fix: spks_of_all_keychains() shouldn't return an infinite iterator for non-wildcard descriptors

[danielabrozzoni]

Description

When you pass a non-wildcard descriptor in new_with_range, we make
sure that the range length is at most 1; if that's not the case, we
shorten it.
We would previously use new_with_range without this check and with a
non-wildcard descriptor in spks_of_all_keychains, this meant creating
a spkiterator that would go on producing the same spks over and over
again, causing some issues with syncing on electrum/esplora.

To reproduce the bug, run in example-crates/example_electrum:

cargo run "sh(wsh(or_d(c:pk_k(cPGudvRLDSgeV4hH9NUofLvYxYBSRjju3cpiXmBg9K8G9k1ikCMp),c:pk_k(cSBSBHRrzqSXFmrBhLkZMzQB9q4P9MnAq92v8d9a5UveBc9sLX32))))#zp9pcfs9" scan

Changelog notice

• Fixed a bug where KeychainTxOutIndex::spks_of_all_keychains/KeychainTxOutIndex::spks_of_keychain would return an iterator yielding infinite spks even for non-wildcard descriptors.

Checklists

All Submissions:

• I've signed all my commits
• I followed the contribution guidelines
• I ran cargo fmt and cargo clippy before committing

[evanlinjin]

Thank you for spotting and fixing this @danielabrozzoni!

I think SpkIterator::new_with_range should also detect non-wildcard descriptors, and the implementation of SpkIterator::new can be changed to this:

    /// Creates a new script pubkey iterator starting at 0 from a descriptor.
    pub fn new(descriptor: D) -> Self {
        SpkIterator::new_with_range(descriptor, 0..=BIP32_MAX_INDEX)
    }

This means, SpkIterator:new_with_range will sometimes return empty iterators.

[LLFourn]

+1 for what @evanlinjin suggested above.

[danielabrozzoni]

I think SpkIterator::new_with_range should also detect non-wildcard descriptors, and the implementation of SpkIterator::new can be changed to this:

I tried to implement this by setting end = next_index in new_with_range for non-wildcard descriptors, but I'm seeing some tests failing, investigating now...

[danielabrozzoni]

I tried to implement this by setting end = next_index in new_with_range for non-wildcard descriptors, but I'm seeing some tests failing, investigating now...

Turns out that this was wrong. I'm now shortening whichever range passed to new_with_range to have length at most 1 if the descriptor doesn't have a wildcard, and it works perfectly. Lmk what you think :)

[evanlinjin]

Thanks for fixing this. This was my oversight.

As discussed (via DM), it'll be better to have non-wildcard descriptors only returns something on derivation index 0.

The follow tests should pass:

// non index-0 should NOT return an spk
assert_eq!(SpkIterator::new_with_range(&no_wildcard_descriptor, 1..1).next(), None);
assert_eq!(SpkIterator::new_with_range(&no_wildcard_descriptor, 1..=1).next(), None);
assert_eq!(SpkIterator::new_with_range(&no_wildcard_descriptor, 1..2).next(), None);
assert_eq!(SpkIterator::new_with_range(&no_wildcard_descriptor, 1..=2).next(), None);

[evanlinjin]

ACK e48b911

Tested with example_electrum with non-wildcard descriptor.
Left a non-critical nit.