1169 sync engine with block cache trait

zcash_client_backend/CHANGELOG.md

@@ -24,6 +24,7 @@ and this library adheres to Rust's notion of
   - `BlockMetadata::orchard_tree_size`
   - `WalletSummary::next_orchard_subtree_index`
   - `chain::ScanSummary::{spent_orchard_note_count, received_orchard_note_count}`
+  - `chain::BlockCache` trait
 - `zcash_client_backend::fees`:
   - `orchard`
   - `ChangeValue::orchard`
@@ -37,6 +38,8 @@ and this library adheres to Rust's notion of
   - `Nullifiers::{orchard, extend_orchard, retain_orchard}`
   - `TaggedOrchardBatch`
   - `TaggedOrchardBatchRunner`
+  - `testing` module
+  - `testing::{'fake_compact_block`, `random_compact_tx`} (moved from `tests` module).
 - `zcash_client_backend::wallet`:
   - `Note::Orchard`
   - `WalletOrchardSpend`
@@ -76,6 +79,8 @@ and this library adheres to Rust's notion of
     constraint on its `<AccountId>` parameter has been strengthened to `Copy`.
 - `zcash_client_backend::fees`:
   - Arguments to `ChangeStrategy::compute_balance` have changed.
+- `zcash_client_backend::scanning`:
+  - `testing::fake_compact_block` is now public.
 - `zcash_client_backend::proto`:
   - `ProposalDecodingError` has a new variant `TransparentMemo`.
 - `zcash_client_backend::zip321::render::amount_str` now takes a

zcash_client_backend/Cargo.toml

@@ -78,12 +78,18 @@ group.workspace = true
 orchard = { workspace = true, optional = true }
 sapling.workspace = true
 
+# - Sync engine
+anyhow = { version = "1", optional = true }
+futures-util = { version = "0.3", optional = true }
+tokio = { version = "1.21.0", features = ["fs", "macros"] }
+
 # - Note commitment trees
 incrementalmerkletree.workspace = true
 shardtree.workspace = true
 
 # - Test dependencies
 proptest = { workspace = true, optional = true }
+jubjub = { workspace = true, optional = true }
 
 # - ZIP 321
 nom = "7"
@@ -116,6 +122,7 @@ shardtree = { workspace = true, features = ["test-dependencies"] }
 zcash_proofs.workspace = true
 zcash_address = { workspace = true, features = ["test-dependencies"] }
 zcash_keys = { workspace = true, features = ["test-dependencies"] }
+tokio = { version = "1.21.0", features = ["rt-multi-thread"] }
 
 time = ">=0.3.22, <0.3.24" # time 0.3.24 has MSRV 1.67
 
@@ -133,9 +140,17 @@ transparent-inputs = [
 ## Enables receiving and spending Orchard funds.
 orchard = ["dep:orchard", "zcash_keys/orchard"]
 
+## Exposes a wallet synchronization function that implements the necessary state machine.
+sync = [
+    "lightwalletd-tonic",
+    "dep:anyhow",
+    "dep:futures-util",
+]
+
 ## Exposes APIs that are useful for testing, such as `proptest` strategies.
 test-dependencies = [
     "dep:proptest",
+    "dep:jubjub",
     "orchard?/test-dependencies",
     "zcash_keys/test-dependencies",
     "zcash_primitives/test-dependencies",