fix: Only ask for blocks within the reorg limit

Instead of creating a block locator all the way back to the genesis
block, only ask for blocks within the reorg limit (99 blocks).

Use the reorg limit as the final locator. (Or if the chain is less
than 99 blocks, use the genesis block.)

Fixes some instances of #818 at very small block heights.

zebra-state/src/lib.rs

@@ -31,6 +31,19 @@ use zebra_chain::{
 pub mod in_memory;
 pub mod on_disk;
 
+/// The maturity threshold for transparent coinbase outputs.
+///
+/// A transaction MUST NOT spend a transparent output of a coinbase transaction
+/// from a block less than 100 blocks prior to the spend. Note that transparent
+/// outputs of coinbase transactions include Founders' Reward outputs.
+const MIN_TRASPARENT_COINBASE_MATURITY: BlockHeight = BlockHeight(100);
+
+/// The maximum chain reorganisation height.
+///
+/// Allowing reorganisations past this height could allow double-spends of
+/// coinbase transactions.
+const MAX_BLOCK_REORG_HEIGHT: BlockHeight = BlockHeight(MIN_TRASPARENT_COINBASE_MATURITY.0 - 1);
+
 /// Configuration for the state service.
 #[derive(Clone, Debug, Deserialize, Serialize)]
 #[serde(deny_unknown_fields)]
@@ -142,13 +155,27 @@ pub enum Response {
 
 /// Get the heights of the blocks for constructing a block_locator list
 fn block_locator_heights(tip_height: BlockHeight) -> impl Iterator<Item = BlockHeight> {
+    // Stop at the reorg limit, or the genesis block.
+    let min_locator_height = tip_height
+        .0
+        .checked_sub(MAX_BLOCK_REORG_HEIGHT.0)
+        .map(BlockHeight)
+        .unwrap_or(BlockHeight(0));
     let locators = iter::successors(Some(1u32), |h| h.checked_mul(2))
         .flat_map(move |step| tip_height.0.checked_sub(step))
-        .filter(|&height| height != 0)
+        .filter(move |&height| height > min_locator_height.0)
         .map(BlockHeight);
-    iter::once(tip_height)
+    let locators = iter::once(tip_height)
         .chain(locators)
-        .chain(iter::once(BlockHeight(0)))
+        .chain(iter::once(min_locator_height));
+    let locators: Vec<_> = locators.collect();
+    tracing::info!(
+        ?tip_height,
+        ?min_locator_height,
+        ?locators,
+        "created block locator"
+    );
+    locators.into_iter()
 }
 
 /// The error type for the State Service.