IBD check uses minimumchain work instead of checkpoints.

This introduces a 'minimum chain work' chainparam which is intended
 to be the known amount of work in the chain for the network at the
 time of software release.  If you don't have this much work, you're
 not yet caught up.

This is used instead of the count of blocks test from checkpoints.

This criteria is trivial to keep updated as there is no element of
subjectivity, trust, or position dependence to it. It is also a more
reliable metric of sync status than a block count.

doc/release-process.md

@@ -24,6 +24,8 @@ Check that there are no surprising performance regressions:
 
 Ensure that new performance metrics appear on that site.
 
+Update `src/chainparams.cpp` nMinimumChainWork with information from the getblockchaininfo rpc.
+
 ### Protocol Safety Checks:
 
 If this release changes the behavior of the protocol or fixes a serious

src/chainparams.cpp

@@ -106,6 +106,9 @@ public:
         consensus.vUpgrades[Consensus::UPGRADE_SAPLING].nActivationHeight =
             Consensus::NetworkUpgrade::NO_ACTIVATION_HEIGHT;
 
+        // The best chain should have at least this much work.
+        consensus.nMinimumChainWork = uint256S("0x00000000000000000000000000000000000000000000000000281b32ff3198a1");
+
         /**
          * The message start string should be awesome! ⓩ❤
          */
@@ -268,6 +271,9 @@ public:
         consensus.vUpgrades[Consensus::UPGRADE_SAPLING].nActivationHeight =
             Consensus::NetworkUpgrade::NO_ACTIVATION_HEIGHT;
 
+        // The best chain should have at least this much work.
+        consensus.nMinimumChainWork = uint256S("0x00000000000000000000000000000000000000000000000000000001d0c4d9cd");
+
         pchMessageStart[0] = 0xfa;
         pchMessageStart[1] = 0x1a;
         pchMessageStart[2] = 0xf9;
@@ -317,6 +323,7 @@ public:
         fMineBlocksOnDemand = false;
         fTestnetToBeDeprecatedFieldRPC = true;
 
+
         checkpointData = (CCheckpointData) {
             boost::assign::map_list_of
             (0, consensus.hashGenesisBlock)
@@ -380,6 +387,9 @@ public:
         consensus.vUpgrades[Consensus::UPGRADE_SAPLING].nActivationHeight =
             Consensus::NetworkUpgrade::NO_ACTIVATION_HEIGHT;
 
+        // The best chain should have at least this much work.
+        consensus.nMinimumChainWork = uint256S("0x00");
+
         pchMessageStart[0] = 0xaa;
         pchMessageStart[1] = 0xe8;
         pchMessageStart[2] = 0x3f;

src/consensus/params.h

@@ -98,6 +98,7 @@ struct Params {
     int64_t AveragingWindowTimespan() const { return nPowAveragingWindow * nPowTargetSpacing; }
     int64_t MinActualTimespan() const { return (AveragingWindowTimespan() * (100 - nPowMaxAdjustUp  )) / 100; }
     int64_t MaxActualTimespan() const { return (AveragingWindowTimespan() * (100 + nPowMaxAdjustDown)) / 100; }
+    uint256 nMinimumChainWork;
 };
 } // namespace Consensus
 

src/main.cpp

@@ -1740,7 +1740,9 @@ bool IsInitialBlockDownload()
         return false;
     if (fImporting || fReindex)
         return true;
-    if (fCheckpointsEnabled && chainActive.Height() < Checkpoints::GetTotalBlocksEstimate(chainParams.Checkpoints()))
+    if (chainActive.Tip() == NULL)
+        return true;
+    if (chainActive.Tip()->nChainWork < UintToArith256(chainParams.GetConsensus().nMinimumChainWork))
         return true;
     bool state = (chainActive.Height() < pindexBestHeader->nHeight - 24 * 6 ||
             pindexBestHeader->GetBlockTime() < GetTime() - nMaxTipAge);
@@ -1758,7 +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;