rpc: add next to getmininginfo

Obtain difficulty and target for the next block without having to call
getblocktemplate.

Author: Sjors

src/rpc/mining.cpp

@@ -428,6 +428,13 @@ static RPCHelpMan getmininginfo()
                         {RPCResult::Type::NUM, "pooledtx", "The size of the mempool"},
                         {RPCResult::Type::STR, "chain", "current network name (" LIST_CHAIN_NAMES ")"},
                         {RPCResult::Type::STR_HEX, "signet_challenge", /*optional=*/true, "The block challenge (aka. block script), in hexadecimal (only present if the current network is a signet)"},
+                        {RPCResult::Type::OBJ, "next", "The next block",
+                        {
+                            {RPCResult::Type::NUM, "height", "The next height"},
+                            {RPCResult::Type::STR_HEX, "bits", "The next target nBits"},
+                            {RPCResult::Type::NUM, "difficulty", "The next difficulty"},
+                            {RPCResult::Type::STR_HEX, "target", "The next target"}
+                        }},
                         (IsDeprecatedRPCEnabled("warnings") ?
                             RPCResult{RPCResult::Type::STR, "warnings", "any network and blockchain warnings (DEPRECATED)"} :
                             RPCResult{RPCResult::Type::ARR, "warnings", "any network and blockchain warnings (run with `-deprecatedrpc=warnings` to return the latest warning as a single string)",
@@ -460,9 +467,20 @@ static RPCHelpMan getmininginfo()
     obj.pushKV("networkhashps",    getnetworkhashps().HandleRequest(request));
     obj.pushKV("pooledtx",         (uint64_t)mempool.size());
     obj.pushKV("chain", chainman.GetParams().GetChainTypeString());
+
+    UniValue next(UniValue::VOBJ);
+    CBlockIndex next_index;
+    NextEmptyBlockIndex(tip, chainman.GetConsensus(), next_index);
+
+    next.pushKV("height", next_index.nHeight);
+    next.pushKV("bits", strprintf("%08x", next_index.nBits));
+    next.pushKV("difficulty", GetDifficulty(next_index));
+    next.pushKV("target", GetTarget(next_index, chainman.GetConsensus().powLimit).GetHex());
+    obj.pushKV("next", next);
+
     if (chainman.GetParams().GetChainType() == ChainType::SIGNET) {
         const std::vector<uint8_t>& signet_challenge =
-            chainman.GetParams().GetConsensus().signet_challenge;
+            chainman.GetConsensus().signet_challenge;
         obj.pushKV("signet_challenge", HexStr(signet_challenge));
     }
     obj.pushKV("warnings", node::GetWarningsForRpc(*CHECK_NONFATAL(node.warnings), IsDeprecatedRPCEnabled("warnings")));

src/rpc/server_util.cpp

@@ -4,10 +4,13 @@
 
 #include <rpc/server_util.h>
 
+#include <chain.h>
 #include <common/args.h>
 #include <net_processing.h>
 #include <node/context.h>
+#include <node/miner.h>
 #include <policy/fees.h>
+#include <pow.h>
 #include <rpc/protocol.h>
 #include <rpc/request.h>
 #include <txmempool.h>
@@ -17,6 +20,7 @@
 #include <any>
 
 using node::NodeContext;
+using node::UpdateTime;
 
 NodeContext& EnsureAnyNodeContext(const std::any& context)
 {
@@ -129,3 +133,18 @@ AddrMan& EnsureAnyAddrman(const std::any& context)
 {
     return EnsureAddrman(EnsureAnyNodeContext(context));
 }
+
+void NextEmptyBlockIndex(CBlockIndex& tip, const Consensus::Params& consensusParams, CBlockIndex& next_index)
+{
+    CBlockHeader next_header{};
+    next_header.hashPrevBlock  = tip.GetBlockHash();
+    UpdateTime(&next_header, consensusParams, &tip);
+    next_header.nBits = GetNextWorkRequired(&tip, &next_header, consensusParams);
+    next_header.nNonce = 0;
+
+    next_index.pprev = &tip;
+    next_index.nTime = next_header.nTime;
+    next_index.nBits = next_header.nBits;
+    next_index.nNonce = next_header.nNonce;
+    next_index.nHeight = tip.nHeight + 1;
+}

src/rpc/server_util.h

@@ -7,8 +7,11 @@
 
 #include <any>
 
+#include <consensus/params.h>
+
 class AddrMan;
 class ArgsManager;
+class CBlockIndex;
 class CBlockPolicyEstimator;
 class CConnman;
 class CTxMemPool;
@@ -39,4 +42,7 @@ PeerManager& EnsurePeerman(const node::NodeContext& node);
 AddrMan& EnsureAddrman(const node::NodeContext& node);
 AddrMan& EnsureAnyAddrman(const std::any& context);
 
+/** Return an empty block index on top of the tip, with height, time and nBits set */
+void NextEmptyBlockIndex(CBlockIndex& tip, const Consensus::Params& consensusParams, CBlockIndex& next_index);
+
 #endif // BITCOIN_RPC_SERVER_UTIL_H

test/functional/mining_basic.py

@@ -213,6 +213,12 @@ def assert_submitblock(block, result_str_1, result_str_2=None):
         assert_equal(mining_info['bits'], nbits_str(REGTEST_N_BITS))
         assert_equal(mining_info['target'], target_str(REGTEST_TARGET))
         assert_equal(mining_info['difficulty'], Decimal('4.656542373906925E-10'))
+        assert_equal(mining_info['next'], {
+            'height': 201,
+            'target': target_str(REGTEST_TARGET),
+            'bits': nbits_str(REGTEST_N_BITS),
+            'difficulty': Decimal('4.656542373906925E-10')
+        })
         assert_equal(mining_info['networkhashps'], Decimal('0.003333333333333334'))
         assert_equal(mining_info['pooledtx'], 0)