switch Iterator to Send + Iterator in return types

This changes the SQLite implementation quite a bit. It may potentially
reduce performance, but allows using the new async API and removes some
usage of unsafe

src/database/abstraction.rs

@@ -50,13 +50,13 @@ pub trait KvTree: Send + Sync {
 
     fn remove(&self, key: &[u8]) -> Result<()>;
 
-    fn iter<'a>(&'a self) -> Box<dyn Iterator<Item = (Vec<u8>, Vec<u8>)> + 'a>;
+    fn iter<'a>(&'a self) -> Box<dyn Send + Iterator<Item = (Vec<u8>, Vec<u8>)> + 'a>;
 
     fn iter_from<'a>(
         &'a self,
         from: &[u8],
         backwards: bool,
-    ) -> Box<dyn Iterator<Item = (Vec<u8>, Vec<u8>)> + 'a>;
+    ) -> Box<dyn Send + Iterator<Item = (Vec<u8>, Vec<u8>)> + 'a>;
 
     fn increment(&self, key: &[u8]) -> Result<Vec<u8>>;
     fn increment_batch(&self, iter: &mut dyn Iterator<Item = Vec<u8>>) -> Result<()>;
@@ -64,7 +64,7 @@ pub trait KvTree: Send + Sync {
     fn scan_prefix<'a>(
         &'a self,
         prefix: Vec<u8>,
-    ) -> Box<dyn Iterator<Item = (Vec<u8>, Vec<u8>)> + 'a>;
+    ) -> Box<dyn Send + Iterator<Item = (Vec<u8>, Vec<u8>)> + 'a>;
 
     fn watch_prefix<'a>(&'a self, prefix: &[u8]) -> Pin<Box<dyn Future<Output = ()> + Send + 'a>>;
 

src/database/abstraction/heed.rs

@@ -74,7 +74,7 @@ impl EngineTree {
         tree: Arc<heed::UntypedDatabase>,
         from: Vec<u8>,
         backwards: bool,
-    ) -> Box<dyn Iterator<Item = TupleOfBytes> + Send + Sync> {
+    ) -> Box<dyn Send + Iterator<Item = TupleOfBytes> + Send + Sync> {
         let (s, r) = bounded::<TupleOfBytes>(100);
         let engine = Arc::clone(&self.engine);
 
@@ -150,7 +150,7 @@ impl Tree for EngineTree {
         Ok(())
     }
 
-    fn iter<'a>(&'a self) -> Box<dyn Iterator<Item = (Vec<u8>, Vec<u8>)> + Send + 'a> {
+    fn iter<'a>(&'a self) -> Box<dyn Send + Iterator<Item = (Vec<u8>, Vec<u8>)> + Send + 'a> {
         self.iter_from(&[], false)
     }
 
@@ -158,7 +158,7 @@ impl Tree for EngineTree {
         &self,
         from: &[u8],
         backwards: bool,
-    ) -> Box<dyn Iterator<Item = (Vec<u8>, Vec<u8>)> + Send> {
+    ) -> Box<dyn Send + Iterator<Item = (Vec<u8>, Vec<u8>)> + Send> {
         self.iter_from_thread(Arc::clone(&self.tree), from.to_vec(), backwards)
     }
 
@@ -181,7 +181,7 @@ impl Tree for EngineTree {
     fn scan_prefix<'a>(
         &'a self,
         prefix: Vec<u8>,
-    ) -> Box<dyn Iterator<Item = (Vec<u8>, Vec<u8>)> + Send + 'a> {
+    ) -> Box<dyn Send + Iterator<Item = (Vec<u8>, Vec<u8>)> + Send + 'a> {
         Box::new(
             self.iter_from(&prefix, false)
                 .take_while(move |(key, _)| key.starts_with(&prefix)),

