Use 2 hour grace period for key timestamps in importmulti rescans

Gregory Maxwell <greg@xiph.org> pointed out the lack of grace period in
https://github.com/bitcoin/bitcoin/pull/9490#issue-199407998.

The importwallet RPC which uses key timestamps in a similar way already has a 2
hour grace period.

qa/rpc-tests/import-rescan.py

@@ -54,7 +54,7 @@ class Variant(collections.namedtuple("Variant", "call data rescan prune")):
                 "scriptPubKey": {
                     "address": self.address["address"]
                 },
-                "timestamp": timestamp + (1 if self.rescan == Rescan.late_timestamp else 0),
+                "timestamp": timestamp + RESCAN_WINDOW + (1 if self.rescan == Rescan.late_timestamp else 0),
                 "pubkeys": [self.address["pubkey"]] if self.data == Data.pub else [],
                 "keys": [self.key] if self.data == Data.priv else [],
                 "label": self.label,
@@ -99,6 +99,9 @@ IMPORT_VARIANTS = [Variant(*variants) for variants in itertools.product(Call, Da
 ImportNode = collections.namedtuple("ImportNode", "prune rescan")
 IMPORT_NODES = [ImportNode(*fields) for fields in itertools.product((False, True), repeat=2)]
 
+# Rescans start at the earliest block up to 2 hours before the key timestamp.
+RESCAN_WINDOW = 2 * 60 * 60
+
 
 class ImportRescanTest(BitcoinTestFramework):
     def __init__(self):
@@ -130,7 +133,7 @@ class ImportRescanTest(BitcoinTestFramework):
         self.nodes[0].generate(1)
         assert_equal(self.nodes[0].getrawmempool(), [])
         timestamp = self.nodes[0].getblockheader(self.nodes[0].getbestblockhash())["time"]
-        set_node_times(self.nodes, timestamp + 1)
+        set_node_times(self.nodes, timestamp + RESCAN_WINDOW + 1)
         self.nodes[0].generate(1)
         sync_blocks(self.nodes)
 

src/wallet/rpcdump.cpp

@@ -988,7 +988,8 @@ UniValue importmulti(const JSONRPCRequest& mainRequest)
             "                                                              or the string \"now\" to substitute the current synced blockchain time. The timestamp of the oldest\n"
             "                                                              key will determine how far back blockchain rescans need to begin for missing wallet transactions.\n"
             "                                                              \"now\" can be specified to bypass scanning, for keys which are known to never have been used, and\n"
-            "                                                              0 can be specified to scan the entire blockchain.\n"
+            "                                                              0 can be specified to scan the entire blockchain. Blocks up to 2 hours before the earliest key\n"
+            "                                                              creation time of all keys being imported by the importmulti call will be scanned.\n"
             "      \"redeemscript\": \"<script>\"                            , (string, optional) Allowed only if the scriptPubKey is a P2SH address or a P2SH scriptPubKey\n"
             "      \"pubkeys\": [\"<pubKey>\", ... ]                         , (array, optional) Array of strings giving pubkeys that must occur in the output or redeemscript\n"
             "      \"keys\": [\"<key>\", ... ]                               , (array, optional) Array of strings giving private keys whose corresponding public keys must occur in the output or redeemscript\n"
@@ -1072,7 +1073,7 @@ UniValue importmulti(const JSONRPCRequest& mainRequest)
     }
 
     if (fRescan && fRunScan && requests.size() && nLowestTimestamp <= chainActive.Tip()->GetBlockTimeMax()) {
-        CBlockIndex* pindex = nLowestTimestamp > minimumTimestamp ? chainActive.FindEarliestAtLeast(nLowestTimestamp) : chainActive.Genesis();
+        CBlockIndex* pindex = nLowestTimestamp > minimumTimestamp ? chainActive.FindEarliestAtLeast(std::max<int64_t>(nLowestTimestamp - 7200, 0)) : chainActive.Genesis();
 
         if (pindex) {
             pwalletMain->ScanForWalletTransactions(pindex, true);