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 bitcoindevkit#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 bitcoindevkit/bdk#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 bitcoindevkit/bdk#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 bitcoindevkit/bdk#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

---

Generated by Claude Code