src/database/abstraction/persy.rs

@@ -113,7 +113,7 @@ impl KvTree for PersyTree {
         Ok(())
     }
 
-    fn iter<'a>(&'a self) -> Box<dyn Iterator<Item = (Vec<u8>, Vec<u8>)> + 'a> {
+    fn iter<'a>(&'a self) -> Box<dyn Send + Iterator<Item = (Vec<u8>, Vec<u8>)> + 'a> {
         let iter = self.persy.range::<ByteVec, ByteVec, _>(&self.name, ..);
         match iter {
             Ok(iter) => Box::new(iter.filter_map(|(k, v)| {
@@ -132,7 +132,7 @@ impl KvTree for PersyTree {
         &'a self,
         from: &[u8],
         backwards: bool,
-    ) -> Box<dyn Iterator<Item = (Vec<u8>, Vec<u8>)> + 'a> {
+    ) -> Box<dyn Send + Iterator<Item = (Vec<u8>, Vec<u8>)> + 'a> {
         let range = if backwards {
             self.persy
                 .range::<ByteVec, ByteVec, _>(&self.name, ..=ByteVec::from(from))
@@ -168,7 +168,7 @@ impl KvTree for PersyTree {
     fn scan_prefix<'a>(
         &'a self,
         prefix: Vec<u8>,
-    ) -> Box<dyn Iterator<Item = (Vec<u8>, Vec<u8>)> + 'a> {
+    ) -> Box<dyn Send + Iterator<Item = (Vec<u8>, Vec<u8>)> + 'a> {
         let range_prefix = ByteVec::from(prefix.clone());
         let range = self
             .persy

src/database/abstraction/rocksdb.rs

@@ -175,7 +175,7 @@ impl KvTree for RocksDbEngineTree<'_> {
             .delete_cf_opt(&self.cf(), key, &writeoptions)?)
     }
 
-    fn iter<'a>(&'a self) -> Box<dyn Iterator<Item = (Vec<u8>, Vec<u8>)> + 'a> {
+    fn iter<'a>(&'a self) -> Box<dyn Send + Iterator<Item = (Vec<u8>, Vec<u8>)> + 'a> {
         let readoptions = rocksdb::ReadOptions::default();
 
         Box::new(
@@ -191,7 +191,7 @@ impl KvTree for RocksDbEngineTree<'_> {
         &'a self,
         from: &[u8],
         backwards: bool,
-    ) -> Box<dyn Iterator<Item = (Vec<u8>, Vec<u8>)> + 'a> {
+    ) -> Box<dyn Send + Iterator<Item = (Vec<u8>, Vec<u8>)> + 'a> {
         let readoptions = rocksdb::ReadOptions::default();
 
         Box::new(
@@ -252,7 +252,7 @@ impl KvTree for RocksDbEngineTree<'_> {
     fn scan_prefix<'a>(
         &'a self,
         prefix: Vec<u8>,
-    ) -> Box<dyn Iterator<Item = (Vec<u8>, Vec<u8>)> + 'a> {
+    ) -> Box<dyn Send + Iterator<Item = (Vec<u8>, Vec<u8>)> + 'a> {
         let readoptions = rocksdb::ReadOptions::default();
 
         Box::new(

src/database/abstraction/sled.rs

@@ -52,7 +52,7 @@ impl Tree for SledEngineTree {
         Ok(())
     }
 
-    fn iter<'a>(&'a self) -> Box<dyn Iterator<Item = (Vec<u8>, Vec<u8>)> + 'a> {
+    fn iter<'a>(&'a self) -> Box<dyn Send + Iterator<Item = (Vec<u8>, Vec<u8>)> + 'a> {
         Box::new(
             self.0
                 .iter()
@@ -70,7 +70,7 @@ impl Tree for SledEngineTree {
         &self,
         from: &[u8],
         backwards: bool,
-    ) -> Box<dyn Iterator<Item = (Vec<u8>, Vec<u8>)>> {
+    ) -> Box<dyn Send + Iterator<Item = (Vec<u8>, Vec<u8>)>> {
         let iter = if backwards {
             self.0.range(..=from)
         } else {
@@ -103,7 +103,7 @@ impl Tree for SledEngineTree {
     fn scan_prefix<'a>(
         &'a self,
         prefix: Vec<u8>,
-    ) -> Box<dyn Iterator<Item = (Vec<u8>, Vec<u8>)> + 'a> {
+    ) -> Box<dyn Send + Iterator<Item = (Vec<u8>, Vec<u8>)> + 'a> {
         let iter = self
             .0
             .scan_prefix(prefix)

