Add multi_a descriptor inference

Author: sipa

src/script/descriptor.cpp

@@ -1287,13 +1287,33 @@ std::unique_ptr<PubkeyProvider> InferXOnlyPubkey(const XOnlyPubKey& xkey, ParseS
     return key_provider;
 }
 
+std::unique_ptr<DescriptorImpl> InferMultiA(const CScript& script, ParseScriptContext ctx, const SigningProvider& provider)
+{
+    auto match = MatchMultiA(script);
+    if (!match) return {};
+    std::vector<std::unique_ptr<PubkeyProvider>> keys;
+    keys.reserve(match->second.size());
+    for (const auto keyspan : match->second) {
+        if (keyspan.size() != 32) return {};
+        auto key = InferXOnlyPubkey(XOnlyPubKey{keyspan}, ctx, provider);
+        if (!key) return {};
+        keys.push_back(std::move(key));
+    }
+    return std::make_unique<MultiADescriptor>(match->first, std::move(keys));
+}
+
 std::unique_ptr<DescriptorImpl> InferScript(const CScript& script, ParseScriptContext ctx, const SigningProvider& provider)
 {
     if (ctx == ParseScriptContext::P2TR && script.size() == 34 && script[0] == 32 && script[33] == OP_CHECKSIG) {
         XOnlyPubKey key{Span{script}.subspan(1, 32)};
         return std::make_unique<PKDescriptor>(InferXOnlyPubkey(key, ctx, provider));
     }
 
+    if (ctx == ParseScriptContext::P2TR) {
+        auto ret = InferMultiA(script, ctx, provider);
+        if (ret) return ret;
+    }
+
     std::vector<std::vector<unsigned char>> data;
     TxoutType txntype = Solver(script, data);