refactor: Get rid of more redundant chain methods

This just drops three interfaces::Chain methods replacing them with other calls.

Motivation for removing these chain methods:

- Need to get rid of findFirstBlockWithTimeAndHeight for #10102, which doesn't
  support overloaded methods
- Followup from
  https://github.com/bitcoin/bitcoin/pull/16426#discussion_r412487403
- phantomcircuit comments about findNextBlock test
  http://www.erisian.com.au/bitcoin-core-dev/log-2020-06-06.html#l-214

Behavior is not changing in any way here. A TODO comment in
ScanForWalletTransactions was removed, but just because it was invalid (see
https://github.com/bitcoin/bitcoin/pull/19195#discussion_r448020762), not
because it was implemented.

4 files changed, 40 insertions, 76 deletions

diff --git a/src/interfaces/chain.h b/src/interfaces/chain.h
index 85d09be0f3..1a49518d69 100644
--- a/src/interfaces/chain.h
+++ b/src/interfaces/chain.h
@@ -44,6 +44,10 @@ public:
     FoundBlock& time(int64_t& time) { m_time = &time; return *this; }
     FoundBlock& maxTime(int64_t& max_time) { m_max_time = &max_time; return *this; }
     FoundBlock& mtpTime(int64_t& mtp_time) { m_mtp_time = &mtp_time; return *this; }
+    //! Return whether block is in the active (most-work) chain.
+    FoundBlock& inActiveChain(bool& in_active_chain) { m_in_active_chain = &in_active_chain; return *this; }
+    //! Return next block in the active chain if current block is in the active chain.
+    FoundBlock& nextBlock(const FoundBlock& next_block) { m_next_block = &next_block; return *this; }
     //! Read block data from disk. If the block exists but doesn't have data
     //! (for example due to pruning), the CBlock variable will be set to null.
     FoundBlock& data(CBlock& data) { m_data = &data; return *this; }
@@ -53,6 +57,8 @@ public:
     int64_t* m_time = nullptr;
     int64_t* m_max_time = nullptr;
     int64_t* m_mtp_time = nullptr;
+    bool* m_in_active_chain = nullptr;
+    const FoundBlock* m_next_block = nullptr;
     CBlock* m_data = nullptr;
 };
 
@@ -77,9 +83,9 @@ public:
 //!   wallet cache it, fee estimation being driven by node mempool, wallet
 //!   should be the consumer.
 //!
-//! * The `guessVerificationProgress`, `getBlockHeight`, `getBlockHash`, etc
-//!   methods can go away if rescan logic is moved on the node side, and wallet
-//!   only register rescan request.
+//! * `guessVerificationProgress` and similar methods can go away if rescan
+//!   logic moves out of the wallet, and the wallet just requests scans from the
+//!   node (https://github.com/bitcoin/bitcoin/issues/11756)
 class Chain
 {
 public:
@@ -90,11 +96,6 @@ public:
     //! any blocks)
     virtual Optional<int> getHeight() = 0;
 
-    //! Get block height above genesis block. Returns 0 for genesis block,
-    //! 1 for following block, and so on. Returns nullopt for a block not
-    //! included in the current chain.
-    virtual Optional<int> getBlockHeight(const uint256& hash) = 0;
-
     //! Get block hash. Height must be valid or this function will abort.
     virtual uint256 getBlockHash(int height) = 0;
 
@@ -102,13 +103,6 @@ public:
     //! pruned), and contains transactions.
     virtual bool haveBlockOnDisk(int height) = 0;
 
-    //! Return height of the first block in the chain with timestamp equal
-    //! or greater than the given time and height equal or greater than the
-    //! given height, or nullopt if there is no block with a high enough
-    //! timestamp and height. Also return the block hash as an optional output parameter
-    //! (to avoid the cost of a second lookup in case this information is needed.)
-    virtual Optional<int> findFirstBlockWithTimeAndHeight(int64_t time, int height, uint256* hash) = 0;
-
     //! Get locator for the current chain tip.
     virtual CBlockLocator getTipLocator() = 0;
 
@@ -130,11 +124,6 @@ public:
     //! information.
     virtual bool findFirstBlockWithTimeAndHeight(int64_t min_time, int min_height, const FoundBlock& block={}) = 0;
 