src/database/abstraction/sqlite.rs

@@ -8,7 +8,7 @@ use std::{
     future::Future,
     path::{Path, PathBuf},
     pin::Pin,
-    sync::Arc,
+    sync::{mpsc, Arc},
 };
 use thread_local::ThreadLocal;
 use tracing::debug;
@@ -18,26 +18,6 @@ thread_local! {
     static READ_CONNECTION_ITERATOR: RefCell<Option<&'static Connection>> = const { RefCell::new(None) };
 }
 
-struct PreparedStatementIterator<'a> {
-    pub iterator: Box<dyn Iterator<Item = TupleOfBytes> + 'a>,
-    pub _statement_ref: NonAliasingBox<rusqlite::Statement<'a>>,
-}
-
-impl Iterator for PreparedStatementIterator<'_> {
-    type Item = TupleOfBytes;
-
-    fn next(&mut self) -> Option<Self::Item> {
-        self.iterator.next()
-    }
-}
-
-struct NonAliasingBox<T>(*mut T);
-impl<T> Drop for NonAliasingBox<T> {
-    fn drop(&mut self) {
-        drop(unsafe { Box::from_raw(self.0) });
-    }
-}
-
 pub struct Engine {
     writer: Mutex<Connection>,
     read_conn_tls: ThreadLocal<Connection>,
@@ -135,6 +115,22 @@ pub struct SqliteTable {
 
 type TupleOfBytes = (Vec<u8>, Vec<u8>);
 
+fn iter_stmt(
+    from: impl rusqlite::Params,
+    mut statement: rusqlite::Statement,
+    out: mpsc::SyncSender<TupleOfBytes>,
+) {
+    for item in statement
+        .query_map(from, |row| Ok((row.get_unwrap(0), row.get_unwrap(1))))
+        .unwrap()
+        .map(|r| r.unwrap())
+    {
+        if out.send(item).is_err() {
+            break;
+        }
+    }
+}
+
 impl SqliteTable {
     fn get_with_guard(&self, guard: &Connection, key: &[u8]) -> Result<Option<Vec<u8>>> {
         Ok(guard
@@ -155,34 +151,18 @@ impl SqliteTable {
         Ok(())
     }
 
-    pub fn iter_with_guard<'a>(
-        &'a self,
-        guard: &'a Connection,
-    ) -> Box<dyn Iterator<Item = TupleOfBytes> + 'a> {
-        let statement = Box::leak(Box::new(
-            guard
-                .prepare(&format!(
-                    "SELECT key, value FROM {} ORDER BY key ASC",
-                    &self.name
-                ))
-                .unwrap(),
-        ));
-
-        let statement_ref = NonAliasingBox(statement);
+    pub fn iter_with_guard(
+        name: &str,
+        guard: &Connection,
+        out: mpsc::SyncSender<TupleOfBytes>,
+    ) {
+        let statement = guard
+            .prepare(&format!("SELECT key, value FROM {} ORDER BY key ASC", name))
+            .unwrap();
 
         //let name = self.name.clone();
 
-        let iterator = Box::new(
-            statement
-                .query_map([], |row| Ok((row.get_unwrap(0), row.get_unwrap(1))))
-                .unwrap()
-                .map(move |r| r.unwrap()),
-        );
-
-        Box::new(PreparedStatementIterator {
-            iterator,
-            _statement_ref: statement_ref,
-        })
+        iter_stmt([], statement, out)
     }
 }
 
