Replace FindLatestBefore used by importmuti with FindEarliestAtLeast.

In spite of the name FindLatestBefore used std::lower_bound to try
 to find the earliest block with a nTime greater or equal to the
 the requested value.  But lower_bound uses bisection and requires
 the input to be ordered with respect to the comparison operation.
 Block times are not well ordered.

I don't know what lower_bound is permitted to do when the data
 is not sufficiently ordered, but it's probably not good.
 (I could construct an implementation which would infinite loop...)

To resolve the issue this commit introduces a maximum-so-far to the
 block indexes and searches that.

For clarity the function is renamed to reflect what it actually does.

An issue that remains is that there is no grace period in importmulti:
 If a address is created at time T and a send is immediately broadcast
 and included by a miner with a slow clock there may not yet have been
 any block with at least time T.

The normal rescan has a grace period of 7200 seconds, but importmulti
 does not.

5 files changed, 19 insertions, 8 deletions

diff --git a/src/chain.cpp b/src/chain.cpp
index 3cd7b33920..0f4d422b9f 100644
--- a/src/chain.cpp
+++ b/src/chain.cpp
@@ -61,10 +61,10 @@ const CBlockIndex *CChain::FindFork(const CBlockIndex *pindex) const {
     return pindex;
 }
 
-CBlockIndex* CChain::FindLatestBefore(int64_t nTime) const
+CBlockIndex* CChain::FindEarliestAtLeast(int64_t nTime) const
 {
     std::vector<CBlockIndex*>::const_iterator lower = std::lower_bound(vChain.begin(), vChain.end(), nTime,
-        [](CBlockIndex* pBlock, const int64_t& time) -> bool { return pBlock->GetBlockTime() < time; });
+        [](CBlockIndex* pBlock, const int64_t& time) -> bool { return pBlock->GetBlockTimeMax() < time; });
     return (lower == vChain.end() ? NULL : *lower);
 }
 
diff --git a/src/chain.h b/src/chain.h
index 989a71958c..acb29b667b 100644
--- a/src/chain.h
+++ b/src/chain.h
@@ -202,6 +202,9 @@ public:
     //! (memory only) Sequential id assigned to distinguish order in which blocks are received.
     int32_t nSequenceId;
 
+    //! (memory only) Maximum nTime in the chain upto and including this block.
+    unsigned int nTimeMax;
+
     void SetNull()
     {
         phashBlock = NULL;
@@ -216,6 +219,7 @@ public:
         nChainTx = 0;
         nStatus = 0;
         nSequenceId = 0;
+        nTimeMax = 0;
 
         nVersion       = 0;
         hashMerkleRoot = uint256();
@@ -281,6 +285,11 @@ public:
         return (int64_t)nTime;
     }
 
+    int64_t GetBlockTimeMax() const
+    {
+        return (int64_t)nTimeMax;
+    }
+
     enum { nMedianTimeSpan=11 };
 
     int64_t GetMedianTimePast() const
@@ -461,8 +470,8 @@ public:
     /** Find the last common block between this chain and a block index entry. */
     const CBlockIndex *FindFork(const CBlockIndex *pindex) const;
 
-    /** Find the most recent block with timestamp lower than the given. */
-    CBlockIndex* FindLatestBefore(int64_t nTime) const;
+    /** Find the earliest block with timestamp equal or greater than the given. */
+    CBlockIndex* FindEarliestAtLeast(int64_t nTime) const;
 };
 
 #endif // BITCOIN_CHAIN_H
diff --git a/src/rpc/blockchain.cpp b/src/rpc/blockchain.cpp
index bbcbde71cd..368654bfa6 100644
--- a/src/rpc/blockchain.cpp
+++ b/src/rpc/blockchain.cpp
@@ -839,9 +839,9 @@ UniValue pruneblockchain(const JSONRPCRequest& request)
     // Height value more than a billion is too high to be a block height, and
     // too low to be a block time (corresponds to timestamp from Sep 2001).
     if (heightParam > 1000000000) {
-        CBlockIndex* pindex = chainActive.FindLatestBefore(heightParam);
+        CBlockIndex* pindex = chainActive.FindEarliestAtLeast(heightParam);
         if (!pindex) {
-            throw JSONRPCError(RPC_INTERNAL_ERROR, "Could not find block before specified timestamp.");
+            throw JSONRPCError(RPC_INTERNAL_ERROR, "Could not find block with at least the specified timestamp.");
         }
         heightParam = pindex->nHeight;
     }
diff --git a/src/validation.cpp b/src/validation.cpp
index 3faa1bf005..625307b6a7 100644
--- a/src/validation.cpp
+++ b/src/validation.cpp
@@ -2606,6 +2606,7 @@ CBlockIndex* AddToBlockIndex(const CBlockHeader& block)
         pindexNew->nHeight = pindexNew->pprev->nHeight + 1;
         pindexNew->BuildSkip();
     }
+    pindexNew->nTimeMax = (pindexNew->pprev ? std::max(pindexNew->pprev->nTimeMax, pindexNew->nTime) : pindexNew->nTime);
     pindexNew->nChainWork = (pindexNew->pprev ? pindexNew->pprev->nChainWork : 0) + GetBlockProof(*pindexNew);
     pindexNew->RaiseValidity(BLOCK_VALID_TREE);
     if (pindexBestHeader == NULL || pindexBestHeader->nChainWork < pindexNew->nChainWork)
@@ -3416,6 +3417,7 @@ bool static LoadBlockIndexDB(const CChainParams& chainparams)
     {
         CBlockIndex* pindex = item.second;
         pindex->nChainWork = (pindex->pprev ? pindex->pprev->nChainWork : 0) + GetBlockProof(*pindex);
+        pindex->nTimeMax = (pindex->pprev ? std::max(pindex->pprev->nTimeMax, pindex->nTime) : pindex->nTime);
         // We can link the chain of blocks for which we've received transactions at some point.
         // Pruned nodes may have deleted the block.
         if (pindex->nTx > 0) {
diff --git a/src/wallet/rpcdump.cpp b/src/wallet/rpcdump.cpp
index a1912a78ec..7d4ed70ed9 100644
--- a/src/wallet/rpcdump.cpp
+++ b/src/wallet/rpcdump.cpp
@@ -1048,8 +1048,8 @@ UniValue importmulti(const JSONRPCRequest& mainRequest)
         }
     }
 
-    if (fRescan && fRunScan && requests.size() && nLowestTimestamp <= chainActive.Tip()->GetBlockTime()) {
-        CBlockIndex* pindex = nLowestTimestamp > minimumTimestamp ? chainActive.FindLatestBefore(nLowestTimestamp) : chainActive.Genesis();
+    if (fRescan && fRunScan && requests.size() && nLowestTimestamp <= chainActive.Tip()->GetBlockTimeMax()) {
+        CBlockIndex* pindex = nLowestTimestamp > minimumTimestamp ? chainActive.FindEarliestAtLeast(nLowestTimestamp) : chainActive.Genesis();
 
         if (pindex) {
             pwalletMain->ScanForWalletTransactions(pindex, true);