Fix importmulti failure to return rescan errors

An off-by-one-block bug in importmulti rescan logic could cause it to return
success in an edge case even when a rescan was not successful. The case where
this would happen is if there were multiple blocks in a row with the same
GetBlockTimeMax() value, and the last block was scanned successfully, but one
or more of the earlier blocks was not readable.

src/wallet/rpcdump.cpp

@@ -1121,13 +1121,13 @@ UniValue importmulti(const JSONRPCRequest& mainRequest)
 
     if (fRescan && fRunScan && requests.size()) {
         CBlockIndex* pindex = nLowestTimestamp > minimumTimestamp ? chainActive.FindEarliestAtLeast(std::max<int64_t>(nLowestTimestamp - TIMESTAMP_WINDOW, 0)) : chainActive.Genesis();
-        CBlockIndex* scannedRange = nullptr;
+        CBlockIndex* scanFailed = nullptr;
         if (pindex) {
-            scannedRange = pwallet->ScanForWalletTransactions(pindex, true);
+            scanFailed = pwallet->ScanForWalletTransactions(pindex, true);
             pwallet->ReacceptWalletTransactions();
         }
 
-        if (!scannedRange || scannedRange->nHeight > pindex->nHeight) {
+        if (scanFailed) {
             std::vector<UniValue> results = response.getValues();
             response.clear();
             response.setArray();
@@ -1137,12 +1137,23 @@ UniValue importmulti(const JSONRPCRequest& mainRequest)
                 // range, or if the import result already has an error set, let
                 // the result stand unmodified. Otherwise replace the result
                 // with an error message.
-                if (GetImportTimestamp(request, now) - TIMESTAMP_WINDOW >= scannedRange->GetBlockTimeMax() || results.at(i).exists("error")) {
+                if (GetImportTimestamp(request, now) - TIMESTAMP_WINDOW > scanFailed->GetBlockTimeMax() || results.at(i).exists("error")) {
                     response.push_back(results.at(i));
                 } else {
                     UniValue result = UniValue(UniValue::VOBJ);
                     result.pushKV("success", UniValue(false));
-                    result.pushKV("error", JSONRPCError(RPC_MISC_ERROR, strprintf("Failed to rescan before time %d, transactions may be missing.", scannedRange->GetBlockTimeMax())));
+                    result.pushKV(
+                        "error",
+                        JSONRPCError(
+                            RPC_MISC_ERROR,
+                            strprintf("Rescan failed for key with creation timestamp %d. There was an error reading a "
+                                      "block from time %d, which is after or within %d seconds of key creation, and "
+                                      "could contain transactions pertaining to the key. As a result, transactions "
+                                      "and coins using this key may not appear in the wallet. This error could be "
+                                      "caused by pruning or data corruption (see bitcoind log for details) and could "
+                                      "be dealt with by downloading and rescanning the relevant blocks (see -reindex "
+                                      "and -rescan options).",
+                                GetImportTimestamp(request, now), scanFailed->GetBlockTimeMax(), TIMESTAMP_WINDOW)));
                     response.push_back(std::move(result));
                 }
                 ++i;

src/wallet/test/wallet_tests.cpp

@@ -364,6 +364,7 @@ BOOST_FIXTURE_TEST_CASE(rescan, TestChain100Setup)
     LOCK(cs_main);
 
     // Cap last block file size, and mine new block in a new block file.
+    CBlockIndex* const nullBlock = nullptr;
     CBlockIndex* oldTip = chainActive.Tip();
     GetBlockFileInfo(oldTip->GetBlockPos().nFile)->nSize = MAX_BLOCKFILE_SIZE;
     CreateAndProcessBlock({}, GetScriptForRawPubKey(coinbaseKey.GetPubKey()));
@@ -375,7 +376,7 @@ BOOST_FIXTURE_TEST_CASE(rescan, TestChain100Setup)
         CWallet wallet;
         LOCK(wallet.cs_wallet);
         wallet.AddKeyPubKey(coinbaseKey, coinbaseKey.GetPubKey());
-        BOOST_CHECK_EQUAL(oldTip, wallet.ScanForWalletTransactions(oldTip));
+        BOOST_CHECK_EQUAL(nullBlock, wallet.ScanForWalletTransactions(oldTip));
         BOOST_CHECK_EQUAL(wallet.GetImmatureBalance(), 100 * COIN);
     }
 
