execute mempool scanning in separate thread

The mempool scanning procedure can now be launched
in a separate thread, while mempool updates are
applied synchronously in the main thread.

src/electrum.rs

@@ -9,15 +9,17 @@ use rayon::prelude::*;
 use serde_derive::Deserialize;
 use serde_json::{self, json, Value};
 
-use std::collections::{hash_map::Entry, HashMap};
+use std::collections::{hash_map::Entry, HashMap, HashSet};
 use std::fmt;
 use std::iter::FromIterator;
 use std::str::FromStr;
+use std::sync::Arc;
 
 use crate::{
     cache::Cache,
     config::{Config, ELECTRS_VERSION},
     daemon::{self, extract_bitcoind_error, Daemon},
+    mempool::MempoolSyncUpdate,
     merkle::Proof,
     metrics::{self, Histogram, Metrics},
     signals::Signal,
@@ -123,7 +125,7 @@ pub struct Rpc {
     tracker: Tracker,
     cache: Cache,
     rpc_duration: Histogram,
-    daemon: Daemon,
+    daemon: Arc<Daemon>,
     signal: Signal,
     banner: String,
     port: u16,
@@ -147,7 +149,7 @@ impl Rpc {
             tracker,
             cache,
             rpc_duration,
-            daemon,
+            daemon: Arc::new(daemon),
             signal,
             banner: config.server_banner.clone(),
             port: config.electrum_rpc_addr.port(),
@@ -158,12 +160,25 @@ impl Rpc {
         &self.signal
     }
 
+    pub(crate) fn daemon(&self) -> &Arc<Daemon> {
+        &self.daemon
+    }
+
     pub fn new_block_notification(&self) -> Receiver<()> {
         self.daemon.new_block_notification()
     }
 
-    pub fn sync(&mut self) -> Result<bool> {
+    pub fn sync_chain(&mut self) -> Result<bool> {
-        self.tracker.sync(&self.daemon, self.signal.exit_flag())
+        self.tracker
+            .sync_chain(&self.daemon, self.signal.exit_flag())
+    }
+
+    pub(crate) fn mempool_txids(&self) -> HashSet<Txid> {
+        self.tracker.mempool.all_txids()
+    }
+
+    pub(crate) fn mempool_apply(&mut self, sync_update: MempoolSyncUpdate) {
+        self.tracker.mempool.apply_sync_update(sync_update)
     }
 
     pub fn update_client(&self, client: &mut Client) -> Result<Vec<String>> {
@@ -432,7 +447,7 @@ impl Rpc {
     }
 
     fn get_fee_histogram(&self) -> Result<Value> {
-        Ok(json!(self.tracker.fees_histogram()))
+        Ok(json!(self.tracker.mempool.fees_histogram()))
     }
 
     fn server_id(&self) -> String {

src/mempool.rs

@@ -127,6 +127,10 @@ impl Mempool {
         &self.fees
     }
 
+    pub(crate) fn all_txids(&self) -> HashSet<Txid> {
+        HashSet::<Txid>::from_iter(self.entries.keys().copied())
+    }
+
     pub(crate) fn get(&self, txid: &Txid) -> Option<&Entry> {
         self.entries.get(txid)
     }
@@ -186,22 +190,6 @@ impl Mempool {
         }
     }
 
-    pub fn sync(&mut self, daemon: &Daemon, exit_flag: &ExitFlag) {
-        let old_txids = HashSet::<Txid>::from_iter(self.entries.keys().copied());
-
-        let poll_result = MempoolSyncUpdate::poll(daemon, old_txids, exit_flag);
-
-        let sync_update = match poll_result {
-            Ok(sync_update) => sync_update,
-            Err(e) => {
-                warn!("mempool sync failed: {}", e);
-                return;
-            }
-        };
-
-        self.apply_sync_update(sync_update);
-    }
-
     /// Add a transaction entry to the mempool and update the fee histogram.
     fn add_entry(&mut self, entry: Entry) {
         for txi in &entry.tx.input {

src/server.rs

@@ -1,19 +1,23 @@
 use anyhow::{Context, Result};
-use crossbeam_channel::{select, unbounded, Sender};
+use bitcoin::Txid;
+use crossbeam_channel::{bounded, select, unbounded, Receiver, Sender};
 use rayon::prelude::*;
 
 use std::{
-    collections::hash_map::HashMap,
+    collections::{HashMap, HashSet},
     io::{BufRead, BufReader, Write},
     iter::once,
     net::{Shutdown, TcpListener, TcpStream},
+    sync::Arc,
 };
 
 use crate::{
     config::Config,
+    daemon::Daemon,
     electrum::{Client, Rpc},
+    mempool::MempoolSyncUpdate,
     metrics::{self, Metrics},
-    signals::ExitError,
+    signals::{ExitError, ExitFlag},
     thread::spawn,
 };
 
