(BLOCK_VALID_TRANSACTIONS) && pindex->nChainTx) {
+            setBlockIndexCandidates.insert(pindex);
+            PruneBlockIndexCandidates();
+        }
+    }
+
+    return ActivateBestChain(state, params);
+}
+
 bool InvalidateBlock(CValidationState& state, const CChainParams& chainparams, CBlockIndex *pindex)
 {
     AssertLockHeld(cs_main);
@@ -4501,7 +4535,7 @@ void static CheckBlockIndex(const Consensus::Params& consensusParams)
             assert(pindex->GetBlockHash() == consensusParams.hashGenesisBlock); // Genesis block's hash must match.
             assert(pindex == chainActive.Genesis()); // The current active chain's genesis block must be this block.
         }
-        if (pindex->nChainTx == 0) assert(pindex->nSequenceId == 0);  // nSequenceId can't be set for blocks that aren't linked
+        if (pindex->nChainTx == 0) assert(pindex->nSequenceId <= 0);  // nSequenceId can't be set positive for blocks that aren't linked (negative is used for preciousblock)
         // VALID_TRANSACTIONS is equivalent to nTx > 0 for all nodes (whether or not pruning has occurred).
         // HAVE_DATA is only equivalent to nTx > 0 (or VALID_TRANSACTIONS) if no pruning has occurred.
         if (!fHavePruned) {

src/main.h

@@ -499,6 +499,9 @@ public:
 /** Find the last common block between the parameter chain and a locator. */
 CBlockIndex* FindForkInGlobalIndex(const CChain& chain, const CBlockLocator& locator);
 
+/** Mark a block as precious and reorganize. */
+bool PreciousBlock(CValidationState& state, const CChainParams& params, CBlockIndex *pindex);
+
 /** Mark a block as invalid. */
 bool InvalidateBlock(CValidationState& state, const CChainParams& chainparams, CBlockIndex *pindex);
 

src/rpc/blockchain.cpp

@@ -1105,6 +1105,44 @@ UniValue getmempoolinfo(const UniValue& params, bool fHelp)
     return mempoolInfoToJSON();
 }
 
+UniValue preciousblock(const UniValue& params, bool fHelp)
+{
+    if (fHelp || params.size() != 1)
+        throw runtime_error(
+            "preciousblock \"hash\"\n"
+            "\nTreats a block as if it were received before others with the same work.\n"
+            "\nA later preciousblock call can override the effect of an earlier one.\n"
+            "\nThe effects of preciousblock are not retained across restarts.\n"
+            "\nArguments:\n"
+            "1. hash   (string, required) the hash of the block to mark as precious\n"
+            "\nResult:\n"
+            "\nExamples:\n"
+            + HelpExampleCli("preciousblock", "\"blockhash\"")
+            + HelpExampleRpc("preciousblock", "\"blockhash\"")
+        );
+
+    std::string strHash = params[0].get_str();
+    uint256 hash(uint256S(strHash));
+    CBlockIndex* pblockindex;
+
+    {
+        LOCK(cs_main);
+        if (mapBlockIndex.count(hash) == 0)
+            throw JSONRPCError(RPC_INVALID_ADDRESS_OR_KEY, "Block not found");
+
+        pblockindex = mapBlockIndex[hash];
+    }
+
+    CValidationState state;
+    PreciousBlock(state, Params(), pblockindex);
+
+    if (!state.IsValid()) {
+        throw JSONRPCError(RPC_DATABASE_ERROR, state.GetRejectReason());
+    }
+
+    return NullUniValue;
+}
+
 UniValue invalidateblock(const UniValue& params, bool fHelp)
 {
     if (fHelp || params.size() != 1)
@@ -1200,6 +1238,8 @@ static const CRPCCommand commands[] =
     { "blockchain",         "gettxoutsetinfo",        &gettxoutsetinfo,        true  },
     { "blockchain",         "verifychain",            &verifychain,            true  },
 
+    { "blockchain",         "preciousblock",          &preciousblock,          true  },
+
     /* Not shown in help */
     { "hidden",             "invalidateblock",        &invalidateblock,        true  },
     { "hidden",             "reconsiderblock",        &reconsiderblock,        true  },