Refactor Cache merging and writing

Instead of having a large blob of cache merging code in TopUp, refactor
this into DescriptorCache so that it can merge and provide a diff
(another DescriptorCache containing just the items that were added).
Then TopUp can just write everything that was in the diff.

src/script/descriptor.cpp

@@ -1418,6 +1418,36 @@ bool DescriptorCache::GetCachedDerivedExtPubKey(uint32_t key_exp_pos, uint32_t d
     return true;
 }
 
+DescriptorCache DescriptorCache::MergeAndDiff(const DescriptorCache& other)
+{
+    DescriptorCache diff;
+    for (const auto& parent_xpub_pair : other.GetCachedParentExtPubKeys()) {
+        CExtPubKey xpub;
+        if (GetCachedParentExtPubKey(parent_xpub_pair.first, xpub)) {
+            if (xpub != parent_xpub_pair.second) {
+                throw std::runtime_error(std::string(__func__) + ": New cached parent xpub does not match already cached parent xpub");
+            }
+            continue;
+        }
+        CacheParentExtPubKey(parent_xpub_pair.first, parent_xpub_pair.second);
+        diff.CacheParentExtPubKey(parent_xpub_pair.first, parent_xpub_pair.second);
+    }
+    for (const auto& derived_xpub_map_pair : other.GetCachedDerivedExtPubKeys()) {
+        for (const auto& derived_xpub_pair : derived_xpub_map_pair.second) {
+            CExtPubKey xpub;
+            if (GetCachedDerivedExtPubKey(derived_xpub_map_pair.first, derived_xpub_pair.first, xpub)) {
+                if (xpub != derived_xpub_pair.second) {
+                    throw std::runtime_error(std::string(__func__) + ": New cached derived xpub does not match already cached derived xpub");
+                }
+                continue;
+            }
+            CacheDerivedExtPubKey(derived_xpub_map_pair.first, derived_xpub_pair.first, derived_xpub_pair.second);
+            diff.CacheDerivedExtPubKey(derived_xpub_map_pair.first, derived_xpub_pair.first, derived_xpub_pair.second);
+        }
+    }
+    return diff;
+}
+
 const ExtPubKeyMap DescriptorCache::GetCachedParentExtPubKeys() const
 {
     return m_parent_xpubs;

src/script/descriptor.h

@@ -55,6 +55,11 @@ public:
     const ExtPubKeyMap GetCachedParentExtPubKeys() const;
     /** Retrieve all cached derived xpubs */
     const std::unordered_map<uint32_t, ExtPubKeyMap> GetCachedDerivedExtPubKeys() const;
+
+    /** Combine another DescriptorCache into this one.
+     * Returns a cache containing the items from the other cache unknown to current cache
+     */
+    DescriptorCache MergeAndDiff(const DescriptorCache& other);
 };
 
 /** \brief Interface for parsed descriptor objects.

src/wallet/scriptpubkeyman.cpp

@@ -1805,33 +1805,18 @@ bool DescriptorScriptPubKeyMan::TopUp(unsigned int size)
             }
             m_map_pubkeys[pubkey] = i;
         }
-        // Write the cache
+        // Merge and write the cache
-        for (const auto& parent_xpub_pair : temp_cache.GetCachedParentExtPubKeys()) {
+        DescriptorCache new_items = m_wallet_descriptor.cache.MergeAndDiff(temp_cache);
-            CExtPubKey xpub;
+        for (const auto& parent_xpub_pair : new_items.GetCachedParentExtPubKeys()) {
-            if (m_wallet_descriptor.cache.GetCachedParentExtPubKey(parent_xpub_pair.first, xpub)) {
-                if (xpub != parent_xpub_pair.second) {
-                    throw std::runtime_error(std::string(__func__) + ": New cached parent xpub does not match already cached parent xpub");
-                }
-                continue;
-            }
             if (!batch.WriteDescriptorParentCache(parent_xpub_pair.second, id, parent_xpub_pair.first)) {
                 throw std::runtime_error(std::string(__func__) + ": writing cache item failed");
             }
-            m_wallet_descriptor.cache.CacheParentExtPubKey(parent_xpub_pair.first, parent_xpub_pair.second);
         }
-        for (const auto& derived_xpub_map_pair : temp_cache.GetCachedDerivedExtPubKeys()) {
+        for (const auto& derived_xpub_map_pair : new_items.GetCachedDerivedExtPubKeys()) {
             for (const auto& derived_xpub_pair : derived_xpub_map_pair.second) {
-                CExtPubKey xpub;
-                if (m_wallet_descriptor.cache.GetCachedDerivedExtPubKey(derived_xpub_map_pair.first, derived_xpub_pair.first, xpub)) {
-                    if (xpub != derived_xpub_pair.second) {
-                        throw std::runtime_error(std::string(__func__) + ": New cached derived xpub does not match already cached derived xpub");
-                    }
-                    continue;
-                }
                 if (!batch.WriteDescriptorDerivedCache(derived_xpub_pair.second, id, derived_xpub_map_pair.first, derived_xpub_pair.first)) {
                     throw std::runtime_error(std::string(__func__) + ": writing cache item failed");
                 }
-                m_wallet_descriptor.cache.CacheDerivedExtPubKey(derived_xpub_map_pair.first, derived_xpub_pair.first, derived_xpub_pair.second);
             }
         }
         m_max_cached_index++;