@@ -87,19 +91,58 @@ fn serve() -> Result<()> {
 
     let new_block_rx = rpc.new_block_notification();
     let mut peers = HashMap::<usize, Peer>::new();
+
+    let (mempool_update_tx, mempool_update_rx) = bounded(0);
+    let (mempool_run_tx, mempool_run_rx) = bounded(0);
+
+    let daemon = Arc::clone(rpc.daemon());
+    let exit_flag = rpc.signal().exit_flag().clone();
+    if !config.ignore_mempool {
+        spawn("mempool_sync", move || {
+            mempool_sync_loop(daemon, mempool_update_tx, mempool_run_rx, exit_flag)
+        });
+    }
+
     loop {
         // initial sync and compaction may take a few hours
         while server_rx.is_empty() {
-            let done = duration.observe_duration("sync", || rpc.sync().context("sync failed"))?; // sync a batch of blocks
+            let done =
+                duration.observe_duration("sync", || rpc.sync_chain().context("sync failed"))?; // sync a batch of blocks
             peers = duration.observe_duration("notify", || notify_peers(&rpc, peers)); // peers are disconnected on error
             if !done {
                 continue; // more blocks to sync
             }
+
             if config.sync_once {
                 return Ok(()); // exit after initial sync is done
             }
+
             break;
         }
+
+        select! {
+            // Start an asynchronous scan of the mempool if we aren't already doing so.
+            // You'd expect this might cause unneeded allocations copying mempool TXIDs, but
+            // `rpc.mempool_txids()` is only evaluated if the channel send is doesn't block.
+            send(mempool_run_tx, rpc.mempool_txids()) -> res => {
+                match res {
+                    Ok(_) => (),
+                    Err(_) => warn!("disconnected from mempool scan thread"),
+                };
+            }
+
+            // Received an update to the mempool state. Apply it.
+            recv(mempool_update_rx) -> res => {
+                match res {
+                    Ok(sync_update) => rpc.mempool_apply(sync_update),
+                    Err(_) => warn!("disconnected from mempool scan thread"),
+                };
+            }
+
+            // Mempool scanning in progress, or `ignore_mempool` is true.
+            default => {}
+        };
+
         duration.observe_duration("select", || -> Result<()> {
             select! {
                 // Handle signals for graceful shutdown
@@ -247,3 +290,24 @@ fn recv_loop(peer_id: usize, stream: &TcpStream, server_tx: Sender<Event>) -> Re
     server_tx.send(Event { peer_id, msg })?;
     Ok(())
 }
+
+fn mempool_sync_loop(
+    daemon: Arc<Daemon>,
+    mempool_update_tx: Sender<MempoolSyncUpdate>,
+    mempool_run_rx: Receiver<HashSet<Txid>>,
+    exit_flag: ExitFlag,
+) -> Result<()> {
+    while let Ok(old_txids) = mempool_run_rx.recv() {
+        match MempoolSyncUpdate::poll(&daemon, old_txids, &exit_flag) {
+            Ok(sync_update) => {
+                let _ = mempool_update_tx.send(sync_update);
+            }
+            Err(e) => {
+                warn!("mempool sync failed: {}", e);
+                continue;
+            }
+        };
+    }
+    debug!("mempool sync thread exiting");
+    Ok(())
+}

src/tracker.rs

@@ -13,7 +13,7 @@ use crate::{
     daemon::Daemon,
     db::DBStore,
     index::Index,
-    mempool::{FeeHistogram, Mempool},
+    mempool::Mempool,
     metrics::Metrics,
     signals::ExitFlag,
     status::{Balance, ScriptHashStatus, UnspentEntry},
@@ -21,10 +21,9 @@ use crate::{
 
 /// Electrum protocol subscriptions' tracker
 pub struct Tracker {
+    pub(crate) mempool: Mempool,
     index: Index,
-    mempool: Mempool,
     metrics: Metrics,
-    ignore_mempool: bool,
 }
 
 pub(crate) enum Error {
@@ -51,7 +50,6 @@ impl Tracker {
             .context("failed to open index")?,
             mempool: Mempool::new(&metrics),
             metrics,
-            ignore_mempool: config.ignore_mempool,
         })
     }
 
@@ -59,10 +57,6 @@ impl Tracker {
         self.index.chain()
     }
 
-    pub(crate) fn fees_histogram(&self) -> &FeeHistogram {
-        self.mempool.fees_histogram()
-    }
-
     pub(crate) fn metrics(&self) -> &Metrics {
         &self.metrics
     }
@@ -71,12 +65,8 @@ impl Tracker {
         status.get_unspent(self.index.chain())
     }
 
-    pub(crate) fn sync(&mut self, daemon: &Daemon, exit_flag: &ExitFlag) -> Result<bool> {
+    pub(crate) fn sync_chain(&mut self, daemon: &Daemon, exit_flag: &ExitFlag) -> Result<bool> {
         let done = self.index.sync(daemon, exit_flag)?;
-        if done && !self.ignore_mempool {
-            self.mempool.sync(daemon, exit_flag);
-            // TODO: double check tip - and retry on diff
-        }
         Ok(done)
     }