fix: define and document stop_gap

- changes the code implementation to "the maximum number of consecutive unused addresses"
  in esplora async and blocking extensions.
- renames `past_gap_limit` to `gap_limit_reached` to indicate we want to break once the gap
  limit is reached and not go further in `full_scan`, as suggested in
  bitcoindevkit#1227 (comment)
- change the tests according to the new implementation.
- add notes on what `stop_gap` means and links to convergent definition in other
  Bitcoin-related software.

Closes bitcoindevkit#1227.

crates/esplora/src/async_ext.rs

@@ -52,6 +52,17 @@ pub trait EsploraAsyncExt {
     /// The full scan for each keychain stops after a gap of `stop_gap` script pubkeys with no associated
     /// transactions. `parallel_requests` specifies the max number of HTTP requests to make in
     /// parallel.
+    ///
+    /// ## Note
+    ///
+    /// `stop_gap` is defined as "the maximum number of consecutive unused addresses".
+    /// For example, with `stop_gap = 3`, `full_scan` will keep searching for script pubkeys
+    /// until it encounters 3 consecutive script pubkeys with no associated transactions.
+    ///
+    /// This follows the same approach as other Bitcoin-related software,
+    /// such as [Electrum](https://electrum.readthedocs.io/en/latest/faq.html#what-is-the-gap-limit),
+    /// [BTCPay Server](https://docs.btcpayserver.org/FAQ/Wallet/#the-gap-limit-problem),
+    /// and [Sparrow](https://www.sparrowwallet.com/docs/faq.html#ive-restored-my-wallet-but-some-of-my-funds-are-missing).
     async fn full_scan<K: Ord + Clone + Send>(
         &self,
         keychain_spks: BTreeMap<
@@ -226,12 +237,12 @@ impl EsploraAsyncExt for esplora_client::AsyncClient {
                 }
 
                 let last_index = last_index.expect("Must be set since handles wasn't empty.");
-                let past_gap_limit = if let Some(i) = last_active_index {
+                let gap_limit_reached = if let Some(i) = last_active_index {
                     last_index > i.saturating_add(stop_gap as u32)
                 } else {
-                    last_index >= stop_gap as u32
+                    last_index + 1 >= stop_gap as u32
                 };
-                if past_gap_limit {
+                if gap_limit_reached {
                     break;
                 }
             }

crates/esplora/src/blocking_ext.rs

@@ -50,6 +50,17 @@ pub trait EsploraExt {
     /// The full scan for each keychain stops after a gap of `stop_gap` script pubkeys with no associated
     /// transactions. `parallel_requests` specifies the max number of HTTP requests to make in
     /// parallel.
+    ///
+    /// ## Note
+    ///
+    /// `stop_gap` is defined as "the maximum number of consecutive unused addresses".
+    /// For example, with `stop_gap = 3`, `full_scan` will keep searching for script pubkeys
+    /// until it encounters 3 consecutive script pubkeys with no associated transactions.
+    ///
+    /// This follows the same approach as other Bitcoin-related software,
+    /// such as [Electrum](https://electrum.readthedocs.io/en/latest/faq.html#what-is-the-gap-limit),
+    /// [BTCPay Server](https://docs.btcpayserver.org/FAQ/Wallet/#the-gap-limit-problem),
+    /// and [Sparrow](https://www.sparrowwallet.com/docs/faq.html#ive-restored-my-wallet-but-some-of-my-funds-are-missing).
     fn full_scan<K: Ord + Clone>(
         &self,
         keychain_spks: BTreeMap<K, impl IntoIterator<Item = (u32, ScriptBuf)>>,
@@ -216,12 +227,12 @@ impl EsploraExt for esplora_client::BlockingClient {
                 }
 
                 let last_index = last_index.expect("Must be set since handles wasn't empty.");
-                let past_gap_limit = if let Some(i) = last_active_index {
+                let gap_limit_reached = if let Some(i) = last_active_index {
                     last_index > i.saturating_add(stop_gap as u32)
                 } else {
-                    last_index >= stop_gap as u32
+                    last_index + 1 >= stop_gap as u32
                 };
-                if past_gap_limit {
+                if gap_limit_reached {
                     break;
                 }
             }

crates/esplora/tests/async_ext.rs

@@ -186,12 +186,12 @@ pub async fn test_async_update_tx_graph_gap_limit() -> anyhow::Result<()> {
         sleep(Duration::from_millis(10))
     }
 
-    // A scan with a gap limit of 2 won't find the transaction, but a scan with a gap limit of 3
+    // A scan with a gap limit of 3 won't find the transaction, but a scan with a gap limit of 4
     // will.
-    let (graph_update, active_indices) = env.client.full_scan(keychains.clone(), 2, 1).await?;
+    let (graph_update, active_indices) = env.client.full_scan(keychains.clone(), 3, 1).await?;
     assert!(graph_update.full_txs().next().is_none());
     assert!(active_indices.is_empty());
-    let (graph_update, active_indices) = env.client.full_scan(keychains.clone(), 3, 1).await?;
+    let (graph_update, active_indices) = env.client.full_scan(keychains.clone(), 4, 1).await?;
     assert_eq!(graph_update.full_txs().next().unwrap().txid, txid_4th_addr);
     assert_eq!(active_indices[&0], 3);
 

crates/esplora/tests/blocking_ext.rs

@@ -214,12 +214,12 @@ pub fn test_update_tx_graph_gap_limit() -> anyhow::Result<()> {
         sleep(Duration::from_millis(10))
     }
 
-    // A scan with a gap limit of 2 won't find the transaction, but a scan with a gap limit of 3
+    // A scan with a gap limit of 3 won't find the transaction, but a scan with a gap limit of 4
     // will.
-    let (graph_update, active_indices) = env.client.full_scan(keychains.clone(), 2, 1)?;
+    let (graph_update, active_indices) = env.client.full_scan(keychains.clone(), 3, 1)?;
     assert!(graph_update.full_txs().next().is_none());
     assert!(active_indices.is_empty());
-    let (graph_update, active_indices) = env.client.full_scan(keychains.clone(), 3, 1)?;
+    let (graph_update, active_indices) = env.client.full_scan(keychains.clone(), 4, 1)?;
     assert_eq!(graph_update.full_txs().next().unwrap().txid, txid_4th_addr);
     assert_eq!(active_indices[&0], 3);