doc: Use multipath descriptors in descriptors.md and linked test

Updates documentation and wallet_miniscript_decaying_multisig_descriptor_psbt.py
to use single multipath descriptors with <0;1> syntax instead of separate
external/internal descriptors.

Changes:
- doc/descriptors.md: Update examples to use /<0;1>/* multipath syntax with /0 and /1 notation
- doc/descriptors.md: Update Basic Multisig Example instructions
- test: Refactor to use single multipath descriptor pattern, matching wallet_multisig_descriptor_psbt.py

Implementation:
- _get_xpub() extracts external descriptor and converts to multipath format
- create_multisig() builds descriptor string, gets checksum, imports descriptor#checksum
- Multipath descriptor properly expands to external and internal/change descriptors

Fixes #34086

doc/descriptors.md

@@ -67,8 +67,8 @@ Output descriptors currently support:
 - `wsh(sortedmulti(1,xpub661MyMwAqRbcFW31YEwpkMuc5THy2PSt5bDMsktWQcFF8syAmRUapSCGu8ED9W6oDMSgv6Zz8idoc4a6mr8BDzTJY47LJhkJ8UB7WEGuduB/1/0/*,xpub69H7F5d8KSRgmmdJg2KhpAK8SR3DjMwAdkxj3ZuxV27CprR9LgpeyGmXUbC6wb7ERfvrnKZjXoUmmDznezpbZb7ap6r1D3tgFxHmwMkQTPH/0/0/*))` describes a set of *1-of-2* P2WSH multisig outputs where one multisig key is the *1/0/`i`* child of the first specified xpub and the other multisig key is the *0/0/`i`* child of the second specified xpub, and `i` is any number in a configurable range (`0-1000` by default). The order of public keys in the resulting witnessScripts is determined by the lexicographic order of the public keys at that index.
 - `tr(c6047f9441ed7d6d3045406e95c07cd85c778e4b8cef3ca7abac09b95c709ee5,{pk(fff97bd5755eeea420453a14355235d382f6472f8568a18b2f057a1460297556),pk(e493dbf1c10d80f3581e4904930b1404cc6c13900ee0758474fa94abe8c4cd13)})` describes a P2TR output with the `c6...` x-only pubkey as internal key, and two script paths.
 - `tr(c6047f9441ed7d6d3045406e95c07cd85c778e4b8cef3ca7abac09b95c709ee5,sortedmulti_a(2,2f8bde4d1a07209355b4a7250a5c5128e88b84bddc619ab7cba8d569b240efe4,5cbdf0646e5db4eaa398f365f2ea7a0e3d419b7e0330e39ce92bddedcac4f9bc))` describes a P2TR output with the `c6...` x-only pubkey as internal key, and a single `multi_a` script that needs 2 signatures with 2 specified x-only keys, which will be sorted lexicographically.
-- `wsh(sortedmulti(2,[6f53d49c/44h/1h/0h]tpubDDjsCRDQ9YzyaAq9rspCfq8RZFrWoBpYnLxK6sS2hS2yukqSczgcYiur8Scx4Hd5AZatxTuzMtJQJhchufv1FRFanLqUP7JHwusSSpfcEp2/0/*,[e6807791/44h/1h/0h]tpubDDAfvogaaAxaFJ6c15ht7Tq6ZmiqFYfrSmZsHu7tHXBgnjMZSHAeHSwhvjARNA6Qybon4ksPksjRbPDVp7yXA1KjTjSd5x18KHqbppnXP1s/0/*,[367c9cfa/44h/1h/0h]tpubDDtPnSgWYk8dDnaDwnof4ehcnjuL5VoUt1eW2MoAed1grPHuXPDnkX1fWMvXfcz3NqFxPbhqNZ3QBdYjLz2hABeM9Z2oqMR1Gt2HHYDoCgh/0/*))#av0kxgw0` describes a *2-of-3* multisig. For brevity, the internal "change" descriptor accompanying the above external "receiving" descriptor is not included here, but it typically differs only in the xpub derivation steps, ending in `/1/*` for change addresses.
