Shorten mempool lock holding for update

src/bin/electrs.rs

@@ -74,7 +74,18 @@ fn run_server(config: Arc<Config>) -> Result<()> {
         &metrics,
         Arc::clone(&config),
     )));
-    mempool.write().unwrap().update(&daemon)?;
+    loop {
+        match Mempool::update(&mempool, &daemon) {
+            Ok(_) => break,
+            Err(e) => {
+                warn!(
+                    "Error performing initial mempool update, trying again in 5 seconds: {}",
+                    e.display_chain()
+                );
+                signal.wait(Duration::from_secs(5), false)?;
+            }
+        }
+    }
 
     #[cfg(feature = "liquid")]
     let asset_db = config.asset_db_path.as_ref().map(|db_dir| {
@@ -136,7 +147,13 @@ fn run_server(config: Arc<Config>) -> Result<()> {
         };
 
         // Update mempool
-        mempool.write().unwrap().update(&daemon)?;
+        if let Err(e) = Mempool::update(&mempool, &daemon) {
+            // Log the error if the result is an Err
+            warn!(
+                "Error updating mempool, skipping mempool update: {}",
+                e.display_chain()
+            );
+        }
 
         // Update subscribed clients
         electrum_server.notify();

src/new_index/mempool.rs

@@ -9,7 +9,7 @@ use elements::{encode::serialize, AssetId};
 use std::collections::{BTreeMap, BTreeSet, HashMap, HashSet};
 use std::iter::FromIterator;
 use std::ops::Bound::{Excluded, Unbounded};
-use std::sync::Arc;
+use std::sync::{Arc, RwLock};
 use std::time::{Duration, Instant};
 
 use crate::chain::{deserialize, Network, OutPoint, Transaction, TxOut, Txid};
@@ -343,46 +343,67 @@ impl Mempool {
         &self.backlog_stats.0
     }
 
-    pub fn update(&mut self, daemon: &Daemon) -> Result<()> {
-        let _timer = self.latency.with_label_values(&["update"]).start_timer();
-        let new_txids = daemon
+    pub fn unique_txids(&self) -> HashSet<Txid> {
+        return HashSet::from_iter(self.txstore.keys().cloned());
+    }
+
+    pub fn update(mempool: &RwLock<Mempool>, daemon: &Daemon) -> Result<()> {
+        // 1. Start the metrics timer and get the current mempool txids
+        // [LOCK] Takes read lock for whole scope.
+        let (_timer, old_txids) = {
+            let mempool = mempool.read().unwrap();
+            (
+                mempool.latency.with_label_values(&["update"]).start_timer(),
+                mempool.unique_txids(),
+            )
+        };
+
+        // 2. Get all the mempool txids from the RPC.
+        // [LOCK] No lock taken. Wait for RPC request. Get lists of remove/add txes.
+        let all_txids = daemon
             .getmempooltxids()
             .chain_err(|| "failed to update mempool from daemon")?;
-        let old_txids = HashSet::from_iter(self.txstore.keys().cloned());
-        let to_remove: HashSet<&Txid> = old_txids.difference(&new_txids).collect();
-
-        // Download and add new transactions from bitcoind's mempool
-        let txids: Vec<&Txid> = new_txids.difference(&old_txids).collect();
-        let to_add = match daemon.gettransactions(&txids) {
-            Ok(txs) => txs,
-            Err(err) => {
-                warn!("failed to get {} transactions: {}", txids.len(), err); // e.g. new block or RBF
-                return Ok(()); // keep the mempool until next update()
+        let txids_to_remove: HashSet<&Txid> = old_txids.difference(&all_txids).collect();
+        let txids_to_add: Vec<&Txid> = all_txids.difference(&old_txids).collect();
+
+        // 3. Remove missing transactions. Even if we are unable to download new transactions from
+        // the daemon, we still want to remove the transactions that are no longer in the mempool.
+        // [LOCK] Write lock is released at the end of the call to remove().
+        mempool.write().unwrap().remove(txids_to_remove);
+
+        // 4. Download the new transactions from the daemon's mempool
+        // [LOCK] No lock taken, waiting for RPC response.
+        let txs_to_add = daemon
+            .gettransactions(&txids_to_add)
+            .chain_err(|| format!("failed to get {} transactions", txids_to_add.len()))?;
+
+        // 4. Update local mempool to match daemon's state
+        // [LOCK] Takes Write lock for whole scope.
+        {
+            let mut mempool = mempool.write().unwrap();
+            // Add new transactions
+            if txs_to_add.len() > mempool.add(txs_to_add) {
+                debug!("Mempool update added less transactions than expected");
             }
-        };
-        // Add new transactions
-        if to_add.len() > self.add(to_add) {
-            debug!("Mempool update added less transactions than expected");
-        }
-        // Remove missing transactions
-        self.remove(to_remove);
 
-        self.count
-            .with_label_values(&["txs"])
-            .set(self.txstore.len() as f64);
+            mempool
+                .count
+                .with_label_values(&["txs"])
+                .set(mempool.txstore.len() as f64);
+
+            // Update cached backlog stats (if expired)
+            if mempool.backlog_stats.1.elapsed()
+                > Duration::from_secs(mempool.config.mempool_backlog_stats_ttl)
+            {
+                let _timer = mempool
+                    .latency
+                    .with_label_values(&["update_backlog_stats"])
+                    .start_timer();
+                mempool.backlog_stats = (BacklogStats::new(&mempool.feeinfo), Instant::now());
+            }
 
-        // Update cached backlog stats (if expired)
-        if self.backlog_stats.1.elapsed()
-            > Duration::from_secs(self.config.mempool_backlog_stats_ttl)
-        {
-            let _timer = self
-                .latency
-                .with_label_values(&["update_backlog_stats"])
-                .start_timer();
-            self.backlog_stats = (BacklogStats::new(&self.feeinfo), Instant::now());
+            Ok(())
         }
-
-        Ok(())
     }
 
     pub fn add_by_txid(&mut self, daemon: &Daemon, txid: &Txid) -> Result<()> {
@@ -418,8 +439,12 @@ impl Mempool {
         // Phase 1: add to txstore
         for tx in txs {
             let txid = tx.txid();
-            txids.push(txid);
-            self.txstore.insert(txid, tx);
+            // Only push if it doesn't already exist.
+            // This is important now that update doesn't lock during
+            // the entire function body.
+            if self.txstore.insert(txid, tx).is_none() {
+                txids.push(txid);
+            }
         }
 
         // Phase 2: index history and spend edges (some txos can be missing)