Merge #15655: Resolve the checkpoints <-> validation circular dependency

418d3230f8 Resolve the checkpoints <-> validation CD. (251)

Pull request description:

  This pull request attempts to resolve the `checkpoints -> validation -> checkpoints` circular dependency.

  The circular dependency is resolved by moving the `CheckPoints::GetLastCheckpoint(const CCheckpointData& data)` function to `validation.cpp` where it used exclusively by the private function `ContextualCheckBlockHeader(const CBlockHeader& block, CValidationState& state, const CChainParams& params, const CBlockIndex* pindexPrev, int64_t nAdjustedTime)`.

ACKs for commit 418d32:
  promag:
    utACK 418d323, only `GetLastCheckpoint` usage is in `validation.cpp` and so makes sense to move it there.
  practicalswift:
    utACK 418d3230f8
  MarcoFalke:
    utACK 418d3230f8
  sipa:
    utACK 418d3230f8

Tree-SHA512: 03c3556bc192e65f5e3fa76fd545d4ee7d63d3fb06b132f7a1fa6131aa21ddd2e5b2d19e2222dfe524f422daaca30efde219bed188db8c74ff4b088876b5bc16

src/validation.cpp

@@ -8,7 +8,6 @@
 #include <arith_uint256.h>
 #include <chain.h>
 #include <chainparams.h>
-#include <checkpoints.h>
 #include <checkqueue.h>
 #include <consensus/consensus.h>
 #include <consensus/merkle.h>
@@ -37,6 +36,7 @@
 #include <txdb.h>
 #include <txmempool.h>
 #include <ui_interface.h>
+#include <uint256.h>
 #include <undo.h>
 #include <util/moneystr.h>
 #include <util/rbf.h>
@@ -3188,6 +3188,22 @@ std::vector<unsigned char> GenerateCoinbaseCommitment(CBlock& block, const CBloc
     return commitment;
 }
 
+//! Returns last CBlockIndex* that is a checkpoint
+static CBlockIndex* GetLastCheckpoint(const CCheckpointData& data)
+{
+    const MapCheckpoints& checkpoints = data.mapCheckpoints;
+
+    for (const MapCheckpoints::value_type& i : reverse_iterate(checkpoints))
+    {
+        const uint256& hash = i.second;
+        CBlockIndex* pindex = LookupBlockIndex(hash);
+        if (pindex) {
+            return pindex;
+        }
+    }
+    return nullptr;
+}
+
 /** Context-dependent validity checks.
  *  By "context", we mean only the previous block headers, but not the UTXO
  *  set; UTXO-related validity checks are done in ConnectBlock().
@@ -3212,7 +3228,7 @@ static bool ContextualCheckBlockHeader(const CBlockHeader& block, CValidationSta
         // Don't accept any forks from the main chain prior to last checkpoint.
         // GetLastCheckpoint finds the last checkpoint in MapCheckpoints that's in our
         // MapBlockIndex.
-        CBlockIndex* pcheckpoint = Checkpoints::GetLastCheckpoint(params.Checkpoints());
+        CBlockIndex* pcheckpoint = GetLastCheckpoint(params.Checkpoints());
         if (pcheckpoint && nHeight < pcheckpoint->nHeight)
             return state.DoS(100, error("%s: forked chain older than last checkpoint (height %d)", __func__, nHeight), REJECT_CHECKPOINT, "bad-fork-prior-to-checkpoint");
     }