+- `wsh(sortedmulti(2,[6f53d49c/44h/1h/0h]tpubDDjsCRDQ9YzyaAq9rspCfq8RZFrWoBpYnLxK6sS2hS2yukqSczgcYiur8Scx4Hd5AZatxTuzMtJQJhchufv1FRFanLqUP7JHwusSSpfcEp2/<0;1>/*,[e6807791/44h/1h/0h]tpubDDAfvogaaAxaFJ6c15ht7Tq6ZmiqFYfrSmZsHu7tHXBgnjMZSHAeHSwhvjARNA6Qybon4ksPksjRbPDVp7yXA1KjTjSd5x18KHqbppnXP1s/<0;1>/*,[367c9cfa/44h/1h/0h]tpubDDtPnSgWYk8dDnaDwnof4ehcnjuL5VoUt1eW2MoAed1grPHuXPDnkX1fWMvXfcz3NqFxPbhqNZ3QBdYjLz2hABeM9Z2oqMR1Gt2HHYDoCgh/<0;1>/*))` describes a *2-of-3* multisig with a multipath descriptor specifying both receiving (/0) and change (/1) address derivation paths.
-- `wsh(thresh(4,pk([7258e4f9/44h/1h/0h]tpubDCZrkQoEU3845aFKUu9VQBYWZtrTwxMzcxnBwKFCYXHD6gEXvtFcxddCCLFsEwmxQaG15izcHxj48SXg1QS5FQGMBx5Ak6deXKPAL7wauBU/0/*),s:pk([c80b1469/44h/1h/0h]tpubDD3UwwHoNUF4F3Vi5PiUVTc3ji1uThuRfFyBexTSHoAcHuWW2z8qEE2YujegcLtgthr3wMp3ZauvNG9eT9xfJyxXCfNty8h6rDBYU8UU1qq/0/*),s:pk([4e5024fe/44h/1h/0h]tpubDDLrpPymPLSCJyCMLQdmcWxrAWwsqqssm5NdxT2WSdEBPSXNXxwbeKtsHAyXPpLkhUyKovtZgCi47QxVpw9iVkg95UUgeevyAqtJ9dqBqa1/0/*),s:pk([3b1d1ee9/44h/1h/0h]tpubDCmDTANBWPzf6d8Ap1J5Ku7J1Ay92MpHMrEV7M5muWxCrTBN1g5f1NPcjMEL6dJHxbvEKNZtYCdowaSTN81DAyLsmv6w6xjJHCQNkxrsrfu/0/*),sln:after(840000),sln:after(1050000),sln:after(1260000)))#k28080kv` describes a Miniscript multisig with spending policy: `thresh(4,pk(key_1),pk(key_2),pk(key_3),pk(key_4),after(t1),after(t2),after(t3))` that starts as 4-of-4 and "decays" to 3-of-4, 2-of-4, and finally 1-of-4 at each future halvening block height. For brevity, the internal "change" descriptor accompanying the above external "receiving" descriptor is not included here, but it typically differs only in the xpub derivation steps, ending in `/1/*` for change addresses.
+- `wsh(thresh(4,pk([7258e4f9/44h/1h/0h]tpubDCZrkQoEU3845aFKUu9VQBYWZtrTwxMzcxnBwKFCYXHD6gEXvtFcxddCCLFsEwmxQaG15izcHxj48SXg1QS5FQGMBx5Ak6deXKPAL7wauBU/<0;1>/*),s:pk([c80b1469/44h/1h/0h]tpubDD3UwwHoNUF4F3Vi5PiUVTc3ji1uThuRfFyBexTSHoAcHuWW2z8qEE2YujegcLtgthr3wMp3ZauvNG9eT9xfJyxXCfNty8h6rDBYU8UU1qq/<0;1>/*),s:pk([4e5024fe/44h/1h/0h]tpubDDLrpPymPLSCJyCMLQdmcWxrAWwsqqssm5NdxT2WSdEBPSXNXxwbeKtsHAyXPpLkhUyKovtZgCi47QxVpw9iVkg95UUgeevyAqtJ9dqBqa1/<0;1>/*),s:pk([3b1d1ee9/44h/1h/0h]tpubDCmDTANBWPzf6d8Ap1J5Ku7J1Ay92MpHMrEV7M5muWxCrTBN1g5f1NPcjMEL6dJHxbvEKNZtYCdowaSTN81DAyLsmv6w6xjJHCQNkxrsrfu/<0;1>/*),sln:after(840000),sln:after(1050000),sln:after(1260000)))` describes a Miniscript multisig with spending policy: `thresh(4,pk(key_1),pk(key_2),pk(key_3),pk(key_4),after(t1),after(t2),after(t3))` that starts as 4-of-4 and "decays" to 3-of-4, 2-of-4, and finally 1-of-4 at each future halvening block height. This uses a multipath descriptor specifying both receiving (/0) and change (/1) address derivation paths.
 - `tr(musig(xpub6ERApfZwUNrhLCkDtcHTcxd75RbzS1ed54G1LkBUHQVHQKqhMkhgbmJbZRkrgZw4koxb5JaHWkY4ALHY2grBGRjaDMzQLcgJvLJuZZvRcEL,xpub68NZiKmJWnxxS6aaHmn81bvJeTESw724CRDs6HbuccFQN9Ku14VQrADWgqbhhTHBaohPX4CjNLf9fq9MYo6oDaPPLPxSb7gwQN3ih19Zm4Y)/0/*)` describes a MuSig2 multisig with key derivation. The internal keys are derived at `m/0/*` from the aggregate key computed from the 2 participants.
 