@@ -241,68 +221,51 @@ impl KvTree for SqliteTable {
         Ok(())
     }
 
-    fn iter<'a>(&'a self) -> Box<dyn Iterator<Item = TupleOfBytes> + 'a> {
-        let guard = self.engine.read_lock_iterator();
+    fn iter<'a>(&'a self) -> Box<dyn Send + Iterator<Item = TupleOfBytes> + 'a> {
+        let (tx, rx) = mpsc::sync_channel(1);
+        let engine = self.engine.clone();
+        let name = self.name.clone();
+        tokio::task::spawn_blocking(move || {
+            let guard = engine.read_lock_iterator();
 
-        self.iter_with_guard(guard)
+            Self::iter_with_guard(&name, guard, tx)
+        });
+        Box::new(rx.into_iter())
     }
 
     fn iter_from<'a>(
         &'a self,
         from: &[u8],
         backwards: bool,
-    ) -> Box<dyn Iterator<Item = TupleOfBytes> + 'a> {
-        let guard = self.engine.read_lock_iterator();
+    ) -> Box<dyn Send + Iterator<Item = TupleOfBytes> + 'a> {
+        let (tx, rx) = mpsc::sync_channel(1);
+        let engine = self.engine.clone();
+        let name = self.name.clone();
         let from = from.to_vec(); // TODO change interface?
-
-        //let name = self.name.clone();
+        tokio::task::spawn_blocking(move || {
+            let guard = engine.read_lock_iterator();
 
             if backwards {
-            let statement = Box::leak(Box::new(
-                guard
+                let statement = guard
                     .prepare(&format!(
                         "SELECT key, value FROM {} WHERE key <= ? ORDER BY key DESC",
-                        &self.name
+                        &name
                     ))
-                    .unwrap(),
-            ));
+                    .unwrap();
 
-            let statement_ref = NonAliasingBox(statement);
-
-            let iterator = Box::new(
-                statement
-                    .query_map([from], |row| Ok((row.get_unwrap(0), row.get_unwrap(1))))
-                    .unwrap()
-                    .map(move |r| r.unwrap()),
-            );
-            Box::new(PreparedStatementIterator {
-                iterator,
-                _statement_ref: statement_ref,
-            })
+                iter_stmt([from], statement, tx)
             } else {
-            let statement = Box::leak(Box::new(
-                guard
+                let statement = guard
                     .prepare(&format!(
                         "SELECT key, value FROM {} WHERE key >= ? ORDER BY key ASC",
-                        &self.name
+                        &name
                     ))
-                    .unwrap(),
-            ));
+                    .unwrap();
 
-            let statement_ref = NonAliasingBox(statement);
-
-            let iterator = Box::new(
-                statement
-                    .query_map([from], |row| Ok((row.get_unwrap(0), row.get_unwrap(1))))
-                    .unwrap()
-                    .map(move |r| r.unwrap()),
-            );
-
-            Box::new(PreparedStatementIterator {
-                iterator,
-                _statement_ref: statement_ref,
-            })
+                iter_stmt([from], statement, tx)
             }
+        });
+        Box::new(rx.into_iter())
     }
 
     fn increment(&self, key: &[u8]) -> Result<Vec<u8>> {
@@ -318,7 +281,10 @@ impl KvTree for SqliteTable {
         Ok(new)
     }
 
-    fn scan_prefix<'a>(&'a self, prefix: Vec<u8>) -> Box<dyn Iterator<Item = TupleOfBytes> + 'a> {
+    fn scan_prefix<'a>(
+        &'a self,
+        prefix: Vec<u8>,
+    ) -> Box<dyn Send + Iterator<Item = TupleOfBytes> + 'a> {
         Box::new(
             self.iter_from(&prefix, false)
                 .take_while(move |(key, _)| key.starts_with(&prefix)),

src/database/key_value/appservice.rs

@@ -36,7 +36,7 @@ impl service::appservice::Data for KeyValueDatabase {
             .transpose()
     }
 
