Add signing support for (sorted)multi_a scripts

Author: sipa

src/script/sign.cpp

@@ -174,6 +174,29 @@ static bool SignTaprootScript(const SigningProvider& provider, const BaseSignatu
             result = Vector(std::move(sig));
             return true;
         }
+        return false;
+    }
+
+    // multi_a scripts (<key> OP_CHECKSIG <key> OP_CHECKSIGADD <key> OP_CHECKSIGADD <k> OP_NUMEQUAL)
+    if (auto match = MatchMultiA(script)) {
+        std::vector<std::vector<unsigned char>> sigs;
+        int good_sigs = 0;
+        for (size_t i = 0; i < match->second.size(); ++i) {
+            XOnlyPubKey pubkey{*(match->second.rbegin() + i)};
+            std::vector<unsigned char> sig;
+            bool good_sig = CreateTaprootScriptSig(creator, sigdata, provider, sig, pubkey, leaf_hash, sigversion);
+            if (good_sig && good_sigs < match->first) {
+                ++good_sigs;
+                sigs.push_back(std::move(sig));
+            } else {
+                sigs.emplace_back();
+            }
+        }
+        if (good_sigs == match->first) {
+            result = std::move(sigs);
+            return true;
+        }
+        return false;
     }
 
     return false;