Implement simple initialblockdownload estimation

src/rpc/blockchain.cpp

@@ -1079,6 +1079,20 @@ UniValue getblockchaininfo(const JSONRPCRequest& request)
     obj.push_back(Pair("signblock_asm", ScriptToAsmStr(tip->proof.challenge)));
     obj.push_back(Pair("signblock_hex", HexStr(tip->proof.challenge.begin(), tip->proof.challenge.end())));
 
+    // Since we have signed fixed-interval blocks, we can estimate initialblockdownload based
+    // on the time since the last block.
+    bool initialblockdownload = true;
+    if (tip->nHeight > 2) {
+        int64_t nNow = time(NULL);
+        int64_t sinceLast = nNow - tip->GetBlockTime();
+
+        // We assume we're done if the last blocks is less than 5 intervals ago.
+        if (sinceLast < 5 * Params().GetConsensus().nPowTargetSpacing) {
+            initialblockdownload = false;
+        }
+    }
+    obj.push_back(Pair("initialblockdownload", initialblockdownload));
+
     const Consensus::Params& consensusParams = Params().GetConsensus();
     UniValue bip9_softforks(UniValue::VOBJ);
     BIP9SoftForkDescPushBack(bip9_softforks, "csv", consensusParams, Consensus::DEPLOYMENT_CSV);