@@ -389,7 +390,7 @@ BOOST_FIXTURE_TEST_CASE(rescan, TestChain100Setup)
         CWallet wallet;
         LOCK(wallet.cs_wallet);
         wallet.AddKeyPubKey(coinbaseKey, coinbaseKey.GetPubKey());
-        BOOST_CHECK_EQUAL(newTip, wallet.ScanForWalletTransactions(oldTip));
+        BOOST_CHECK_EQUAL(oldTip, wallet.ScanForWalletTransactions(oldTip));
         BOOST_CHECK_EQUAL(wallet.GetImmatureBalance(), 50 * COIN);
     }
 
@@ -413,7 +414,7 @@ BOOST_FIXTURE_TEST_CASE(rescan, TestChain100Setup)
         CKey futureKey;
         futureKey.MakeNewKey(true);
         key.pushKV("scriptPubKey", HexStr(GetScriptForRawPubKey(futureKey.GetPubKey())));
-        key.pushKV("timestamp", newTip->GetBlockTimeMax() + TIMESTAMP_WINDOW);
+        key.pushKV("timestamp", newTip->GetBlockTimeMax() + TIMESTAMP_WINDOW + 1);
         key.pushKV("internal", UniValue(true));
         keys.push_back(key);
         JSONRPCRequest request;
@@ -421,20 +422,17 @@ BOOST_FIXTURE_TEST_CASE(rescan, TestChain100Setup)
         request.params.push_back(keys);
 
         UniValue response = importmulti(request);
-        BOOST_CHECK_EQUAL(response.write(), strprintf("[{\"success\":false,\"error\":{\"code\":-1,\"message\":\"Failed to rescan before time %d, transactions may be missing.\"}},{\"success\":true}]", newTip->GetBlockTimeMax()));
+        BOOST_CHECK_EQUAL(response.write(),
+            strprintf("[{\"success\":false,\"error\":{\"code\":-1,\"message\":\"Rescan failed for key with creation "
+                      "timestamp %d. There was an error reading a block from time %d, which is after or within %d "
+                      "seconds of key creation, and could contain transactions pertaining to the key. As a result, "
+                      "transactions and coins using this key may not appear in the wallet. This error could be caused "
+                      "by pruning or data corruption (see bitcoind log for details) and could be dealt with by "
+                      "downloading and rescanning the relevant blocks (see -reindex and -rescan "
+                      "options).\"}},{\"success\":true}]",
+                              0, oldTip->GetBlockTimeMax(), TIMESTAMP_WINDOW));
         ::pwalletMain = backup;
     }
-
-    // Verify ScanForWalletTransactions does not return null when the scan is
-    // elided due to the nTimeFirstKey optimization.
-    {
-        CWallet wallet;
-        {
-            LOCK(wallet.cs_wallet);
-            wallet.UpdateTimeFirstKey(newTip->GetBlockTime() + 7200 + 1);
-        }
-        BOOST_CHECK_EQUAL(newTip, wallet.ScanForWalletTransactions(newTip));
-    }
 }
 
 // Check that GetImmatureCredit() returns a newly calculated value instead of

src/wallet/wallet.cpp

@@ -1456,10 +1456,9 @@ void CWalletTx::GetAmounts(std::list<COutputEntry>& listReceived,
  * from or to us. If fUpdate is true, found transactions that already
  * exist in the wallet will be updated.
  *
- * Returns pointer to the first block in the last contiguous range that was
+ * Returns null if scan was successful. Otherwise, if a complete rescan was not
- * successfully scanned or elided (elided if pIndexStart points at a block
+ * possible (due to pruning or corruption), returns pointer to the most recent
- * before CWallet::nTimeFirstKey). Returns null if there is no such range, or
+ * block that could not be scanned.
- * the range doesn't include chainActive.Tip().
  */
 CBlockIndex* CWallet::ScanForWalletTransactions(CBlockIndex* pindexStart, bool fUpdate)
 {
@@ -1467,7 +1466,7 @@ CBlockIndex* CWallet::ScanForWalletTransactions(CBlockIndex* pindexStart, bool f
     const CChainParams& chainParams = Params();
 
     CBlockIndex* pindex = pindexStart;
-    CBlockIndex* ret = pindexStart;
+    CBlockIndex* ret = nullptr;
     {
         LOCK2(cs_main, cs_wallet);
         fAbortRescan = false;
@@ -1495,11 +1494,8 @@ CBlockIndex* CWallet::ScanForWalletTransactions(CBlockIndex* pindexStart, bool f
                 for (size_t posInBlock = 0; posInBlock < block.vtx.size(); ++posInBlock) {
                     AddToWalletIfInvolvingMe(block.vtx[posInBlock], pindex, posInBlock, fUpdate);
                 }
-                if (!ret) {
-                    ret = pindex;
-                }
             } else {
-                ret = nullptr;
+                ret = pindex;
             }
             pindex = chainActive.Next(pindex);
         }