descriptors: check duplicate keys in all multipath Miniscript branches #35742

Fixes #35629.

Multipath Miniscript descriptors performed duplicate-key sanity checking while parsing the template, when key comparison used the first multipath branch. As a result, duplicate keys present only in a later branch were accepted.

Select each expanded multipath branch and rerun the existing duplicate-key check before constructing the final descriptors. If a branch contains a duplicate, report the offending concrete Miniscript expression.

The regression test covers:

• a valid two-branch wsh control;
• a duplicate in the second wsh branch;
• a duplicate in the third of three tr branches.

Tested with:

build/bin/test_bitcoin --run_test=descriptor_tests/multipath_miniscript_duplicate_keys
build/bin/test_bitcoin --run_test=descriptor_tests
build/bin/test_bitcoin

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Code Coverage & Benchmarks

For details see: https://corecheck.dev/bitcoin/bitcoin/pulls/35742.

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Conflicts

Reviewers, this pull request conflicts with the following ones:

• #35918 (descriptor: Expose key expression parsing by pseudoramdom)
• #34697 (descriptor: fix musig() duplicate key checks and doubled PSBT origin paths by shuv-amp)

If you consider this pull request important, please also help to review the conflicting pull requests. Ideally, start with the one that should be merged first.

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I ran this on master and on pr-35742,

bitcoin-cli getdescriptorinfo "wsh(or_i(pk(xpub6BgBgsespWvERF3LHQu6CnqdvfEvtMcQjYrcRzx53QJjSxarj2afYWcLteoGVky7D3UKDP9QyrLprQ3VCECoY49yfdDEHGCtMMj92pReUsQ/<0;1>),pk(xpub6BgBgsespWvERF3LHQu6CnqdvfEvtMcQjYrcRzx53QJjSxarj2afYWcLteoGVky7D3UKDP9QyrLprQ3VCECoY49yfdDEHGCtMMj92pReUsQ/<2;1>)))"

and this is what I got:

On master Same public key is generated

{
  "descriptor": "wsh(or_i(pk(xpub6BgBgsespWvERF3LHQu6CnqdvfEvtMcQjYrcRzx53QJjSxarj2afYWcLteoGVky7D3UKDP9QyrLprQ3VCECoY49yfdDEHGCtMMj92pReUsQ/0),pk(xpub6BgBgsespWvERF3LHQu6CnqdvfEvtMcQjYrcRzx53QJjSxarj2afYWcLteoGVky7D3UKDP9QyrLprQ3VCECoY49yfdDEHGCtMMj92pReUsQ/2)))#kzwg47ay",
  "multipath_expansion": [
    "wsh(or_i(pk(xpub6BgBgsespWvERF3LHQu6CnqdvfEvtMcQjYrcRzx53QJjSxarj2afYWcLteoGVky7D3UKDP9QyrLprQ3VCECoY49yfdDEHGCtMMj92pReUsQ/0),pk(xpub6BgBgsespWvERF3LHQu6CnqdvfEvtMcQjYrcRzx53QJjSxarj2afYWcLteoGVky7D3UKDP9QyrLprQ3VCECoY49yfdDEHGCtMMj92pReUsQ/2)))#kzwg47ay",
    "wsh(or_i(pk(xpub6BgBgsespWvERF3LHQu6CnqdvfEvtMcQjYrcRzx53QJjSxarj2afYWcLteoGVky7D3UKDP9QyrLprQ3VCECoY49yfdDEHGCtMMj92pReUsQ/1),pk(xpub6BgBgsespWvERF3LHQu6CnqdvfEvtMcQjYrcRzx53QJjSxarj2afYWcLteoGVky7D3UKDP9QyrLprQ3VCECoY49yfdDEHGCtMMj92pReUsQ/1)))#jkt2efkp"
  ],
  "checksum": "mn7hqkkh",
  "isrange": false,
  "issolvable": true,
  "hasprivatekeys": false
}

On branch

error code: -5
error message:
or_i(pk(xpub6BgBgsespWvERF3LHQu6CnqdvfEvtMcQjYrcRzx53QJjSxarj2afYWcLteoGVky7D3UKDP9QyrLprQ3VCECoY49yfdDEHGCtMMj92pReUsQ/1),pk(xpub6BgBgsespWvERF3LHQu6CnqdvfEvtMcQjYrcRzx53QJjSxarj2afYWcLteoGVky7D3UKDP9QyrLprQ3VCECoY49yfdDEHGCtMMj92pReUsQ/1)) is not sane: contains duplicate public keys

I also checked using importdescriptors to observe wallet action.

On master, the unsafe branch is added to the wallet:

bitcoin-cli -rpcwallet=testwallet_master importdescriptors '[{"desc": "wsh(or_i(pk(xpub6BgBgsespWvERF3LHQu6CnqdvfEvtMcQjYrcRzx53QJjSxarj2afYWcLteoGVky7D3UKDP9QyrLprQ3VCECoY49yfdDEHGCtMMj92pReUsQ/<0;1>),pk(xpub6BgBgsespWvERF3LHQu6CnqdvfEvtMcQjYrcRzx53QJjSxarj2afYWcLteoGVky7D3UKDP9QyrLprQ3VCECoY49yfdDEHGCtMMj92pReUsQ/<2;1>)))#mn7hqkkh", "timestamp": "now", "active": false}]'

[
  {
    "success": true
  }
]

On the PR:

[
  {
    "success": false,
    "error": {
      "code": -5,
      "message": "or_i(pk(xpub6BgBgsespWvERF3LHQu6CnqdvfEvtMcQjYrcRzx53QJjSxarj2afYWcLteoGVky7D3UKDP9QyrLprQ3VCECoY49yfdDEHGCtMMj92pReUsQ/1),pk(xpub6BgBgsespWvERF3LHQu6CnqdvfEvtMcQjYrcRzx53QJjSxarj2afYWcLteoGVky7D3UKDP9QyrLprQ3VCECoY49yfdDEHGCtMMj92pReUsQ/1)) is not sane: contains duplicate public keys"
    }
  }
]

This shows exactly how the PR fixes the silent passing on master