 ## Reference
@@ -175,9 +175,9 @@ The basic steps are:
      the participant's signer wallet. Avoid reusing this wallet for any purpose other than signing transactions from the
      corresponding multisig we are about to create. Hint: extract the wallet's xpubs using `listdescriptors` and pick the one from the
      `pkh` descriptor since it's least likely to be accidentally reused (legacy addresses)
-  2. Create a watch-only descriptor wallet (blank, private keys disabled). Now the multisig is created by importing the external and internal descriptors:
+  2. Create a watch-only descriptor wallet (blank, private keys disabled). Now the multisig is created by importing a single multipath descriptor:
-     `wsh(sortedmulti(<M>,XPUB1/0/*,XPUB2/0/*,…,XPUBN/0/*))` and `wsh(sortedmulti(<M>,XPUB1/1/*,XPUB2/1/*,…,XPUBN/1/*))`
+     `wsh(sortedmulti(<M>,XPUB1/<0;1>/*,XPUB2/<0;1>/*,…,XPUBN/<0;1>/*))`
-     (one descriptor w/ `0` for receiving addresses and another w/ `1` for change). Every participant does this. All key origin information (master key fingerprint and all derivation steps) should be included with xpubs for proper support of hardware devices / external signers
+     This single descriptor specifies both receiving (`/0`) and change (`/1`) addresses. Every participant does this. All key origin information (master key fingerprint and all derivation steps) should be included with xpubs for proper support of hardware devices / external signers
   3. A receiving address is generated for the multisig. As a check to ensure step 2 was done correctly, every participant
      should verify they get the same addresses
   4. Funds are sent to the resulting address

test/functional/wallet_miniscript_decaying_multisig_descriptor_psbt.py

@@ -28,32 +28,26 @@ class WalletMiniscriptDecayingMultisigDescriptorPSBTTest(BitcoinTestFramework):
         self.skip_if_no_wallet()
 
