feat(core): allow serialization of SyncResponse

[evanlinjin]

Description

Adds serde::Serialize / serde::Deserialize for SyncResponse, enabling air-gapped workflows: a connected (online) device can sync, serialize the SyncResponse, and hand it to a disconnected (offline) device which deserializes and applies it independently. This is the goal of #1954, implemented without recursion.

SyncResponse is made up of a TxUpdate and an Option<CheckPoint>, so both gain serde support:

• TxUpdate — a plain #[derive(Serialize, Deserialize)]. Its fields are flat collections, so there is nothing self-referential to recurse over. An Ord bound is added on A for deserialization so that anchors: BTreeSet<(A, Txid)> can be rebuilt.

• CheckPoint — this is the interesting one. A CheckPoint is a reference-counted linked list (prev + skip pointers), so a derived implementation would recurse one stack frame per checkpoint and overflow the stack on long chains — the exact hazard that already forced a hand-written Drop (example_bitcoind_rpc_polling sync command ends with stack overflow #1634). It is therefore implemented by hand:

  • Serialize: walk the chain iteratively via CheckPoint::iter() and emit a flat sequence of (height, data) pairs.
  • Deserialize: collect the sequence and rebuild with CheckPoint::from_blocks(..), which re-derives the skip/index topology deterministically.

  The skip/index topology is intentionally not serialized — it is reconstructed on load, keeping the encoding minimal and both directions iterative.

Notes to the reviewers

• This is an alternative take on core: allow serialization of SyncResponse #1954 that addresses the "serializing CheckPoints is a bit odd since it is a linked list" concern raised in review: the linked list is flattened to a (height, data) sequence and rebuilt, so neither serialization nor deserialization recurses.
• checkpoint_serde_is_not_recursive round-trips a 10k-checkpoint chain on a deliberately small (128 KiB) thread stack, mirroring the existing checkpoint_drop_is_not_recursive test — a recursive impl would overflow it.
• serde_json is added only as a dev-dependency of bdk_core, for the round-trip tests.
• Scope is intentionally limited to SyncResponse to match core: allow serialization of SyncResponse #1954. FullScanResponse can follow the exact same pattern as a trivial follow-up if desired.

Changelog notice

• Added serde::Serialize and serde::Deserialize implementations for SyncResponse, TxUpdate, and CheckPoint (under the serde feature). CheckPoint is (de)serialized iteratively as a flat list of (height, data) blocks to avoid recursing over its linked-list structure.

Checklists

All Submissions:

• I followed the contribution guidelines

New Features:

• I've added tests for the new feature
• I've added docs for the new feature

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[codecov]

Codecov Report

❌ Patch coverage is 98.55072% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 79.11%. Comparing base (47556ab) to head (b62c35c).
⚠️ Report is 10 commits behind head on master.

Files with missing lines	Patch %	Lines
crates/core/src/checkpoint.rs	96.29%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2232      +/-   ##
==========================================
+ Coverage   77.69%   79.11%   +1.42%     
==========================================
  Files          29       30       +1     
  Lines        5801     6047     +246     
  Branches      271      279       +8     
==========================================
+ Hits         4507     4784     +277     
+ Misses       1223     1187      -36     
- Partials       71       76       +5     

Flag	Coverage Δ
rust	79.11% <98.55%> (+1.42%)	⬆️

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