descriptors: Have GetPrivKey fill keys directly

Instead of GetPrivKey returning a key and having the caller fill the
FlatSigningProvider, have GetPrivKey take the FlatSigningProvider and
fill it by itself. This will be necessary for descriptors such as
musig() where there are private keys that need to be added to the
FlatSigningProvider but do not directly appear in any resulting scripts.

GetPrivKey is now changed to void as the caller no longer cares whether
it succeeds or fails.

Author: achow101

src/script/descriptor.cpp

@@ -211,8 +211,8 @@ struct PubkeyProvider
      */
     virtual bool ToNormalizedString(const SigningProvider& arg, std::string& out, const DescriptorCache* cache = nullptr) const = 0;
 
-    /** Derive a private key, if private data is available in arg. */
-    virtual bool GetPrivKey(int pos, const SigningProvider& arg, CKey& key) const = 0;
+    /** Derive a private key, if private data is available in arg and put it into out. */
+    virtual void GetPrivKey(int pos, const SigningProvider& arg, FlatSigningProvider& out) const = 0;
 
     /** Return the non-extended public key for this PubkeyProvider, if it has one. */
     virtual std::optional<CPubKey> GetRootPubKey() const = 0;
@@ -274,9 +274,9 @@ class OriginPubkeyProvider final : public PubkeyProvider
         }
         return true;
     }
-    bool GetPrivKey(int pos, const SigningProvider& arg, CKey& key) const override
+    void GetPrivKey(int pos, const SigningProvider& arg, FlatSigningProvider& out) const override
     {
-        return m_provider->GetPrivKey(pos, arg, key);
+        m_provider->GetPrivKey(pos, arg, out);
     }
     std::optional<CPubKey> GetRootPubKey() const override
     {
@@ -298,6 +298,14 @@ class ConstPubkeyProvider final : public PubkeyProvider
     CPubKey m_pubkey;
     bool m_xonly;
 
+    std::optional<CKey> GetPrivKey(const SigningProvider& arg) const
+    {
+        CKey key;
+        if (!(m_xonly ? arg.GetKeyByXOnly(XOnlyPubKey(m_pubkey), key) :
+                        arg.GetKey(m_pubkey.GetID(), key))) return std::nullopt;
+        return key;
+    }
+
 public:
     ConstPubkeyProvider(uint32_t exp_index, const CPubKey& pubkey, bool xonly) : PubkeyProvider(exp_index), m_pubkey(pubkey), m_xonly(xonly) {}
     std::optional<CPubKey> GetPubKey(int pos, const SigningProvider&, FlatSigningProvider& out, const DescriptorCache* read_cache = nullptr, DescriptorCache* write_cache = nullptr) const override
@@ -314,20 +322,21 @@ class ConstPubkeyProvider final : public PubkeyProvider
     std::string ToString(StringType type) const override { return m_xonly ? HexStr(m_pubkey).substr(2) : HexStr(m_pubkey); }
     bool ToPrivateString(const SigningProvider& arg, std::string& ret) const override
     {
-        CKey key;
-        if (!GetPrivKey(/*pos=*/0, arg, key)) return false;
-        ret = EncodeSecret(key);
+        std::optional<CKey> key = GetPrivKey(arg);
+        if (!key) return false;
+        ret = EncodeSecret(*key);
         return true;
     }
     bool ToNormalizedString(const SigningProvider& arg, std::string& ret, const DescriptorCache* cache) const override
     {
         ret = ToString(StringType::PUBLIC);
         return true;
     }
-    bool GetPrivKey(int pos, const SigningProvider& arg, CKey& key) const override
+    void GetPrivKey(int pos, const SigningProvider& arg, FlatSigningProvider& out) const override
     {
-        return m_xonly ? arg.GetKeyByXOnly(XOnlyPubKey(m_pubkey), key) :
-                         arg.GetKey(m_pubkey.GetID(), key);
+        std::optional<CKey> key = GetPrivKey(arg);
+        if (!key) return;
+        out.keys.emplace(key->GetPubKey().GetID(), *key);
     }
     std::optional<CPubKey> GetRootPubKey() const override
     {
@@ -542,15 +551,14 @@ class BIP32PubkeyProvider final : public PubkeyProvider
         }
         return true;
     }
-    bool GetPrivKey(int pos, const SigningProvider& arg, CKey& key) const override
+    void GetPrivKey(int pos, const SigningProvider& arg, FlatSigningProvider& out) const override
     {
         CExtKey extkey;
         CExtKey dummy;
-        if (!GetDerivedExtKey(arg, extkey, dummy)) return false;
-        if (m_derive == DeriveType::UNHARDENED && !extkey.Derive(extkey, pos)) return false;
-        if (m_derive == DeriveType::HARDENED && !extkey.Derive(extkey, pos | 0x80000000UL)) return false;
-        key = extkey.key;
-        return true;
+        if (!GetDerivedExtKey(arg, extkey, dummy)) return;
+        if (m_derive == DeriveType::UNHARDENED && !extkey.Derive(extkey, pos)) return;
+        if (m_derive == DeriveType::HARDENED && !extkey.Derive(extkey, pos | 0x80000000UL)) return;
+        out.keys.emplace(extkey.key.GetPubKey().GetID(), extkey.key);
     }
     std::optional<CPubKey> GetRootPubKey() const override
     {
@@ -736,9 +744,7 @@ class DescriptorImpl : public Descriptor
     void ExpandPrivate(int pos, const SigningProvider& provider, FlatSigningProvider& out) const final
     {
         for (const auto& p : m_pubkey_args) {
-            CKey key;
-            if (!p->GetPrivKey(pos, provider, key)) continue;
-            out.keys.emplace(key.GetPubKey().GetID(), key);
+            p->GetPrivKey(pos, provider, out);
         }
         for (const auto& arg : m_subdescriptor_args) {
             arg->ExpandPrivate(pos, provider, out);