descriptors: Have GetPubKey fill origins directly

Instead of having ExpandHelper fill in the origins in the
FlatSigningProvider output, have GetPubKey do it by itself. This reduces
the extra variables needed in order to track and set origins in
ExpandHelper.

Also changes GetPubKey to return a std::optional<CPubKey> rather than
using a bool and output parameters.

Author: achow101

src/script/descriptor.cpp

@@ -174,22 +174,20 @@ struct PubkeyProvider
      * Used by the Miniscript descriptors to check for duplicate keys in the script.
      */
     bool operator<(PubkeyProvider& other) const {
-        CPubKey a, b;
-        SigningProvider dummy;
-        KeyOriginInfo dummy_info;
+        FlatSigningProvider dummy;
 
-        GetPubKey(0, dummy, a, dummy_info);
-        other.GetPubKey(0, dummy, b, dummy_info);
+        std::optional<CPubKey> a = GetPubKey(0, dummy, dummy);
+        std::optional<CPubKey> b = other.GetPubKey(0, dummy, dummy);
 
         return a < b;
     }
 
-    /** Derive a public key.
+    /** Derive a public key and put it into out.
      *  read_cache is the cache to read keys from (if not nullptr)
      *  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) const = 0;
+    virtual std::optional<CPubKey> GetPubKey(int pos, const SigningProvider& arg, FlatSigningProvider& out, 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;
@@ -240,12 +238,15 @@ class OriginPubkeyProvider final : public PubkeyProvider
 
 public:
     OriginPubkeyProvider(uint32_t exp_index, KeyOriginInfo info, std::unique_ptr<PubkeyProvider> provider, bool apostrophe) : PubkeyProvider(exp_index), m_origin(std::move(info)), m_provider(std::move(provider)), m_apostrophe(apostrophe) {}
-    bool GetPubKey(int pos, const SigningProvider& arg, CPubKey& key, KeyOriginInfo& info, const DescriptorCache* read_cache = nullptr, DescriptorCache* write_cache = nullptr) const override
+    std::optional<CPubKey> GetPubKey(int pos, const SigningProvider& arg, FlatSigningProvider& out, 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);
-        info.path.insert(info.path.begin(), m_origin.path.begin(), m_origin.path.end());
-        return true;
+        std::optional<CPubKey> pub = m_provider->GetPubKey(pos, arg, out, read_cache, write_cache);
+        if (!pub) return std::nullopt;
+        auto& [pubkey, suborigin] = out.origins[pub->GetID()];
+        Assert(pubkey == *pub); // m_provider must have a valid origin by this point.
+        std::copy(std::begin(m_origin.fingerprint), std::end(m_origin.fingerprint), suborigin.fingerprint);
+        suborigin.path.insert(suborigin.path.begin(), m_origin.path.begin(), m_origin.path.end());
+        return pub;
     }
     bool IsRange() const override { return m_provider->IsRange(); }
     size_t GetSize() const override { return m_provider->GetSize(); }
@@ -298,13 +299,13 @@ 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) const override
+    std::optional<CPubKey> GetPubKey(int pos, const SigningProvider&, FlatSigningProvider& out, const DescriptorCache* read_cache = nullptr, DescriptorCache* write_cache = nullptr) const override
     {
-        key = m_pubkey;
-        info.path.clear();
+        KeyOriginInfo info;
         CKeyID keyid = m_pubkey.GetID();
         std::copy(keyid.begin(), keyid.begin() + sizeof(info.fingerprint), info.fingerprint);
-        return true;
+        out.origins.emplace(keyid, std::make_pair(m_pubkey, info));
+        return m_pubkey;
     }
     bool IsRange() const override { return false; }
     size_t GetSize() const override { return m_pubkey.size(); }
@@ -394,7 +395,7 @@ class BIP32PubkeyProvider final : public PubkeyProvider
     BIP32PubkeyProvider(uint32_t exp_index, const CExtPubKey& extkey, KeyPath path, DeriveType derive, bool apostrophe) : PubkeyProvider(exp_index), m_root_extkey(extkey), m_path(std::move(path)), m_derive(derive), m_apostrophe(apostrophe) {}
     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) const override
+    std::optional<CPubKey> GetPubKey(int pos, const SigningProvider& arg, FlatSigningProvider& out, const DescriptorCache* read_cache = nullptr, DescriptorCache* write_cache = nullptr) const override
     {
         KeyOriginInfo info;
         CKeyID keyid = m_root_extkey.pubkey.GetID();
@@ -410,16 +411,16 @@ class BIP32PubkeyProvider final : public PubkeyProvider
         bool der = true;
         if (read_cache) {
             if (!read_cache->GetCachedDerivedExtPubKey(m_expr_index, pos, final_extkey)) {
-                if (m_derive == DeriveType::HARDENED) return false;
+                if (m_derive == DeriveType::HARDENED) return std::nullopt;
                 // Try to get the derivation parent
-                if (!read_cache->GetCachedParentExtPubKey(m_expr_index, parent_extkey)) return false;
+                if (!read_cache->GetCachedParentExtPubKey(m_expr_index, parent_extkey)) return std::nullopt;
                 final_extkey = parent_extkey;
                 if (m_derive == DeriveType::UNHARDENED) der = parent_extkey.Derive(final_extkey, pos);
             }
         } else if (IsHardened()) {
             CExtKey xprv;
             CExtKey lh_xprv;
-            if (!GetDerivedExtKey(arg, xprv, lh_xprv)) return false;
+            if (!GetDerivedExtKey(arg, xprv, lh_xprv)) return std::nullopt;
             parent_extkey = xprv.Neuter();
             if (m_derive == DeriveType::UNHARDENED) der = xprv.Derive(xprv, pos);
             if (m_derive == DeriveType::HARDENED) der = xprv.Derive(xprv, pos | 0x80000000UL);
@@ -429,16 +430,15 @@ class BIP32PubkeyProvider final : public PubkeyProvider
             }
         } else {
             for (auto entry : m_path) {
-                if (!parent_extkey.Derive(parent_extkey, entry)) return false;
+                if (!parent_extkey.Derive(parent_extkey, entry)) return std::nullopt;
             }
             final_extkey = parent_extkey;
             if (m_derive == DeriveType::UNHARDENED) der = parent_extkey.Derive(final_extkey, pos);
             assert(m_derive != DeriveType::HARDENED);
         }
-        if (!der) return false;
+        if (!der) return std::nullopt;
 
