wallet: Avoid use of Chain::Lock in importwallet and dumpwallet

This is a step toward removing the Chain::Lock class and reducing cs_main
locking.

This change only affects behavior in the case where wallet last block processed
falls behind the chain tip, in which case it will use more accurate backup and
rescan timestamps.

Author: ryanofsky

src/wallet/rpcdump.cpp

@@ -566,8 +566,7 @@ UniValue importwallet(const JSONRPCRequest& request)
         if (!file.is_open()) {
             throw JSONRPCError(RPC_INVALID_PARAMETER, "Cannot open wallet dump file");
         }
-        Optional<int> tip_height = locked_chain->getHeight();
-        nTimeBegin = tip_height ? locked_chain->getBlockTime(*tip_height) : 0;
+        CHECK_NONFATAL(pwallet->chain().findBlock(pwallet->GetLastBlockHash(), FoundBlock().time(nTimeBegin)));
 
         int64_t nFilesize = std::max((int64_t)1, (int64_t)file.tellg());
         file.seekg(0, file.beg);
@@ -791,9 +790,10 @@ UniValue dumpwallet(const JSONRPCRequest& request)
     // produce output
     file << strprintf("# Wallet dump created by Bitcoin %s\n", CLIENT_BUILD);
     file << strprintf("# * Created on %s\n", FormatISO8601DateTime(GetTime()));
-    const Optional<int> tip_height = locked_chain->getHeight();
-    file << strprintf("# * Best block at time of backup was %i (%s),\n", tip_height.get_value_or(-1), tip_height ? locked_chain->getBlockHash(*tip_height).ToString() : "(missing block hash)");
-    file << strprintf("#   mined on %s\n", tip_height ? FormatISO8601DateTime(locked_chain->getBlockTime(*tip_height)) : "(missing block time)");
+    file << strprintf("# * Best block at time of backup was %i (%s),\n", pwallet->GetLastBlockHeight(), pwallet->GetLastBlockHash().ToString());
+    int64_t block_time = 0;
+    CHECK_NONFATAL(pwallet->chain().findBlock(pwallet->GetLastBlockHash(), FoundBlock().time(block_time)));
+    file << strprintf("#   mined on %s\n", FormatISO8601DateTime(block_time));
     file << "\n";
 
     // add the base58check encoded extended master if the wallet uses HD

src/wallet/test/wallet_tests.cpp

@@ -225,6 +225,7 @@ BOOST_FIXTURE_TEST_CASE(importwallet_rescan, TestChain100Setup)
         request.params.setArray();
         request.params.push_back(backup_file);
         AddWallet(wallet);
+        wallet->SetLastBlockProcessed(::ChainActive().Height(), ::ChainActive().Tip()->GetBlockHash());
         ::dumpwallet(request);
         RemoveWallet(wallet);
     }
@@ -233,16 +234,17 @@ BOOST_FIXTURE_TEST_CASE(importwallet_rescan, TestChain100Setup)
     // were scanned, and no prior blocks were scanned.
     {
         std::shared_ptr<CWallet> wallet = std::make_shared<CWallet>(chain.get(), WalletLocation(), WalletDatabase::CreateDummy());
+        LOCK(wallet->cs_wallet);
         wallet->SetupLegacyScriptPubKeyMan();
 
         JSONRPCRequest request;
         request.params.setArray();
         request.params.push_back(backup_file);
         AddWallet(wallet);
+        wallet->SetLastBlockProcessed(::ChainActive().Height(), ::ChainActive().Tip()->GetBlockHash());
         ::importwallet(request);
         RemoveWallet(wallet);
 
-        LOCK(wallet->cs_wallet);
         BOOST_CHECK_EQUAL(wallet->mapWallet.size(), 3U);
         BOOST_CHECK_EQUAL(m_coinbase_txns.size(), 103U);
         for (size_t i = 0; i < m_coinbase_txns.size(); ++i) {