-    //! Find next block if block is part of current chain. Also flag if
-    //! there was a reorg and the specified block hash is no longer in the
-    //! current chain, and optionally return block information.
-    virtual bool findNextBlock(const uint256& block_hash, int block_height, const FoundBlock& next={}, bool* reorg=nullptr) = 0;
-
     //! Find ancestor of block at specified height and optionally return
     //! ancestor information.
     virtual bool findAncestorByHeight(const uint256& block_hash, int ancestor_height, const FoundBlock& ancestor_out={}) = 0;
diff --git a/src/node/interfaces.cpp b/src/node/interfaces.cpp
index da3e759e38..b08fa39786 100644
--- a/src/node/interfaces.cpp
+++ b/src/node/interfaces.cpp
@@ -311,6 +311,8 @@ bool FillBlock(const CBlockIndex* index, const FoundBlock& block, UniqueLock<Rec
     if (block.m_time) *block.m_time = index->GetBlockTime();
     if (block.m_max_time) *block.m_max_time = index->GetBlockTimeMax();
     if (block.m_mtp_time) *block.m_mtp_time = index->GetMedianTimePast();
+    if (block.m_in_active_chain) *block.m_in_active_chain = ChainActive()[index->nHeight] == index;
+    if (block.m_next_block) FillBlock(ChainActive()[index->nHeight] == index ? ChainActive()[index->nHeight + 1] : nullptr, *block.m_next_block, lock);
     if (block.m_data) {
         REVERSE_LOCK(lock);
         if (!ReadBlockFromDisk(*block.m_data, index, Params().GetConsensus())) block.m_data->SetNull();
@@ -419,15 +421,6 @@ public:
         }
         return nullopt;
     }
-    Optional<int> getBlockHeight(const uint256& hash) override
-    {
-        LOCK(::cs_main);
-        CBlockIndex* block = LookupBlockIndex(hash);
-        if (block && ::ChainActive().Contains(block)) {
-            return block->nHeight;
-        }
-        return nullopt;
-    }
     uint256 getBlockHash(int height) override
     {
         LOCK(::cs_main);
@@ -441,16 +434,6 @@ public:
         CBlockIndex* block = ::ChainActive()[height];
         return block && ((block->nStatus & BLOCK_HAVE_DATA) != 0) && block->nTx > 0;
     }
-    Optional<int> findFirstBlockWithTimeAndHeight(int64_t time, int height, uint256* hash) override
-    {
-        LOCK(cs_main);
-        CBlockIndex* block = ::ChainActive().FindEarliestAtLeast(time, height);
-        if (block) {
-            if (hash) *hash = block->GetBlockHash();
-            return block->nHeight;
-        }
-        return nullopt;
-    }
     CBlockLocator getTipLocator() override
     {
         LOCK(cs_main);
@@ -479,13 +462,6 @@ public:
         WAIT_LOCK(cs_main, lock);
         return FillBlock(ChainActive().FindEarliestAtLeast(min_time, min_height), block, lock);
     }
-    bool findNextBlock(const uint256& block_hash, int block_height, const FoundBlock& next, bool* reorg) override {
-        WAIT_LOCK(cs_main, lock);
-        CBlockIndex* block = ChainActive()[block_height];
-        if (block && block->GetBlockHash() != block_hash) block = nullptr;
-        if (reorg) *reorg = !block;
-        return FillBlock(block ? ChainActive()[block_height + 1] : nullptr, next, lock);
-    }
     bool findAncestorByHeight(const uint256& block_hash, int ancestor_height, const FoundBlock& ancestor_out) override
     {
         WAIT_LOCK(cs_main, lock);
diff --git a/src/test/interfaces_tests.cpp b/src/test/interfaces_tests.cpp
index b0d4de89f3..2e8729736c 100644
--- a/src/test/interfaces_tests.cpp
+++ b/src/test/interfaces_tests.cpp
@@ -44,6 +44,18 @@ BOOST_AUTO_TEST_CASE(findBlock)
     BOOST_CHECK(chain->findBlock(active[60]->GetBlockHash(), FoundBlock().mtpTime(mtp_time)));
     BOOST_CHECK_EQUAL(mtp_time, active[60]->GetMedianTimePast());
 
+    bool cur_active{false}, next_active{false};
+    uint256 next_hash;
+    BOOST_CHECK_EQUAL(active.Height(), 100);
+    BOOST_CHECK(chain->findBlock(active[99]->GetBlockHash(), FoundBlock().inActiveChain(cur_active).nextBlock(FoundBlock().inActiveChain(next_active).hash(next_hash))));
+    BOOST_CHECK(cur_active);
+    BOOST_CHECK(next_active);
+    BOOST_CHECK_EQUAL(next_hash, active[100]->GetBlockHash());
+    cur_active = next_active = false;
+    BOOST_CHECK(chain->findBlock(active[100]->GetBlockHash(), FoundBlock().inActiveChain(cur_active).nextBlock(FoundBlock().inActiveChain(next_active))));
+    BOOST_CHECK(cur_active);
+    BOOST_CHECK(!next_active);
+
     BOOST_CHECK(!chain->findBlock({}, FoundBlock()));
 }
 
@@ -59,21 +71,6 @@ BOOST_AUTO_TEST_CASE(findFirstBlockWithTimeAndHeight)
     BOOST_CHECK(!chain->findFirstBlockWithTimeAndHeight(/* min_time= */ active.Tip()->GetBlockTimeMax() + 1, /* min_height= */ 0));
 }
 
