a0abcbd382 doc: Mention multipath specifier (Ava Chow)
0019f61fc5 tests: Test importing of multipath descriptors (Ava Chow)
f97d5c137d wallet, rpc: Allow importdescriptors to import multipath descriptors (Ava Chow)
32dcbca3fb rpc: Allow importmulti to import multipath descriptors correctly (Ava Chow)
64dfe3ce4b wallet: Move internal to be per key when importing (Ava Chow)
1692245525 tests: Multipath descriptors for scantxoutset and deriveaddresses (Ava Chow)
cddc0ba9a9 rpc: Have deriveaddresses derive receiving and change (Ava Chow)
360456cd22 tests: Multipath descriptors for getdescriptorinfo (Ava Chow)
a90eee444c tests: Add unit tests for multipath descriptors (Ava Chow)
1bbf46e2da descriptors: Change Parse to return vector of descriptors (Ava Chow)
0d640c6f02 descriptors: Have ParseKeypath handle multipath specifiers (Ava Chow)
a5f39b1034 descriptors: Change ParseScript to return vector of descriptors (Ava Chow)
0d55deae15 descriptors: Add DescriptorImpl::Clone (Ava Chow)
7e86541f72 descriptors: Add PubkeyProvider::Clone (Ava Chow)
Pull request description:
It is convenient to have a descriptor which specifies both receiving and change addresses in a single string. However, as discussed in https://github.com/bitcoin/bitcoin/issues/17190#issuecomment-895515768, it is not feasible to use a generic multipath specification like BIP 88 due to combinatorial blow up and that it would result in unexpected descriptors.
To resolve that problem, this PR proposes a targeted solution which allows only a single pair of 2 derivation indexes to be inserted in the place of a single derivation index. So instead of two descriptor `wpkh(xpub.../0/0/*)` and `wpkh(xpub.../0/1/*)` to represent receive and change addresses, this could be written as `wpkh(xpub.../0/<0;1>/*)`. The multipath specifier is of the form `<NUM;NUM>`. Each `NUM` can have its own hardened specifier, e.g. `<0;1h>` is valid. The multipath specifier can also only appear in one path index in the derivation path.
This results in the parser returning two descriptors. The first descriptor uses the first `NUM` in all pairs present, and the second uses the second `NUM`. In our implementation, if a multipath descriptor is not provided, a pair is still returned, but the second element is just `nullptr`.
The wallet will not output the multipath descriptors (yet). Furthermore, when a multipath descriptor is imported, it is expanded to the two descriptors and each imported on its own, with the second descriptor being implicitly for internal (change) addresses. There is no change to how the wallet stores or outputs descriptors (yet).
Note that the path specifier is different from what was proposed. It uses angle brackets and the semicolon because these are unused characters available in the character set and I wanted to avoid conflicts with characters already in use in descriptors.
Closes#17190
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