-        final_info_out = info;
-        key_out = final_extkey.pubkey;
+        out.origins.emplace(final_extkey.pubkey.GetID(), std::make_pair(final_extkey.pubkey, info));
 
         if (write_cache) {
             // Only cache parent if there is any unhardened derivation
@@ -448,12 +448,12 @@ class BIP32PubkeyProvider final : public PubkeyProvider
                 if (last_hardened_extkey.pubkey.IsValid()) {
                     write_cache->CacheLastHardenedExtPubKey(m_expr_index, last_hardened_extkey);
                 }
-            } else if (final_info_out.path.size() > 0) {
+            } else if (info.path.size() > 0) {
                 write_cache->CacheDerivedExtPubKey(m_expr_index, pos, final_extkey);
             }
         }
 
-        return true;
+        return final_extkey.pubkey;
     }
     std::string ToString(StringType type, bool normalized) const
     {
@@ -696,16 +696,17 @@ class DescriptorImpl : public Descriptor
     // NOLINTNEXTLINE(misc-no-recursion)
     bool ExpandHelper(int pos, const SigningProvider& arg, const DescriptorCache* read_cache, std::vector<CScript>& output_scripts, FlatSigningProvider& out, DescriptorCache* write_cache) const
     {
-        std::vector<std::pair<CPubKey, KeyOriginInfo>> entries;
-        entries.reserve(m_pubkey_args.size());
+        FlatSigningProvider subprovider;
+        std::vector<CPubKey> pubkeys;
+        pubkeys.reserve(m_pubkey_args.size());
 
-        // Construct temporary data in `entries`, `subscripts`, and `subprovider` to avoid producing output in case of failure.
+        // Construct temporary data in `pubkeys`, `subscripts`, and `subprovider` to avoid producing output in case of failure.
         for (const auto& p : m_pubkey_args) {
-            entries.emplace_back();
-            if (!p->GetPubKey(pos, arg, entries.back().first, entries.back().second, read_cache, write_cache)) return false;
+            std::optional<CPubKey> pubkey = p->GetPubKey(pos, arg, subprovider, read_cache, write_cache);
+            if (!pubkey) return false;
+            pubkeys.push_back(pubkey.value());
         }
         std::vector<CScript> subscripts;
-        FlatSigningProvider subprovider;
         for (const auto& subarg : m_subdescriptor_args) {
             std::vector<CScript> outscripts;
             if (!subarg->ExpandHelper(pos, arg, read_cache, outscripts, subprovider, write_cache)) return false;
@@ -714,13 +715,6 @@ class DescriptorImpl : public Descriptor
         }
         out.Merge(std::move(subprovider));
 
-        std::vector<CPubKey> pubkeys;
-        pubkeys.reserve(entries.size());
-        for (auto& entry : entries) {
-            pubkeys.push_back(entry.first);
-            out.origins.emplace(entry.first.GetID(), std::make_pair<CPubKey, KeyOriginInfo>(CPubKey(entry.first), std::move(entry.second)));
-        }
-
         output_scripts = MakeScripts(pubkeys, std::span{subscripts}, out);
         return true;
     }