wallet: add FastWalletRescanFilter class for speeding up rescans

This only supports wallet descriptors right now.

src/wallet/wallet.cpp

@@ -5,6 +5,7 @@
 
 #include <wallet/wallet.h>
 
+#include <blockfilter.h>
 #include <chain.h>
 #include <consensus/amount.h>
 #include <consensus/consensus.h>
@@ -261,6 +262,64 @@ std::shared_ptr<CWallet> LoadWalletInternal(WalletContext& context, const std::s
         return nullptr;
     }
 }
+
+class FastWalletRescanFilter
+{
+public:
+    FastWalletRescanFilter(const CWallet& wallet) : m_wallet(wallet)
+    {
+        // fast rescanning via block filters is only supported by descriptor wallets right now
+        assert(!m_wallet.IsLegacy());
+
+        // create initial filter with scripts from all ScriptPubKeyMans
+        for (auto spkm : m_wallet.GetAllScriptPubKeyMans()) {
+            auto desc_spkm{dynamic_cast<DescriptorScriptPubKeyMan*>(spkm)};
+            assert(desc_spkm != nullptr);
+            AddScriptPubKeys(desc_spkm);
+            // save each range descriptor's end for possible future filter updates
+            if (desc_spkm->IsHDEnabled()) {
+                m_last_range_ends.emplace(desc_spkm->GetID(), desc_spkm->GetEndRange());
+            }
+        }
+    }
+
+    void UpdateIfNeeded()
+    {
+        // repopulate filter with new scripts if top-up has happened since last iteration
+        for (const auto& [desc_spkm_id, last_range_end] : m_last_range_ends) {
+            auto desc_spkm{dynamic_cast<DescriptorScriptPubKeyMan*>(m_wallet.GetScriptPubKeyMan(desc_spkm_id))};
+            assert(desc_spkm != nullptr);
+            int32_t current_range_end{desc_spkm->GetEndRange()};
+            if (current_range_end > last_range_end) {
+                AddScriptPubKeys(desc_spkm, last_range_end);
+                m_last_range_ends.at(desc_spkm->GetID()) = current_range_end;
+            }
+        }
+    }
+
+    std::optional<bool> MatchesBlock(const uint256& block_hash) const
+    {
+        return m_wallet.chain().blockFilterMatchesAny(BlockFilterType::BASIC, block_hash, m_filter_set);
+    }
+
+private:
+    const CWallet& m_wallet;
+    /** Map for keeping track of each range descriptor's last seen end range.
+      * This information is used to detect whether new addresses were derived
+      * (that is, if the current end range is larger than the saved end range)
+      * after processing a block and hence a filter set update is needed to
+      * take possible keypool top-ups into account.
+      */
+    std::map<uint256, int32_t> m_last_range_ends;
+    GCSFilter::ElementSet m_filter_set;
+
+    void AddScriptPubKeys(const DescriptorScriptPubKeyMan* desc_spkm, int32_t last_range_end = 0)
+    {
+        for (const auto& script_pub_key : desc_spkm->GetScriptPubKeys(last_range_end)) {
+            m_filter_set.emplace(script_pub_key.begin(), script_pub_key.end());
+        }
+    }
+};
 } // namespace
 
 std::shared_ptr<CWallet> LoadWallet(WalletContext& context, const std::string& name, std::optional<bool> load_on_start, const DatabaseOptions& options, DatabaseStatus& status, bilingual_str& error, std::vector<bilingual_str>& warnings)