-BOOST_AUTO_TEST_CASE(findNextBlock)
-{
-    auto chain = interfaces::MakeChain(m_node);
-    auto& active = ChainActive();
-    bool reorg;
-    uint256 hash;
-    BOOST_CHECK(chain->findNextBlock(active[20]->GetBlockHash(), 20, FoundBlock().hash(hash), &reorg));
-    BOOST_CHECK_EQUAL(hash, active[21]->GetBlockHash());
-    BOOST_CHECK_EQUAL(reorg, false);
-    BOOST_CHECK(!chain->findNextBlock(uint256(), 20, {}, &reorg));
-    BOOST_CHECK_EQUAL(reorg, true);
-    BOOST_CHECK(!chain->findNextBlock(active.Tip()->GetBlockHash(), active.Height(), {}, &reorg));
-    BOOST_CHECK_EQUAL(reorg, false);
-}
-
 BOOST_AUTO_TEST_CASE(findAncestorByHeight)
 {
     auto chain = interfaces::MakeChain(m_node);
diff --git a/src/wallet/wallet.cpp b/src/wallet/wallet.cpp
index 65b54f39b4..8350d66fa7 100644
--- a/src/wallet/wallet.cpp
+++ b/src/wallet/wallet.cpp
@@ -946,11 +946,12 @@ bool CWallet::LoadToWallet(const uint256& hash, const UpdateWalletTxFn& fill_wtx
     }
     // If wallet doesn't have a chain (e.g wallet-tool), don't bother to update txn.
     if (HaveChain()) {
-        Optional<int> block_height = chain().getBlockHeight(wtx.m_confirm.hashBlock);
-        if (block_height) {
+        bool active;
+        int height;
+        if (chain().findBlock(wtx.m_confirm.hashBlock, FoundBlock().inActiveChain(active).height(height)) && active) {
             // Update cached block height variable since it not stored in the
             // serialized transaction.
-            wtx.m_confirm.block_height = *block_height;
+            wtx.m_confirm.block_height = height;
         } else if (wtx.isConflicted() || wtx.isConfirmed()) {
             // If tx block (or conflicting block) was reorged out of chain
             // while the wallet was shutdown, change tx status to UNCONFIRMED
@@ -1771,18 +1772,22 @@ CWallet::ScanResult CWallet::ScanForWalletTransactions(const uint256& start_bloc
             WalletLogPrintf("Still rescanning. At block %d. Progress=%f\n", block_height, progress_current);
         }
 
+        // Read block data
         CBlock block;
-        bool next_block;
+        chain().findBlock(block_hash, FoundBlock().data(block));
+
+        // Find next block separately from reading data above, because reading
+        // is slow and there might be a reorg while it is read.
+        bool block_still_active = false;
+        bool next_block = false;
         uint256 next_block_hash;
-        bool reorg = false;
-        if (chain().findBlock(block_hash, FoundBlock().data(block)) && !block.IsNull()) {
+        chain().findBlock(block_hash, FoundBlock().inActiveChain(block_still_active).nextBlock(FoundBlock().inActiveChain(next_block).hash(next_block_hash)));
+
+        if (!block.IsNull()) {
             LOCK(cs_wallet);
-            next_block = chain().findNextBlock(block_hash, block_height, FoundBlock().hash(next_block_hash), &reorg);
-            if (reorg) {
+            if (!block_still_active) {
                 // Abort scan if current block is no longer active, to prevent
                 // marking transactions as coming from the wrong block.
-                // TODO: This should return success instead of failure, see
-                // https://github.com/bitcoin/bitcoin/pull/14711#issuecomment-458342518
                 result.last_failed_block = block_hash;
                 result.status = ScanResult::FAILURE;
                 break;
@@ -1797,13 +1802,12 @@ CWallet::ScanResult CWallet::ScanForWalletTransactions(const uint256& start_bloc
             // could not scan block, keep scanning but record this block as the most recent failure
             result.last_failed_block = block_hash;
             result.status = ScanResult::FAILURE;
-            next_block = chain().findNextBlock(block_hash, block_height, FoundBlock().hash(next_block_hash), &reorg);
         }
         if (max_height && block_height >= *max_height) {
             break;
         }
         {
-            if (!next_block || reorg) {
+            if (!next_block) {
                 // break successfully when rescan has reached the tip, or
                 // previous block is no longer on the chain due to a reorg
                 break;
@@ -4058,9 +4062,7 @@ std::shared_ptr<CWallet> CWallet::Create(interfaces::Chain& chain, const std::st
             if (!time_first_key || time < *time_first_key) time_first_key = time;
         }
         if (time_first_key) {
-            if (Optional<int> first_block = chain.findFirstBlockWithTimeAndHeight(*time_first_key - TIMESTAMP_WINDOW, rescan_height, nullptr)) {
-                rescan_height = *first_block;
-            }
+            chain.findFirstBlockWithTimeAndHeight(*time_first_key - TIMESTAMP_WINDOW, rescan_height, FoundBlock().height(rescan_height));
         }
 
         {