Add watcher prune task for sqlite

src/database.rs

@@ -774,6 +774,7 @@ impl Database {
         #[cfg(feature = "sqlite")]
         {
             Self::start_wal_clean_task(Arc::clone(&db), config).await;
+            Self::start_watcher_prune_task(Arc::clone(&db)).await;
         }
 
         Ok(db)
@@ -961,6 +962,25 @@ impl Database {
             }
         });
     }
+
+    #[cfg(feature = "sqlite")]
+    #[tracing::instrument(skip(db))]
+    pub async fn start_watcher_prune_task(db: Arc<TokioRwLock<Self>>) {
+        use std::time::Duration;
+        use tokio::time::interval;
+
+        tokio::spawn(async move {
+            let mut i = interval(Duration::from_secs(60));
+
+            loop {
+                i.tick().await;
+
+                for table in db.read().await._db.tables.read().values() {
+                    table.prune_dead_watchers();
+                }
+            }
+        });
+    }
 }
 
 pub struct DatabaseGuard(OwnedRwLockReadGuard<Database>);

src/database/abstraction/sqlite.rs

@@ -46,6 +46,8 @@ pub struct Engine {
 
     path: PathBuf,
     cache_size_per_thread: u32,
+
+    pub(in crate::database) tables: RwLock<HashMap<String, Arc<SqliteTable>>>,
 }
 
 impl Engine {
@@ -102,19 +104,29 @@ impl DatabaseEngine for Engine {
             read_iterator_conn_tls: ThreadLocal::new(),
             path,
             cache_size_per_thread,
+            tables: RwLock::new(HashMap::new()),
         });
 
         Ok(arc)
     }
 
     fn open_tree(self: &Arc<Self>, name: &str) -> Result<Arc<dyn Tree>> {
-        self.write_lock().execute(&format!("CREATE TABLE IF NOT EXISTS {} ( \"key\" BLOB PRIMARY KEY, \"value\" BLOB NOT NULL )", name), [])?;
+        Ok(match self.tables.write().entry(name.to_string()) {
+            hash_map::Entry::Occupied(o) => o.get().clone(),
+            hash_map::Entry::Vacant(v) => {
+                self.write_lock().execute(&format!("CREATE TABLE IF NOT EXISTS {} ( \"key\" BLOB PRIMARY KEY, \"value\" BLOB NOT NULL )", name), [])?;
+
+                let table = Arc::new(SqliteTable {
+                    engine: Arc::clone(self),
+                    name: name.to_owned(),
+                    watchers: RwLock::new(HashMap::new()),
+                });
 
-        Ok(Arc::new(SqliteTable {
+                v.insert(table.clone());
-            engine: Arc::clone(self),
+
-            name: name.to_owned(),
+                table
-            watchers: RwLock::new(HashMap::new()),
+            }
-        }))
+        })
     }
 
     fn flush(self: &Arc<Self>) -> Result<()> {
@@ -375,7 +387,8 @@ impl Tree for SqliteTable {
         };
 
         Box::pin(async move {
-            // Tx is never destroyed
+            // Tx will be destroyed if receiver_count = 1,
+            // and self.watchers is held with write lock
             rx.changed().await.unwrap();
         })
     }
@@ -390,3 +403,37 @@ impl Tree for SqliteTable {
         Ok(())
     }
 }
+
+impl SqliteTable {
+    pub fn prune_dead_watchers(&self) {
+        let watchers = self.watchers.read();
+        let mut interesting = Vec::new();
+        for (key, (tx, _)) in watchers.iter() {
+            if tx.receiver_count() == 1 {
+                // We do read mode first to get all "interesting" keys, not blocking other watch threads.
+                interesting.push(key.clone())
+            }
+        }
+
+        drop(watchers);
+
+        if !interesting.is_empty() {
+            let mut watchers = self.watchers.write();
+            let mut cleared = 0;
+            for prefix in interesting {
+                // Test for occupied, there is only the slightest chance this entry has been deleted inbetween then and now.
+                if let hash_map::Entry::Occupied(o) = watchers.entry(prefix) {
+                    // Check one last time, its possible a receiver is cloned inbetween then and now.
+                    if o.get().0.receiver_count() == 1 {
+                        // Receiver == 1, this is the receiver bundled in the entry, so it's okay to drop this.
+                        o.remove_entry();
+
+                        cleared += 1;
+                    }
+                }
+            }
+
+            debug!("Cleared {} dead watchers.", cleared)
+        }
+    }
+}