Revert "Cache parent xpub inside of BIP32PubkeyProvider"

This reverts commit 09e2507.

The changes made in this commit have turned out to be unnecessary and
confusing, so it is being reverted.

src/script/descriptor.cpp

@@ -166,7 +166,7 @@ struct PubkeyProvider
      *  write_cache is the cache to write keys to (if not nullptr)
      *  Caches are not exclusive but this is not tested. Currently we use them exclusively
      */
-    virtual bool GetPubKey(int pos, const SigningProvider& arg, CPubKey& key, KeyOriginInfo& info, const DescriptorCache* read_cache = nullptr, DescriptorCache* write_cache = nullptr) = 0;
+    virtual bool GetPubKey(int pos, const SigningProvider& arg, CPubKey& key, KeyOriginInfo& info, const DescriptorCache* read_cache = nullptr, DescriptorCache* write_cache = nullptr) const = 0;
 
     /** Whether this represent multiple public keys at different positions. */
     virtual bool IsRange() const = 0;
@@ -199,7 +199,7 @@ class OriginPubkeyProvider final : public PubkeyProvider
 
 public:
     OriginPubkeyProvider(uint32_t exp_index, KeyOriginInfo info, std::unique_ptr<PubkeyProvider> provider) : PubkeyProvider(exp_index), m_origin(std::move(info)), m_provider(std::move(provider)) {}
-    bool GetPubKey(int pos, const SigningProvider& arg, CPubKey& key, KeyOriginInfo& info, const DescriptorCache* read_cache = nullptr, DescriptorCache* write_cache = nullptr) override
+    bool GetPubKey(int pos, const SigningProvider& arg, CPubKey& key, KeyOriginInfo& info, const DescriptorCache* read_cache = nullptr, DescriptorCache* write_cache = nullptr) const override
     {
         if (!m_provider->GetPubKey(pos, arg, key, info, read_cache, write_cache)) return false;
         std::copy(std::begin(m_origin.fingerprint), std::end(m_origin.fingerprint), info.fingerprint);
@@ -245,7 +245,7 @@ class ConstPubkeyProvider final : public PubkeyProvider
 
 public:
     ConstPubkeyProvider(uint32_t exp_index, const CPubKey& pubkey, bool xonly) : PubkeyProvider(exp_index), m_pubkey(pubkey), m_xonly(xonly) {}
-    bool GetPubKey(int pos, const SigningProvider& arg, CPubKey& key, KeyOriginInfo& info, const DescriptorCache* read_cache = nullptr, DescriptorCache* write_cache = nullptr) override
+    bool GetPubKey(int pos, const SigningProvider& arg, CPubKey& key, KeyOriginInfo& info, const DescriptorCache* read_cache = nullptr, DescriptorCache* write_cache = nullptr) const override
     {
         key = m_pubkey;
         info.path.clear();
@@ -288,9 +288,6 @@ class BIP32PubkeyProvider final : public PubkeyProvider
     CExtPubKey m_root_extkey;
     KeyPath m_path;
     DeriveType m_derive;
-    // Cache of the parent of the final derived pubkeys.
-    // Primarily useful for situations when no read_cache is provided
-    CExtPubKey m_cached_xpub;
 
     bool GetExtKey(const SigningProvider& arg, CExtKey& ret) const
     {
@@ -327,7 +324,7 @@ class BIP32PubkeyProvider final : public PubkeyProvider
     BIP32PubkeyProvider(uint32_t exp_index, const CExtPubKey& extkey, KeyPath path, DeriveType derive) : PubkeyProvider(exp_index), m_root_extkey(extkey), m_path(std::move(path)), m_derive(derive) {}
     bool IsRange() const override { return m_derive != DeriveType::NO; }
     size_t GetSize() const override { return 33; }
-    bool GetPubKey(int pos, const SigningProvider& arg, CPubKey& key_out, KeyOriginInfo& final_info_out, const DescriptorCache* read_cache = nullptr, DescriptorCache* write_cache = nullptr) override
+    bool GetPubKey(int pos, const SigningProvider& arg, CPubKey& key_out, KeyOriginInfo& final_info_out, const DescriptorCache* read_cache = nullptr, DescriptorCache* write_cache = nullptr) const override
     {
         // Info of parent of the to be derived pubkey
         KeyOriginInfo parent_info;
@@ -352,9 +349,6 @@ class BIP32PubkeyProvider final : public PubkeyProvider
                 final_extkey = parent_extkey;
                 if (m_derive == DeriveType::UNHARDENED) der = parent_extkey.Derive(final_extkey, pos);
             }
-        } else if (m_cached_xpub.pubkey.IsValid() && m_derive != DeriveType::HARDENED) {
-            parent_extkey = final_extkey = m_cached_xpub;
-            if (m_derive == DeriveType::UNHARDENED) der = parent_extkey.Derive(final_extkey, pos);
         } else if (IsHardened()) {
             CExtKey xprv;
             if (!GetDerivedExtKey(arg, xprv)) return false;
@@ -376,11 +370,6 @@ class BIP32PubkeyProvider final : public PubkeyProvider
         final_info_out = final_info_out_tmp;
         key_out = final_extkey.pubkey;
 
-        // We rely on the consumer to check that m_derive isn't HARDENED as above
-        // But we can't have already cached something in case we read something from the cache
-        // and parent_extkey isn't actually the parent.
-        if (!m_cached_xpub.pubkey.IsValid()) m_cached_xpub = parent_extkey;
-
         if (write_cache) {
             // Only cache parent if there is any unhardened derivation
             if (m_derive != DeriveType::HARDENED) {