fix(checkpoints): settle checkpoints past rollback window

zebra-chain/src/parameters/constants.rs

@@ -1,6 +1,7 @@
 //! Definitions of Zebra chain constants, including:
 //! - slow start interval,
-//! - slow start shift
+//! - slow start shift,
+//! - maximum reorg height
 
 use crate::block::Height;
 
@@ -16,6 +17,18 @@ pub const SLOW_START_INTERVAL: Height = Height(20_000);
 /// This calculation is exact, because `SLOW_START_INTERVAL` is divisible by 2.
 pub const SLOW_START_SHIFT: Height = Height(SLOW_START_INTERVAL.0 / 2);
 
+/// The maximum chain reorganisation height.
+///
+/// This threshold determines the maximum length of the best non-finalized
+/// chain. Once the chain grows past this height, Zebra finalizes its oldest
+/// blocks; deeper reorganisations are outside Zebra's rollback window.
+///
+/// This is a local-only node policy; it is not part of consensus. The window is
+/// sized as a defence-in-depth measure against sustained consensus splits.
+//
+// TODO: change to HeightDiff
+pub const MAX_BLOCK_REORG_HEIGHT: u32 = 1000;
+
 /// Magic numbers used to identify different Zcash networks.
 pub mod magics {
     use crate::parameters::network::magic::Magic;

zebra-state/src/constants.rs

@@ -25,7 +25,7 @@ pub use zebra_chain::transparent::MIN_TRANSPARENT_COINBASE_MATURITY;
 /// sized as a defence-in-depth measure against sustained consensus splits.
 //
 // TODO: change to HeightDiff
-pub const MAX_BLOCK_REORG_HEIGHT: u32 = 1000;
+pub const MAX_BLOCK_REORG_HEIGHT: u32 = zebra_chain::parameters::constants::MAX_BLOCK_REORG_HEIGHT;
 
 /// The directory name used to distinguish the state database from Zebra's other databases or flat files.
 pub const STATE_DATABASE_KIND: &str = "state";

zebra-utils/src/bin/zebra-checkpoints/main.rs

@@ -23,6 +23,7 @@ use structopt::StructOpt;
 
 use zebra_chain::{
     block::{self, Block, Height, HeightDiff, TryIntoHeight},
+    parameters::constants::MAX_BLOCK_REORG_HEIGHT,
     serialization::ZcashDeserializeInto,
     transparent::MIN_TRANSPARENT_COINBASE_MATURITY,
 };
@@ -32,6 +33,11 @@ use zebra_node_services::{
 };
 use zebra_utils::init_tracing;
 
+const _: () = assert!(
+    MAX_BLOCK_REORG_HEIGHT >= MIN_TRANSPARENT_COINBASE_MATURITY,
+    "checkpoint settlement margin must be at least the coinbase maturity",
+);
+
 pub mod args;
 
 use args::{Args, Backend, Transport};
@@ -162,14 +168,14 @@ async fn main() -> Result<()> {
         .try_into_height()
         .expect("height: unexpected invalid value, missing field, or field type");
 
-    // Checkpoints must be on the main chain, so we skip blocks that are within the
-    // Zcash reorg limit.
-    let height_limit = height_limit - HeightDiff::from(MIN_TRANSPARENT_COINBASE_MATURITY);
+    // Checkpoints must be on a settled part of the best chain, so skip blocks
+    // within Zebra's rollback window.
+    let height_limit = height_limit - HeightDiff::from(MAX_BLOCK_REORG_HEIGHT);
     let height_limit = height_limit
         .ok_or_else(|| {
             eyre!(
                 "checkpoint generation needs at least {:?} blocks",
-                MIN_TRANSPARENT_COINBASE_MATURITY
+                MAX_BLOCK_REORG_HEIGHT
             )
         })
         .with_suggestion(|| "Hint: wait for the node to sync more blocks")?;