Add support for inferring descriptors from scripts

src/script/descriptor.cpp

@@ -625,6 +625,80 @@ std::unique_ptr<Descriptor> ParseScript(Span<const char>& sp, ParseScriptContext
     return nullptr;
 }
 
+std::unique_ptr<PubkeyProvider> InferPubkey(const CPubKey& pubkey, ParseScriptContext, const SigningProvider& provider)
+{
+    std::unique_ptr<PubkeyProvider> key_provider = MakeUnique<ConstPubkeyProvider>(pubkey);
+    KeyOriginInfo info;
+    if (provider.GetKeyOrigin(pubkey.GetID(), info)) {
+        return MakeUnique<OriginPubkeyProvider>(std::move(info), std::move(key_provider));
+    }
+    return key_provider;
+}
+
+std::unique_ptr<Descriptor> InferScript(const CScript& script, ParseScriptContext ctx, const SigningProvider& provider)
+{
+    std::vector<std::vector<unsigned char>> data;
+    txnouttype txntype = Solver(script, data);
+
+    if (txntype == TX_PUBKEY) {
+        CPubKey pubkey(data[0].begin(), data[0].end());
+        if (pubkey.IsValid()) {
+            return MakeUnique<SingleKeyDescriptor>(InferPubkey(pubkey, ctx, provider), P2PKGetScript, "pk");
+        }
+    }
+    if (txntype == TX_PUBKEYHASH) {
+        uint160 hash(data[0]);
+        CKeyID keyid(hash);
+        CPubKey pubkey;
+        if (provider.GetPubKey(keyid, pubkey)) {
+            return MakeUnique<SingleKeyDescriptor>(InferPubkey(pubkey, ctx, provider), P2PKHGetScript, "pkh");
+        }
+    }
+    if (txntype == TX_WITNESS_V0_KEYHASH && ctx != ParseScriptContext::P2WSH) {
+        uint160 hash(data[0]);
+        CKeyID keyid(hash);
+        CPubKey pubkey;
+        if (provider.GetPubKey(keyid, pubkey)) {
+            return MakeUnique<SingleKeyDescriptor>(InferPubkey(pubkey, ctx, provider), P2WPKHGetScript, "wpkh");
+        }
+    }
+    if (txntype == TX_MULTISIG) {
+        std::vector<std::unique_ptr<PubkeyProvider>> providers;
+        for (size_t i = 1; i + 1 < data.size(); ++i) {
+            CPubKey pubkey(data[i].begin(), data[i].end());
+            providers.push_back(InferPubkey(pubkey, ctx, provider));
+        }
+        return MakeUnique<MultisigDescriptor>((int)data[0][0], std::move(providers));
+    }
+    if (txntype == TX_SCRIPTHASH && ctx == ParseScriptContext::TOP) {
+        uint160 hash(data[0]);
+        CScriptID scriptid(hash);
+        CScript subscript;
+        if (provider.GetCScript(scriptid, subscript)) {
+            auto sub = InferScript(subscript, ParseScriptContext::P2SH, provider);
+            if (sub) return MakeUnique<ConvertorDescriptor>(std::move(sub), ConvertP2SH, "sh");
+        }
+    }
+    if (txntype == TX_WITNESS_V0_SCRIPTHASH && ctx != ParseScriptContext::P2WSH) {
+        CScriptID scriptid;
+        CRIPEMD160().Write(data[0].data(), data[0].size()).Finalize(scriptid.begin());
+        CScript subscript;
+        if (provider.GetCScript(scriptid, subscript)) {
+            auto sub = InferScript(subscript, ParseScriptContext::P2WSH, provider);
+            if (sub) return MakeUnique<ConvertorDescriptor>(std::move(sub), ConvertP2WSH, "wsh");
+        }
+    }
+
+    CTxDestination dest;
+    if (ExtractDestination(script, dest)) {
+        if (GetScriptForDestination(dest) == script) {
+            return MakeUnique<AddressDescriptor>(std::move(dest));
+        }
+    }
+
+    return MakeUnique<RawDescriptor>(script);
+}
+
 } // namespace
 
 std::unique_ptr<Descriptor> Parse(const std::string& descriptor, FlatSigningProvider& out)
@@ -634,3 +708,8 @@ std::unique_ptr<Descriptor> Parse(const std::string& descriptor, FlatSigningProv
     if (sp.size() == 0 && ret) return ret;
     return nullptr;
 }
+
+std::unique_ptr<Descriptor> InferDescriptor(const CScript& script, const SigningProvider& provider)
+{
+    return InferScript(script, ParseScriptContext::TOP, provider);
+}

src/script/descriptor.h

@@ -51,5 +51,20 @@ struct Descriptor {
 /** Parse a descriptor string. Included private keys are put in out. Returns nullptr if parsing fails. */
 std::unique_ptr<Descriptor> Parse(const std::string& descriptor, FlatSigningProvider& out);
 
-#endif // BITCOIN_SCRIPT_DESCRIPTOR_H
+/** Find a descriptor for the specified script, using information from provider where possible.
+ *
+ * A non-ranged descriptor which only generates the specified script will be returned in all
+ * circumstances.
+ *
+ * For public keys with key origin information, this information will be preserved in the returned
+ * descriptor.
+ *
+ * - If all information for solving `script` is present in `provider`, a descriptor will be returned
+ *   which is `IsSolvable()` and encapsulates said information.
+ * - Failing that, if `script` corresponds to a known address type, an "addr()" descriptor will be
+ *   returned (which is not `IsSolvable()`).
+ * - Failing that, a "raw()" descriptor is returned.
+ */
+std::unique_ptr<Descriptor> InferDescriptor(const CScript& script, const SigningProvider& provider);
 
+#endif // BITCOIN_SCRIPT_DESCRIPTOR_H