Have createwalletdescriptor auto-detect an unused(KEY)

src/wallet/rpc/wallet.cpp

@@ -883,11 +883,37 @@ static RPCHelpMan createwalletdescriptor()
 
             CExtPubKey xpub;
             if (hdkey.isNull()) {
+                // First consider the HD key from active descriptors
                 std::set<CExtPubKey> active_xpubs = pwallet->GetActiveHDPubKeys();
-                if (active_xpubs.size() != 1) {
+                if (active_xpubs.size() == 1) {
+                    xpub = *active_xpubs.begin();
+                } else if (active_xpubs.size() > 1) {
                     throw JSONRPCError(RPC_INVALID_ADDRESS_OR_KEY, "Unable to determine which HD key to use from active descriptors. Please specify with 'hdkey'");
+                } else {
+                    // Look for an unused(KEY) descriptor
+                    std::set<DescriptorScriptPubKeyMan*> spkms{pwallet->GetScriptlessSPKMs()};
+
+                    std::vector<CExtPubKey> wallet_xpubs;
+                    for (auto* spkm : spkms) {
+                        // Retrieve the pubkeys from the descriptor
+                        LOCK(spkm->cs_desc_man);
+                        std::set<CPubKey> pubkeys;
+                        std::set<CExtPubKey> extpubs;
+                        spkm->GetWalletDescriptor().descriptor->GetPubKeys(pubkeys, extpubs);
+
+                        for (const CExtPubKey& xpub : extpubs) {
+                            wallet_xpubs.emplace_back(xpub);
+                        }
+                    }
+
+                    if (wallet_xpubs.empty()) {
+                        throw JSONRPCError(RPC_INVALID_ADDRESS_OR_KEY, "No HD key found. Please generate one with 'addhdkey' or import an active descriptor.");
+                    } else if (wallet_xpubs.size() > 1) {
+                        throw JSONRPCError(RPC_INVALID_ADDRESS_OR_KEY, "Unable to determine which HD key to use. Please specify with 'hdkey'");
+                    }
+
+                    xpub = wallet_xpubs[0];
                 }
-                xpub = *active_xpubs.begin();
             } else {
                 xpub = DecodeExtPubKey(hdkey.get_str());
                 if (!xpub.pubkey.IsValid()) {

test/functional/wallet_createwalletdescriptor.py

@@ -25,6 +25,7 @@ def run_test(self):
         self.test_basic()
         self.test_imported_other_keys()
         self.test_encrypted()
+        self.test_from_unused_desc()
 
     def test_basic(self):
         def_wallet = self.nodes[0].get_wallet_rpc(self.default_wallet_name)
@@ -39,7 +40,7 @@ def test_basic(self):
             if desc["desc"].startswith("wpkh("):
                 expected_descs.append(desc["desc"])
 
-        assert_raises_rpc_error(-5, "Unable to determine which HD key to use from active descriptors. Please specify with 'hdkey'", wallet.createwalletdescriptor, "bech32")
+        assert_raises_rpc_error(-5, "No HD key found. Please generate one with 'addhdkey' or import an active descriptor.", wallet.createwalletdescriptor, "bech32")
         assert_raises_rpc_error(-5, f"Private key for {xpub} is not known", wallet.createwalletdescriptor, type="bech32", hdkey=xpub)
 
         self.log.info("Test createwalletdescriptor after importing active descriptor to blank wallet")
@@ -114,6 +115,26 @@ def test_encrypted(self):
         with WalletUnlock(wallet, "pass"):
             wallet.createwalletdescriptor(type="bech32m")
 
+    def test_from_unused_desc(self):
+        self.log.info("Test createwalletdescriptor from only an unused(KEY) descriptor")
+        self.nodes[0].createwallet("w1", blank=True)
+        w1 = self.nodes[0].get_wallet_rpc("w1")
+
+        # Wallet can't be completely empty
+        assert_raises_rpc_error(-5, "No HD key found. Please generate one with 'addhdkey' or import an active descriptor.", w1.createwalletdescriptor, "bech32")
+
+        # Create unused(KEY) descriptor and try again
+        w1.addhdkey()
+        w1.createwalletdescriptor(type="bech32")
+
+        self.nodes[0].createwallet("w2", blank=True)
+        w2 = self.nodes[0].get_wallet_rpc("w2")
+
+        # Multiple unused(KEY) descriptors require user to choose
+        w2.addhdkey()
+        w2.addhdkey()
+
+        assert_raises_rpc_error(-5, "Unable to determine which HD key to use. Please specify with 'hdkey'", w2.createwalletdescriptor, "bech32")
 
 
 if __name__ == '__main__':