-    fn iter_ids<'a>(&'a self) -> Result<Box<dyn Iterator<Item = Result<String>> + 'a>> {
+    fn iter_ids<'a>(&'a self) -> Result<Box<dyn Send + Iterator<Item = Result<String>> + 'a>> {
         Ok(Box::new(self.id_appserviceregistrations.iter().map(
             |(id, _)| {
                 utils::string_from_bytes(&id).map_err(|_| {

src/database/key_value/pusher.rs

@@ -60,7 +60,7 @@ impl service::pusher::Data for KeyValueDatabase {
     fn get_pushkeys<'a>(
         &'a self,
         sender: &UserId,
-    ) -> Box<dyn Iterator<Item = Result<String>> + 'a> {
+    ) -> Box<dyn Send + Iterator<Item = Result<String>> + 'a> {
         let mut prefix = sender.as_bytes().to_vec();
         prefix.push(0xff);
 

src/database/key_value/rooms/alias.rs

@@ -52,7 +52,7 @@ impl service::rooms::alias::Data for KeyValueDatabase {
     fn local_aliases_for_room<'a>(
         &'a self,
         room_id: &RoomId,
-    ) -> Box<dyn Iterator<Item = Result<OwnedRoomAliasId>> + 'a> {
+    ) -> Box<dyn Send + Iterator<Item = Result<OwnedRoomAliasId>> + 'a> {
         let mut prefix = room_id.as_bytes().to_vec();
         prefix.push(0xff);
 

src/database/key_value/rooms/directory.rs

@@ -15,7 +15,7 @@ impl service::rooms::directory::Data for KeyValueDatabase {
         Ok(self.publicroomids.get(room_id.as_bytes())?.is_some())
     }
 
-    fn public_rooms<'a>(&'a self) -> Box<dyn Iterator<Item = Result<OwnedRoomId>> + 'a> {
+    fn public_rooms<'a>(&'a self) -> Box<dyn Send + Iterator<Item = Result<OwnedRoomId>> + 'a> {
         Box::new(self.publicroomids.iter().map(|(bytes, _)| {
             RoomId::parse(
                 utils::string_from_bytes(&bytes).map_err(|_| {

src/database/key_value/rooms/edus/read_receipt.rs

@@ -53,7 +53,8 @@ impl service::rooms::edus::read_receipt::Data for KeyValueDatabase {
         room_id: &RoomId,
         since: u64,
     ) -> Box<
-        dyn Iterator<
+        dyn Send
+            + Iterator<
                 Item = Result<(
                     OwnedUserId,
                     u64,

src/database/key_value/rooms/metadata.rs

@@ -18,7 +18,7 @@ impl service::rooms::metadata::Data for KeyValueDatabase {
             .is_some())
     }
 
-    fn iter_ids<'a>(&'a self) -> Box<dyn Iterator<Item = Result<OwnedRoomId>> + 'a> {
+    fn iter_ids<'a>(&'a self) -> Box<dyn Send + Iterator<Item = Result<OwnedRoomId>> + 'a> {
         Box::new(self.roomid_shortroomid.iter().map(|(bytes, _)| {
             RoomId::parse(
                 utils::string_from_bytes(&bytes).map_err(|_| {

src/database/key_value/rooms/pdu_metadata.rs

@@ -22,7 +22,7 @@ impl service::rooms::pdu_metadata::Data for KeyValueDatabase {
         shortroomid: u64,
         target: u64,
         until: PduCount,
-    ) -> Result<Box<dyn Iterator<Item = Result<(PduCount, PduEvent)>> + 'a>> {
+    ) -> Result<Box<dyn Send + Iterator<Item = Result<(PduCount, PduEvent)>> + 'a>> {
         let prefix = target.to_be_bytes().to_vec();
         let mut current = prefix.clone();
 

src/database/key_value/rooms/search.rs

@@ -46,7 +46,7 @@ impl service::rooms::search::Data for KeyValueDatabase {
         &'a self,
         room_id: &RoomId,
         search_string: &str,
-    ) -> Result<Option<(Box<dyn Iterator<Item = Vec<u8>> + 'a>, Vec<String>)>> {
+    ) -> Result<Option<(Box<dyn Send + Iterator<Item = Vec<u8>> + 'a>, Vec<String>)>> {
         let prefix = services()
             .rooms
             .short

