fix(esplora): don't insert genesis from the server's latest blocks

crates/esplora/src/async_ext.rs

@@ -281,7 +281,12 @@ async fn chain_update<S: Sleeper>(
 
     // insert the most recent blocks at the tip to make sure we update the tip and make the update
     // robust.
-    for (&height, &hash) in latest_blocks.iter() {
+    //
+    // Height 0 is skipped: genesis stays immutable in a `CheckPoint`, so an entry agreeing with our
+    // genesis is a no-op while one disagreeing with it panics `insert`, poisoning every later sync.
+    // A server on a different chain is still rejected above, where we fail to find a point of
+    // agreement.
+    for (&height, &hash) in latest_blocks.range(1..) {
         tip = tip.insert(height, hash);
     }
 
@@ -556,7 +561,10 @@ where
 #[cfg(test)]
 #[cfg_attr(coverage_nightly, coverage(off))]
 mod test {
-    use std::{collections::BTreeSet, time::Duration};
+    use std::{
+        collections::{BTreeMap, BTreeSet},
+        time::Duration,
+    };
 
     use bdk_chain::{
         bitcoin::{hashes::Hash, Txid},
@@ -565,7 +573,7 @@ mod test {
     };
     use bdk_core::{bitcoin, ConfirmationBlockTime};
     use bdk_testenv::{anyhow, TestEnv};
-    use esplora_client::Builder;
+    use esplora_client::{BlockHash, Builder};
 
     use crate::async_ext::{chain_update, fetch_latest_blocks};
 
@@ -610,6 +618,34 @@ mod test {
         Ok(())
     }
 
+    // Test that `chain_update` does not panic when the `/blocks` response carries a height-0
+    // entry conflicting with the local genesis.
+    // See <https://github.com/bitcoindevkit/bdk/issues/2341>.
+    #[tokio::test]
+    async fn test_chain_update_ignores_genesis_in_latest_blocks() -> anyhow::Result<()> {
+        // `latest_blocks` covers the local tip height, so no request is made.
+        let client = Builder::new("http://127.0.0.1:1").build_async()?;
+
+        let genesis_hash: BlockHash = h!("genesis");
+        let block_1_hash: BlockHash = h!("block 1");
+        let local_tip = bdk_chain::CheckPoint::new(0, genesis_hash)
+            .push(1, block_1_hash)
+            .expect("height is above genesis");
+
+        let latest_blocks = BTreeMap::from([(0, h!("other genesis")), (1, block_1_hash)]);
+
+        let update = chain_update(&client, &latest_blocks, &local_tip, &BTreeSet::new()).await?;
+
+        assert_eq!(update.height(), 1);
+        assert_eq!(
+            update.get(0).map(|cp| cp.hash()),
+            Some(genesis_hash),
+            "the local genesis must be left untouched",
+        );
+
+        Ok(())
+    }
+
     /// Ensure that update does not remove heights (from original), and all anchor heights are
     /// included.
     #[tokio::test]

crates/esplora/src/blocking_ext.rs

@@ -266,7 +266,12 @@ fn chain_update(
 
     // insert the most recent blocks at the tip to make sure we update the tip and make the update
     // robust.
-    for (&height, &hash) in latest_blocks.iter() {
+    //
+    // Height 0 is skipped: genesis stays immutable in a `CheckPoint`, so an entry agreeing with our
+    // genesis is a no-op while one disagreeing with it panics `insert`, poisoning every later sync.
+    // A server on a different chain is still rejected above, where we fail to find a point of
+    // agreement.
+    for (&height, &hash) in latest_blocks.range(1..) {
         tip = tip.insert(height, hash);
     }
 
@@ -573,6 +578,34 @@ mod test {
         Ok(())
     }
 
+    // Test that `chain_update` does not panic when the `/blocks` response carries a height-0
+    // entry conflicting with the local genesis.
+    // See <https://github.com/bitcoindevkit/bdk/issues/2341>.
+    #[test]
+    fn test_chain_update_ignores_genesis_in_latest_blocks() -> anyhow::Result<()> {
+        // `latest_blocks` covers the local tip height, so no request is made.
+        let client = Builder::new("http://127.0.0.1:1").build_blocking();
+
+        let genesis_hash: BlockHash = h!("genesis");
+        let block_1_hash: BlockHash = h!("block 1");
+        let local_tip = bdk_chain::CheckPoint::new(0, genesis_hash)
+            .push(1, block_1_hash)
+            .expect("height is above genesis");
+
+        let latest_blocks = BTreeMap::from([(0, h!("other genesis")), (1, block_1_hash)]);
+
+        let update = chain_update(&client, &latest_blocks, &local_tip, &BTreeSet::new())?;
+
+        assert_eq!(update.height(), 1);
+        assert_eq!(
+            update.get(0).map(|cp| cp.hash()),
+            Some(genesis_hash),
+            "the local genesis must be left untouched",
+        );
+
+        Ok(())
+    }
+
     /// Ensure that update does not remove heights (from original), and all anchor heights are
     /// included.
     #[test]