Merge bitcoin/bitcoin#33286: doc: update multisig tutorial to use multipath descriptors

de7c3587cd doc: Update add checksum instructions in tutorial (Ben Westgate)
2a46e94a16 doc: Update multisig-tutorial.md to use multipath descriptors (Ben Westgate)

Pull request description:

  ### Summary

  Update `doc/multisig-tutorial.md` to use multipath descriptor format
  instead of separate external/internal descriptors. The tutorial now:

  - extracts a single `xpub_n` per participant
  - constructs a multipath `wsh(sortedmulti(...))` descriptor with `<0;1>`
    change index semantics
  - uses `getdescriptorinfo` to compute descriptor checksum
  - explains that `importdescriptors` expands the multipath descriptor
    into internal and external descriptors
  - update `/test/functional/wallet_multisig_descriptor_psbt.py` functional test / documentation to use multi-path descriptors

  ---

  ### Motivation

  A single multipath descriptor is the most convenient pattern for multisig; our documentation should use it.

  ---

  ### What changed

  - replaced extraction of `external_xpub_n` and `internal_xpub_n` with
    extraction of a single `xpub_n`
  - removed instructions to create and import separate external/internal
    descriptors
  - added instructions to build a multipath `wsh(sortedmulti(...))`
    descriptor and derive checksum with `getdescriptorinfo`
  - checksum field is parsed and appended as the multipath descriptor is not the canonical "desc" output
  - clarified that `importdescriptors` automatically expands multipath
    descriptors into internal and external forms
  - similar changes to the functional test: wallet_multisig_descriptor_psbt.

  ---

  ### Testing

  I have run the updated shell snippets and confirmed the multipath descriptor produces the same `listdescriptors` output after importing as the two descriptor method in bitcoin:master.

  ---

  ### Related issues / PRs

  This tutorial change references the multipath descriptor
  consolidation (see commit / PR referenced in the change). The commit
  message points to bitcoin#22838 as the upstream change that enables
  this behavior.

  ---

  ### Release note (for changelog)

  Documentation: update multisig tutorial and multisig functional test to use multipath descriptors

ACKs for top commit:
  Sjors:
    utACK de7c3587cd
  kannapoix:
    Core review ACK: de7c3587cd
  achow101:
    ACK de7c3587cd
  rkrux:
    crACK de7c3587cd

Tree-SHA512: ca7275d8ad04922b3fa8d2a3084ff96aa2104265f1fc2f749814dd16776351ab692d67e7e643d08052f7492e3eaa1a9a4dfe3470163e57939a49f782d3df511a

test/functional/wallet_multisig_descriptor_psbt.py

@@ -25,12 +25,13 @@ class WalletMultisigDescriptorPSBTTest(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>/*")
 
     @staticmethod
     def _check_psbt(psbt, to, value, multisig):
@@ -44,26 +45,19 @@ class WalletMultisigDescriptorPSBTTest(BitcoinTestFramework):
                 amount += vout["value"]
         assert_approx(amount, float(value), vspan=0.001)
 
-    def participants_create_multisigs(self, external_xpubs, internal_xpubs):
+    def participants_create_multisigs(self, xpubs):
         """The multisig is created by importing the following descriptors. The resulting wallet is watch-only and every participant can do this."""
         for i, node in enumerate(self.nodes):
             node.createwallet(wallet_name=f"{self.name}_{i}", blank=True, disable_private_keys=True)
             multisig = node.get_wallet_rpc(f"{self.name}_{i}")
-            external = multisig.getdescriptorinfo(f"wsh(sortedmulti({self.M},{','.join(external_xpubs)}))")
-            internal = multisig.getdescriptorinfo(f"wsh(sortedmulti({self.M},{','.join(internal_xpubs)}))")
+            multisig_desc = f"wsh(sortedmulti({self.M},{','.join(xpubs)}))"
+            checksum = multisig.getdescriptorinfo(multisig_desc)["checksum"]
             result = multisig.importdescriptors([
-                {  # receiving addresses (internal: False)
-                    "desc": external["descriptor"],
+                {  # Multipath descriptor expands to receive and change
+                    "desc": f"{multisig_desc}#{checksum}",
                     "active": True,
-                    "internal": False,
                     "timestamp": "now",
-                },
-                {  # change addresses (internal: True)
-                    "desc": internal["descriptor"],
-                    "active": True,
-                    "internal": True,
-                    "timestamp": "now",
-                },
+                }
             ])
             assert all(r["success"] for r in result)
             yield multisig
@@ -84,10 +78,10 @@ class WalletMultisigDescriptorPSBTTest(BitcoinTestFramework):
         }
 
         self.log.info("Generate and exchange xpubs...")
-        external_xpubs, internal_xpubs = [[self._get_xpub(signer, internal) for signer in participants["signers"]] for internal in [False, True]]
+        xpubs = [self._get_xpub(signer) for signer in participants["signers"]]
 
         self.log.info("Every participant imports the following descriptors to create the watch-only multisig...")
-        participants["multisigs"] = list(self.participants_create_multisigs(external_xpubs, internal_xpubs))
+        participants["multisigs"] = list(self.participants_create_multisigs(xpubs))
 
         self.log.info("Check that every participant's multisig generates the same addresses...")
         for _ in range(10):  # we check that the first 10 generated addresses are the same for all participant's multisigs