src/database/key_value/rooms/state_cache.rs

@@ -244,7 +244,7 @@ impl service::rooms::state_cache::Data for KeyValueDatabase {
     fn room_servers<'a>(
         &'a self,
         room_id: &RoomId,
-    ) -> Box<dyn Iterator<Item = Result<OwnedServerName>> + 'a> {
+    ) -> Box<dyn Send + Iterator<Item = Result<OwnedServerName>> + 'a> {
         let mut prefix = room_id.as_bytes().to_vec();
         prefix.push(0xff);
 
@@ -277,7 +277,7 @@ impl service::rooms::state_cache::Data for KeyValueDatabase {
     fn server_rooms<'a>(
         &'a self,
         server: &ServerName,
-    ) -> Box<dyn Iterator<Item = Result<OwnedRoomId>> + 'a> {
+    ) -> Box<dyn Send + Iterator<Item = Result<OwnedRoomId>> + 'a> {
         let mut prefix = server.as_bytes().to_vec();
         prefix.push(0xff);
 
@@ -299,7 +299,7 @@ impl service::rooms::state_cache::Data for KeyValueDatabase {
     fn room_members<'a>(
         &'a self,
         room_id: &RoomId,
-    ) -> Box<dyn Iterator<Item = Result<OwnedUserId>> + 'a> {
+    ) -> Box<dyn Send + Iterator<Item = Result<OwnedUserId>> + 'a> {
         let mut prefix = room_id.as_bytes().to_vec();
         prefix.push(0xff);
 
@@ -345,7 +345,7 @@ impl service::rooms::state_cache::Data for KeyValueDatabase {
     fn room_useroncejoined<'a>(
         &'a self,
         room_id: &RoomId,
-    ) -> Box<dyn Iterator<Item = Result<OwnedUserId>> + 'a> {
+    ) -> Box<dyn Send + Iterator<Item = Result<OwnedUserId>> + 'a> {
         let mut prefix = room_id.as_bytes().to_vec();
         prefix.push(0xff);
 
@@ -375,7 +375,7 @@ impl service::rooms::state_cache::Data for KeyValueDatabase {
     fn room_members_invited<'a>(
         &'a self,
         room_id: &RoomId,
-    ) -> Box<dyn Iterator<Item = Result<OwnedUserId>> + 'a> {
+    ) -> Box<dyn Send + Iterator<Item = Result<OwnedUserId>> + 'a> {
         let mut prefix = room_id.as_bytes().to_vec();
         prefix.push(0xff);
 
@@ -433,7 +433,7 @@ impl service::rooms::state_cache::Data for KeyValueDatabase {
     fn rooms_joined<'a>(
         &'a self,
         user_id: &UserId,
-    ) -> Box<dyn Iterator<Item = Result<OwnedRoomId>> + 'a> {
+    ) -> Box<dyn Send + Iterator<Item = Result<OwnedRoomId>> + 'a> {
         Box::new(
             self.userroomid_joined
                 .scan_prefix(user_id.as_bytes().to_vec())
@@ -459,7 +459,8 @@ impl service::rooms::state_cache::Data for KeyValueDatabase {
     fn rooms_invited<'a>(
         &'a self,
         user_id: &UserId,
-    ) -> Box<dyn Iterator<Item = Result<(OwnedRoomId, Vec<Raw<AnyStrippedStateEvent>>)>> + 'a> {
+    ) -> Box<dyn Send + Iterator<Item = Result<(OwnedRoomId, Vec<Raw<AnyStrippedStateEvent>>)>> + 'a>
+    {
         let mut prefix = user_id.as_bytes().to_vec();
         prefix.push(0xff);
 
