rpc: add next to getdifficulty and gettarget

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

src/rpc/blockchain.cpp

@@ -432,7 +432,9 @@ static RPCHelpMan getdifficulty()
 {
     return RPCHelpMan{"getdifficulty",
                 "\nReturns the proof-of-work difficulty as a multiple of the minimum difficulty.\n",
-                {},
+                {
+                    {"next", RPCArg::Type::BOOL, RPCArg::Default{false}, "difficulty for the next block, if found now"},
+                },
                 RPCResult{
                     RPCResult::Type::NUM, "", "the proof-of-work difficulty as a multiple of the minimum difficulty."},
                 RPCExamples{
@@ -442,8 +444,21 @@ static RPCHelpMan getdifficulty()
         [&](const RPCHelpMan& self, const JSONRPCRequest& request) -> UniValue
 {
     ChainstateManager& chainman = EnsureAnyChainman(request.context);
-    LOCK(cs_main);
-    return GetDifficulty(*CHECK_NONFATAL(chainman.ActiveChain().Tip()));
+    CBlockIndex& tip{*CHECK_NONFATAL(WITH_LOCK(chainman.GetMutex(), return chainman.ActiveChain().Tip()))};
+
+    bool next{false};
+    if (!request.params[0].isNull()) {
+        next = request.params[0].get_bool();
+    }
+
+    if (next) {
+        CBlockIndex next_index;
+        NextEmptyBlockIndex(tip, chainman.GetConsensus(), next_index);
+        return GetDifficulty(next_index);
+    } else {
+        return GetDifficulty(tip);
+    }
+
 },
     };
 }
@@ -452,7 +467,9 @@ static RPCHelpMan gettarget()
 {
     return RPCHelpMan{"gettarget",
                 "\nReturns the proof-of-work target.\n",
-                {},
+                {
+                    {"next", RPCArg::Type::BOOL, RPCArg::Default{false}, "target for the next block, if found now"},
+                },
                 RPCResult{
                     RPCResult::Type::STR_HEX, "", "the proof-of-work target."},
                 RPCExamples{
@@ -463,7 +480,19 @@ static RPCHelpMan gettarget()
 {
     ChainstateManager& chainman = EnsureAnyChainman(request.context);
     CBlockIndex& tip{*CHECK_NONFATAL(WITH_LOCK(chainman.GetMutex(), return chainman.ActiveChain().Tip()))};
-    return GetTarget(tip, chainman.GetParams().GetConsensus().powLimit).GetHex();
+
+    bool next{false};
+    if (!request.params[0].isNull()) {
+        next = request.params[0].get_bool();
+    }
+
+    if (next) {
+        CBlockIndex next_index;
+        NextEmptyBlockIndex(tip, chainman.GetConsensus(), next_index);
+        return GetTarget(next_index, chainman.GetConsensus().powLimit).GetHex();
+    } else {
+        return GetTarget(tip, chainman.GetConsensus().powLimit).GetHex();
+    }
 },
     };
 }

src/rpc/client.cpp

@@ -114,6 +114,8 @@ static const CRPCConvertParam vRPCConvertParams[] =
     { "getblock", 1, "verbose" },
     { "getblockheader", 1, "verbose" },
     { "getchaintxstats", 0, "nblocks" },
+    { "getdifficulty", 0, "next" },
+    { "gettarget", 0, "next" },
     { "gettransaction", 1, "include_watchonly" },
     { "gettransaction", 2, "verbose" },
     { "getrawtransaction", 1, "verbosity" },

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/rpc_blockchain.py

@@ -444,11 +444,14 @@ class BlockchainTest(BitcoinTestFramework):
         # 1 hash in 2 should be valid, so difficulty should be 1/2**31
         # binary => decimal => binary math is why we do this check
         assert abs(difficulty * 2**31 - 1) < 0.0001
+        self.log.info("Next difficulty should be the same as the current (no difficulty adjustment)")
+        assert_equal(self.nodes[0].getdifficulty(next=True), difficulty)
 
     def _test_gettarget(self):
         self.log.info("Test gettarget")
         target = self.nodes[0].gettarget()
         assert_equal(target, target_str(REGTEST_TARGET))
+        assert_equal(self.nodes[0].gettarget(next=True), target_str(REGTEST_TARGET))
 
     def _test_getnetworkhashps(self):
         self.log.info("Test getnetworkhashps")