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.

4 files changed, 16 insertions, 2 deletions

diff --git a/doc/release-process.md b/doc/release-process.md
index 63f75fb399..85cf160f47 100644
--- a/doc/release-process.md
+++ b/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:
 
diff --git a/src/chainparams.cpp b/src/chainparams.cpp
index 5850016ae2..a57ab632e4 100644
--- a/src/chainparams.cpp
+++ b/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;
diff --git a/src/consensus/params.h b/src/consensus/params.h
index 0e73cace83..20efc68ade 100644
--- a/src/consensus/params.h
+++ b/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
 
diff --git a/src/main.cpp b/src/main.cpp
index 5e17ec6251..6b33539a2e 100644
--- a/src/main.cpp
+++ b/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;