Have createwalletdescriptor auto-detect an unused(KEY) #32861

1. 
   Sjors commented at 8:55 am on July 3, 2025: member

   The createwalletdescriptor was introduced in #29130 to let users add a tr() descriptor to an existing SegWit wallet. The new addhdkey method from #29136 introduces a new potential workflow: start from a blank wallet, generate an HD and then add only the descriptors you need, e.g.:

   0bitcoin rpc createwallet TaprootMaxi blank=true
   1bitcoin rpc addhdkey
   2bitcoin rpc createwalletdescriptor bech32m
   

   Before this PR the last line would fail, requiring the user to call gethdkeys and copy-paste the xpub.

   This PR makes createwalletdescriptor a bit smarter so it just finds the unused(KEY) generated by hdkey and uses that.

   If multiple unused(KEY) descriptors are present the user still has to pick one.

   A potential followup is to make our multisig tutorial slightly safer to use. Rather than creating a full wallet, the instruction could be changed to start with a blank wallet and only generate the default legacy descriptor. This avoids accidental use of single sig p2sh-segwit and bech32 addresses.

   (I might do that in #32784, which introduces some other improvements to the tutorial. The situation is still not great; ideally the importdescriptors RPC is enhanced to detect when an xpub is a child of an hd key and then treats it as if an xpriv was imported.)

   Builds on #29136.

2. descriptor: Add unused(KEY) descriptor

   unused() descriptors do not have scriptPubKeys. Instead, the wallet uses
   them to store keys without having any scripts to watch for.

   10b677530e
3. test: Simple test for importing unused(KEY) 92849bdd9b
4. wallet: Add addhdkey RPC 0d24f0af7a
5. wallet, rpc: Disallow import of unused() if key already exists cba3bcb6a7
6. wallet, rpc: Disallow importing unused() to wallets without privkeys 6100108594
7. wallet: add GetScriptlessSPKMs() helper

   A helper method to obtain all unused(key) descriptor SPKMs.

   f5db60a844
8. rpc: make createwalletdescriptor smarter

   When a wallet contains only an unused(KEY) descriptor, use it. Previously the user would have to call listdescriptors and manually specify it.

   e8b1440aa3
9. 
   DrahtBot commented at 8:55 am on July 3, 2025: contributor

   The following sections might be updated with supplementary metadata relevant to reviewers and maintainers.

   Code Coverage & Benchmarks

   For details see: https://corecheck.dev/bitcoin/bitcoin/pulls/32861.

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   Conflicts

   Reviewers, this pull request conflicts with the following ones:

   • #32489 (wallet: Add exportwatchonlywallet RPC to export a watchonly version of a wallet by achow101)
   • #30243 (descriptors: taproot partial descriptors by Eunovo)
   • #28333 (wallet: Construct ScriptPubKeyMans with all data rather than loaded progressively by achow101)

   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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Sjors DrahtBot