Merge bitcoindevkit#2053: Remove panic from stop gap scan loop

745f805 fix: remove panic from stop gap scan loop (Matthew)

Pull request description:

  <!-- You can erase any parts of this template not applicable to your Pull Request. -->

  ### Description

  This PR addresses a panic path in stop gap scan loop. Removes `expect` used to compute gap boundary and instead tracks `consecutive_unused` with `gap_limit = stop_gap.max(1)` to decide when to stop scanning.

  Spurred by trying to address bitcoindevkit/bdk_wallet#30 for Esplora

  ### Notes to the reviewers

  Open to any and all feedback on this.

  Behavior is unchanged for typical cases, it just avoids a panic in the control flow.

  ### Changelog notice

  ```
  ### Fix:
  - Avoid a panic in the Esplora stop‑gap scan loop by tracking consecutive unused scripts to compute the gap boundary.
  ```

  ### Checklists

  #### All Submissions:

  * [x] I followed the [contribution guidelines](https://github.com/bitcoindevkit/bdk/blob/master/CONTRIBUTING.md)

  #### New Features:

  * [ ] I've added tests for the new feature
  * [ ] I've added docs for the new feature

  #### Bugfixes:

  * [ ] This pull request breaks the existing API
  * [ ] I've added tests to reproduce the issue which are now passing
  * [ ] I'm linking the issue being fixed by this PR

ACKs for top commit:
  luisschwab:
    ACK 745f805
  evanlinjin:
    ACK 745f805

Tree-SHA512: 50b45955d7807996d871453dac61caf1ce25e85d31e261ebd670749b6370e92318d43070147aa67df5b4a904bd9b353b08711e043007c79a125cfd7d9a108da3

Author: evanlinjin

crates/esplora/src/async_ext.rs

@@ -314,8 +314,11 @@ where
     type TxsOfSpkIndex = (u32, Vec<esplora_client::Tx>, HashSet<Txid>);
 
     let mut update = TxUpdate::<ConfirmationBlockTime>::default();
-    let mut last_index = Option::<u32>::None;
     let mut last_active_index = Option::<u32>::None;
+    // Use consecutive_unused so unused count drives stop gap.
+    let mut consecutive_unused = 0usize;
+    // Treat stop_gap = 0 as 1 while preserving original semantics for other values.
+    let gap_limit = stop_gap.max(1);
 
     loop {
         let handles = keychain_spks
@@ -352,8 +355,10 @@ where
         }
 
         for (index, txs, evicted) in handles.try_collect::<Vec<TxsOfSpkIndex>>().await? {
-            last_index = Some(index);
-            if !txs.is_empty() {
+            if txs.is_empty() {
+                consecutive_unused = consecutive_unused.saturating_add(1);
+            } else {
+                consecutive_unused = 0;
                 last_active_index = Some(index);
             }
             for tx in txs {
@@ -368,13 +373,7 @@ where
                 .extend(evicted.into_iter().map(|txid| (txid, start_time)));
         }
 
-        let last_index = last_index.expect("Must be set since handles wasn't empty.");
-        let gap_limit_reached = if let Some(i) = last_active_index {
-            last_index >= i.saturating_add(stop_gap as u32)
-        } else {
-            last_index + 1 >= stop_gap as u32
-        };
-        if gap_limit_reached {
+        if consecutive_unused >= gap_limit {
             break;
         }
     }
@@ -596,7 +595,10 @@ mod test {
         let res = chain_update(&client, &latest_blocks, &cp, &anchors).await;
         use esplora_client::Error;
         assert!(
-            matches!(*res.unwrap_err(), Error::HeaderHashNotFound(hash) if hash == genesis_hash),
+            matches!(
+                *res.unwrap_err(),
+                Error::HeaderHashNotFound(hash) if hash == genesis_hash
+            ),
             "`chain_update` should error if it can't connect to the local CP",
         );
 

crates/esplora/src/blocking_ext.rs

@@ -282,8 +282,11 @@ fn fetch_txs_with_keychain_spks<I: Iterator<Item = Indexed<SpkWithExpectedTxids>
     type TxsOfSpkIndex = (u32, Vec<esplora_client::Tx>, HashSet<Txid>);
 
     let mut update = TxUpdate::<ConfirmationBlockTime>::default();
-    let mut last_index = Option::<u32>::None;
     let mut last_active_index = Option::<u32>::None;
+    // Use consecutive_unused so unused count drives stop gap.
+    let mut consecutive_unused = 0usize;
+    // Treat stop_gap = 0 as 1 while preserving original semantics for other values.
+    let gap_limit = stop_gap.max(1);
 
     loop {
         let handles = keychain_spks
@@ -321,8 +324,10 @@ fn fetch_txs_with_keychain_spks<I: Iterator<Item = Indexed<SpkWithExpectedTxids>
 
         for handle in handles {
             let (index, txs, evicted) = handle.join().expect("thread must not panic")?;
-            last_index = Some(index);
-            if !txs.is_empty() {
+            if txs.is_empty() {
+                consecutive_unused = consecutive_unused.saturating_add(1);
+            } else {
+                consecutive_unused = 0;
                 last_active_index = Some(index);
             }
             for tx in txs {
@@ -337,13 +342,7 @@ fn fetch_txs_with_keychain_spks<I: Iterator<Item = Indexed<SpkWithExpectedTxids>
                 .extend(evicted.into_iter().map(|txid| (txid, start_time)));
         }
 
-        let last_index = last_index.expect("Must be set since handles wasn't empty.");
-        let gap_limit_reached = if let Some(i) = last_active_index {
-            last_index >= i.saturating_add(stop_gap as u32)
-        } else {
-            last_index + 1 >= stop_gap as u32
-        };
-        if gap_limit_reached {
+        if consecutive_unused >= gap_limit {
             break;
         }
     }