@@ -538,7 +539,8 @@ impl service::rooms::state_cache::Data for KeyValueDatabase {
     fn rooms_left<'a>(
         &'a self,
         user_id: &UserId,
-    ) -> Box<dyn Iterator<Item = Result<(OwnedRoomId, Vec<Raw<AnySyncStateEvent>>)>> + 'a> {
+    ) -> Box<dyn Send + Iterator<Item = Result<(OwnedRoomId, Vec<Raw<AnySyncStateEvent>>)>> + 'a>
+    {
         let mut prefix = user_id.as_bytes().to_vec();
         prefix.push(0xff);
 

src/database/key_value/rooms/threads.rs

@@ -11,7 +11,7 @@ impl service::rooms::threads::Data for KeyValueDatabase {
         room_id: &'a RoomId,
         until: u64,
         _include: &'a IncludeThreads,
-    ) -> Result<Box<dyn Iterator<Item = Result<(u64, PduEvent)>> + 'a>> {
+    ) -> Result<Box<dyn Send + Iterator<Item = Result<(u64, PduEvent)>> + 'a>> {
         let prefix = services()
             .rooms
             .short

src/database/key_value/rooms/timeline.rs

@@ -228,7 +228,7 @@ impl service::rooms::timeline::Data for KeyValueDatabase {
         user_id: &UserId,
         room_id: &RoomId,
         until: PduCount,
-    ) -> Result<Box<dyn Iterator<Item = Result<(PduCount, PduEvent)>> + 'a>> {
+    ) -> Result<Box<dyn Send + Iterator<Item = Result<(PduCount, PduEvent)>> + 'a>> {
         let (prefix, current) = count_to_id(room_id, until, 1, true)?;
 
         let user_id = user_id.to_owned();
@@ -255,7 +255,7 @@ impl service::rooms::timeline::Data for KeyValueDatabase {
         user_id: &UserId,
         room_id: &RoomId,
         from: PduCount,
-    ) -> Result<Box<dyn Iterator<Item = Result<(PduCount, PduEvent)>> + 'a>> {
+    ) -> Result<Box<dyn Send + Iterator<Item = Result<(PduCount, PduEvent)>> + 'a>> {
         let (prefix, current) = count_to_id(room_id, from, 1, false)?;
 
         let user_id = user_id.to_owned();

src/database/key_value/rooms/user.rs

@@ -111,7 +111,7 @@ impl service::rooms::user::Data for KeyValueDatabase {
     fn get_shared_rooms<'a>(
         &'a self,
         users: Vec<OwnedUserId>,
-    ) -> Result<Box<dyn Iterator<Item = Result<OwnedRoomId>> + 'a>> {
+    ) -> Result<Box<dyn Send + Iterator<Item = Result<OwnedRoomId>> + 'a>> {
         let iterators = users.into_iter().map(move |user_id| {
             let mut prefix = user_id.as_bytes().to_vec();
             prefix.push(0xff);

src/database/key_value/sending.rs

@@ -12,7 +12,8 @@ use crate::{
 impl service::sending::Data for KeyValueDatabase {
     fn active_requests<'a>(
         &'a self,
-    ) -> Box<dyn Iterator<Item = Result<(Vec<u8>, OutgoingKind, SendingEventType)>> + 'a> {
+    ) -> Box<dyn Send + Iterator<Item = Result<(Vec<u8>, OutgoingKind, SendingEventType)>> + 'a>
+    {
         Box::new(
             self.servercurrentevent_data
                 .iter()
@@ -23,7 +24,7 @@ impl service::sending::Data for KeyValueDatabase {
     fn active_requests_for<'a>(
         &'a self,
         outgoing_kind: &OutgoingKind,
-    ) -> Box<dyn Iterator<Item = Result<(Vec<u8>, SendingEventType)>> + 'a> {
+    ) -> Box<dyn Send + Iterator<Item = Result<(Vec<u8>, SendingEventType)>> + 'a> {
         let prefix = outgoing_kind.get_prefix();
         Box::new(
             self.servercurrentevent_data
@@ -87,7 +88,7 @@ impl service::sending::Data for KeyValueDatabase {
     fn queued_requests<'a>(
         &'a self,
         outgoing_kind: &OutgoingKind,
-    ) -> Box<dyn Iterator<Item = Result<(SendingEventType, Vec<u8>)>> + 'a> {
+    ) -> Box<dyn Send + Iterator<Item = Result<(SendingEventType, Vec<u8>)>> + 'a> {
         let prefix = outgoing_kind.get_prefix();
         return Box::new(
             self.servernameevent_data

