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

@@ -12,6 +12,7 @@ Before every minor and major release:
 * Update [bips.md](bips.md) to account for changes since the last release.
 * Update version in sources (see below)
 * Write release notes (see below)
+* Update `src/chainparams.cpp` nMinimumChainWork with information from the getblockchaininfo rpc.
 
 Before every major release:
 

src/chainparams.cpp

@@ -96,6 +96,9 @@ public:
         consensus.vDeployments[Consensus::DEPLOYMENT_SEGWIT].nStartTime = 1479168000; // November 15th, 2016.
         consensus.vDeployments[Consensus::DEPLOYMENT_SEGWIT].nTimeout = 1510704000; // November 15th, 2017.
 
+        // The best chain should have at least this much work.
+        consensus.nMinimumChainWork = uint256S("0x0000000000000000000000000000000000000000002cb971dd56d1c583c20f90");
+
         /**
          * The message start string is designed to be unlikely to occur in normal data.
          * The characters are rarely used upper ASCII, not valid as UTF-8, and produce
@@ -191,6 +194,9 @@ public:
         consensus.vDeployments[Consensus::DEPLOYMENT_SEGWIT].nStartTime = 1462060800; // May 1st 2016
         consensus.vDeployments[Consensus::DEPLOYMENT_SEGWIT].nTimeout = 1493596800; // May 1st 2017
 
+        // The best chain should have at least this much work.
+        consensus.nMinimumChainWork = uint256S("0x0000000000000000000000000000000000000000000000198b4def2baa9338d6");
+
         pchMessageStart[0] = 0x0b;
         pchMessageStart[1] = 0x11;
         pchMessageStart[2] = 0x09;
@@ -224,6 +230,7 @@ public:
         fRequireStandard = false;
         fMineBlocksOnDemand = false;
 
+
         checkpointData = (CCheckpointData) {
             boost::assign::map_list_of
             ( 546, uint256S("000000002a936ca763904c3c35fce2f3556c559c0214345d31b1bcebf76acb70")),
@@ -265,6 +272,9 @@ public:
         consensus.vDeployments[Consensus::DEPLOYMENT_SEGWIT].nStartTime = 0;
         consensus.vDeployments[Consensus::DEPLOYMENT_SEGWIT].nTimeout = 999999999999ULL;
 
+        // The best chain should have at least this much work.
+        consensus.nMinimumChainWork = uint256S("0x00");
+
         pchMessageStart[0] = 0xfa;
         pchMessageStart[1] = 0xbf;
         pchMessageStart[2] = 0xb5;

src/consensus/params.h

@@ -61,6 +61,7 @@ struct Params {
     int64_t nPowTargetSpacing;
     int64_t nPowTargetTimespan;
     int64_t DifficultyAdjustmentInterval() const { return nPowTargetTimespan / nPowTargetSpacing; }
+    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 ||
             std::max(chainActive.Tip()->GetBlockTime(), pindexBestHeader->GetBlockTime()) < GetTime() - nMaxTipAge);
@@ -1774,7 +1776,7 @@ 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;