Auto merge of #3263 - str4d:ibd-upstream-changes, r=bitcartel

InitialBlockDownload upstream changes

Cherry-picked from the following upstream PRs:

- bitcoin/bitcoin#7208
- bitcoin/bitcoin#8007
- bitcoin/bitcoin#9053
  - Excluding second commit (requires bitcoin/bitcoin#8865)
- bitcoin/bitcoin#10388

src/main.cpp

@@ -32,6 +32,7 @@
 #include "wallet/asyncrpcoperation_shieldcoinbase.h"
 
 #include <algorithm>
+#include <atomic>
 #include <sstream>
 
 #include <boost/algorithm/string/replace.hpp>
@@ -75,6 +76,9 @@ bool fCoinbaseEnforcedProtectionEnabled = true;
 size_t nCoinCacheUsage = 5000 * 300;
 uint64_t nPruneTarget = 0;
 bool fAlerts = DEFAULT_ALERTS;
+/* If the tip is older than this (in seconds), the node is considered to be in initial block download.
+ */
+int64_t nMaxTipAge = DEFAULT_MAX_TIP_AGE;
 
 unsigned int expiryDelta = DEFAULT_TX_EXPIRY_DELTA;
 
@@ -1724,19 +1728,27 @@ CAmount GetBlockSubsidy(int nHeight, const Consensus::Params& consensusParams)
 bool IsInitialBlockDownload()
 {
     const CChainParams& chainParams = Params();
+
+    // Once this function has returned false, it must remain false.
+    static std::atomic<bool> latchToFalse{false};
+    // Optimization: pre-test latch before taking the lock.
+    if (latchToFalse.load(std::memory_order_relaxed))
+        return false;
+
     LOCK(cs_main);
+    if (latchToFalse.load(std::memory_order_relaxed))
+        return false;
     if (fImporting || fReindex)
         return true;
-    if (fCheckpointsEnabled && chainActive.Height() < Checkpoints::GetTotalBlocksEstimate(chainParams.Checkpoints()))
+    if (chainActive.Tip() == NULL)
         return true;
-    static bool lockIBDState = false;
-    if (lockIBDState)
-        return false;
-    bool state = (chainActive.Height() < pindexBestHeader->nHeight - 24 * 6 ||
-            pindexBestHeader->GetBlockTime() < GetTime() - chainParams.MaxTipAge());
-    if (!state)
-        lockIBDState = true;
-    return state;
+    if (chainActive.Tip()->nChainWork < UintToArith256(chainParams.GetConsensus().nMinimumChainWork))
+        return true;
+    if (chainActive.Tip()->GetBlockTime() < (GetTime() - nMaxTipAge))
+        return true;
+    LogPrintf("Leaving InitialBlockDownload (latching to false)\n");
+    latchToFalse.store(true, std::memory_order_relaxed);
+    return false;
 }
 
 static bool fLargeWorkForkFound = false;
@@ -1748,7 +1760,7 @@ void CheckForkWarningConditions()
 {
     AssertLockHeld(cs_main);
     // Before we get past initial download, we cannot reliably alert about forks
-    // (we assume we don't get stuck on a fork before the last checkpoint)
+    // (we assume we don't get stuck on a fork before finishing our initial sync)
     if (IsInitialBlockDownload())
         return;