     @staticmethod
-    def _get_xpub(wallet, internal):
+    def _get_xpub(wallet):
         """Extract the wallet's xpubs using `listdescriptors` and pick the one from the `pkh` descriptor since it's least likely to be accidentally reused (legacy addresses)."""
-        pkh_descriptor = next(filter(lambda d: d["desc"].startswith("pkh(") and d["internal"] == internal, wallet.listdescriptors()["descriptors"]))
+        pkh_descriptor = next(filter(lambda d: d["desc"].startswith("pkh(") and not d["internal"], wallet.listdescriptors()["descriptors"]))
         # keep all key origin information (master key fingerprint and all derivation steps) for proper support of hardware devices
         # see section 'Key origin identification' in 'doc/descriptors.md' for more details...
-        return pkh_descriptor["desc"].split("pkh(")[1].split(")")[0]
+        # Replace the change index with the multipath convention
+        return pkh_descriptor["desc"].split("pkh(")[1].split(")")[0].replace("/0/*", "/<0;1>/*")
 
-    def create_multisig(self, external_xpubs, internal_xpubs):
+    def create_multisig(self, xpubs):
-        """The multisig is created by importing the following descriptors. The resulting wallet is watch-only and every signer can do this."""
+        """The multisig is created by importing a single multipath descriptor. The resulting wallet is watch-only and every signer can do this."""
         self.node.createwallet(wallet_name=f"{self.name}", blank=True, disable_private_keys=True)
         multisig = self.node.get_wallet_rpc(f"{self.name}")
         # spending policy: `thresh(4,pk(key_1),pk(key_2),pk(key_3),pk(key_4),after(t1),after(t2),after(t3))`
         # IMPORTANT: when backing up your descriptor, the order of key_1...key_4 must be correct!
-        external = multisig.getdescriptorinfo(f"wsh(thresh({self.N},pk({'),s:pk('.join(external_xpubs)}),sln:after({'),sln:after('.join(map(str, self.locktimes))})))")
+        multisig_desc = f"wsh(thresh({self.N},pk({'),s:pk('.join(xpubs)}),sln:after({'),sln:after('.join(map(str, self.locktimes))})))"
-        internal = multisig.getdescriptorinfo(f"wsh(thresh({self.N},pk({'),s:pk('.join(internal_xpubs)}),sln:after({'),sln:after('.join(map(str, self.locktimes))})))")
+        checksum = multisig.getdescriptorinfo(multisig_desc)["checksum"]
         result = multisig.importdescriptors([
-            {  # receiving addresses (internal: False)
+            {  # Multipath descriptor expands to receive and change
-                "desc": external["descriptor"],
+                "desc": f"{multisig_desc}#{checksum}",
                 "active": True,
-                "internal": False,
-                "timestamp": "now",
-            },
-            {  # change addresses (internal: True)
-                "desc": internal["descriptor"],
-                "active": True,
-                "internal": True,
                 "timestamp": "now",
             },
         ])
@@ -73,10 +67,10 @@ class WalletMiniscriptDecayingMultisigDescriptorPSBTTest(BitcoinTestFramework):
 
         self.log.info("Create the signer wallets and get their xpubs...")
         signers = [self.node.get_wallet_rpc(self.node.createwallet(wallet_name=f"signer_{i}")["name"]) for i in range(self.N)]
-        external_xpubs, internal_xpubs = [[self._get_xpub(signer, internal) for signer in signers] for internal in [False, True]]
+        xpubs = [self._get_xpub(signer) for signer in signers]
 
         self.log.info("Create the watch-only decaying multisig using signers' xpubs...")
-        multisig = self.create_multisig(external_xpubs, internal_xpubs)
+        multisig = self.create_multisig(xpubs)
 
         self.log.info("Get a mature utxo to send to the multisig...")
         coordinator_wallet = self.node.get_wallet_rpc(self.node.createwallet(wallet_name="coordinator")["name"])