Merge branch 'deduplicate-watchers' into 'next'

improvement: deduplicate watchers

See merge request famedly/conduit!201

src/database/abstraction/sqlite.rs

@@ -4,14 +4,14 @@
 use rusqlite::{Connection, DatabaseName::Main, OptionalExtension};
 use std::{
     cell::RefCell,
-    collections::HashMap,
+    collections::{hash_map, HashMap},
     future::Future,
     path::{Path, PathBuf},
     pin::Pin,
     sync::Arc,
 };
 use thread_local::ThreadLocal;
-use tokio::sync::oneshot::Sender;
+use tokio::sync::watch;
 use tracing::debug;
 
 thread_local! {
@@ -126,7 +126,7 @@ fn flush(self: &Arc<Self>) -> Result<()> {
 pub struct SqliteTable {
     engine: Arc<Engine>,
     name: String,
-    watchers: RwLock<HashMap<Vec<u8>, Vec<Sender<()>>>>,
+    watchers: RwLock<HashMap<Vec<u8>, (watch::Sender<()>, watch::Receiver<()>)>>,
 }
 
 type TupleOfBytes = (Vec<u8>, Vec<u8>);
@@ -215,10 +215,8 @@ fn insert(&self, key: &[u8], value: &[u8]) -> Result<()> {
         if !triggered.is_empty() {
             let mut watchers = self.watchers.write();
             for prefix in triggered {
-                if let Some(txs) = watchers.remove(prefix) {
-                    for tx in txs {
-                        let _ = tx.send(());
-                    }
+                if let Some(tx) = watchers.remove(prefix) {
+                    let _ = tx.0.send(());
                 }
             }
         };
@@ -367,17 +365,18 @@ fn scan_prefix<'a>(&'a self, prefix: Vec<u8>) -> Box<dyn Iterator<Item = TupleOf
 
     #[tracing::instrument(skip(self, prefix))]
     fn watch_prefix<'a>(&'a self, prefix: &[u8]) -> Pin<Box<dyn Future<Output = ()> + Send + 'a>> {
-        let (tx, rx) = tokio::sync::oneshot::channel();
-
-        self.watchers
-            .write()
-            .entry(prefix.to_vec())
-            .or_default()
-            .push(tx);
+        let mut rx = match self.watchers.write().entry(prefix.to_vec()) {
+            hash_map::Entry::Occupied(o) => o.get().1.clone(),
+            hash_map::Entry::Vacant(v) => {
+                let (tx, rx) = tokio::sync::watch::channel(());
+                v.insert((tx, rx.clone()));
+                rx
+            }
+        };
 
         Box::pin(async move {
             // Tx is never destroyed
-            rx.await.unwrap();
+            rx.changed().await.unwrap();
         })
     }