src/database/key_value/users.rs

@@ -68,7 +68,7 @@ impl service::users::Data for KeyValueDatabase {
     }
 
     /// Returns an iterator over all users on this homeserver.
-    fn iter<'a>(&'a self) -> Box<dyn Iterator<Item = Result<OwnedUserId>> + 'a> {
+    fn iter<'a>(&'a self) -> Box<dyn Send + Iterator<Item = Result<OwnedUserId>> + 'a> {
         Box::new(self.userid_password.iter().map(|(bytes, _)| {
             UserId::parse(utils::string_from_bytes(&bytes).map_err(|_| {
                 Error::bad_database("User ID in userid_password is invalid unicode.")
@@ -259,7 +259,7 @@ impl service::users::Data for KeyValueDatabase {
     fn all_device_ids<'a>(
         &'a self,
         user_id: &UserId,
-    ) -> Box<dyn Iterator<Item = Result<OwnedDeviceId>> + 'a> {
+    ) -> Box<dyn Send + Iterator<Item = Result<OwnedDeviceId>> + 'a> {
         let mut prefix = user_id.as_bytes().to_vec();
         prefix.push(0xff);
         // All devices have metadata
@@ -584,7 +584,7 @@ impl service::users::Data for KeyValueDatabase {
         user_or_room_id: &str,
         from: u64,
         to: Option<u64>,
-    ) -> Box<dyn Iterator<Item = Result<OwnedUserId>> + 'a> {
+    ) -> Box<dyn Send + Iterator<Item = Result<OwnedUserId>> + 'a> {
         let mut prefix = user_or_room_id.as_bytes().to_vec();
         prefix.push(0xff);
 
@@ -899,7 +899,7 @@ impl service::users::Data for KeyValueDatabase {
     fn all_devices_metadata<'a>(
         &'a self,
         user_id: &UserId,
-    ) -> Box<dyn Iterator<Item = Result<Device>> + 'a> {
+    ) -> Box<dyn Send + Iterator<Item = Result<Device>> + 'a> {
         let mut key = user_id.as_bytes().to_vec();
         key.push(0xff);
 

src/service/appservice/data.rs

@@ -15,7 +15,7 @@ pub trait Data: Send + Sync {
 
     fn get_registration(&self, id: &str) -> Result<Option<Registration>>;
 
-    fn iter_ids<'a>(&'a self) -> Result<Box<dyn Iterator<Item = Result<String>> + 'a>>;
+    fn iter_ids<'a>(&'a self) -> Result<Box<dyn Send + Iterator<Item = Result<String>> + 'a>>;
 
     fn all(&self) -> Result<Vec<(String, Registration)>>;
 }

src/service/pusher/data.rs

@@ -11,6 +11,8 @@ pub trait Data: Send + Sync {
 
     fn get_pushers(&self, sender: &UserId) -> Result<Vec<Pusher>>;
 
-    fn get_pushkeys<'a>(&'a self, sender: &UserId)
-        -> Box<dyn Iterator<Item = Result<String>> + 'a>;
+    fn get_pushkeys<'a>(
+        &'a self,
+        sender: &UserId,
+    ) -> Box<dyn Send + Iterator<Item = Result<String>> + 'a>;
 }

src/service/pusher/mod.rs

@@ -39,7 +39,7 @@ impl Service {
         self.db.get_pushers(sender)
     }
 
-    pub fn get_pushkeys(&self, sender: &UserId) -> Box<dyn Iterator<Item = Result<String>>> {
+    pub fn get_pushkeys(&self, sender: &UserId) -> Box<dyn Send + Iterator<Item = Result<String>>> {
         self.db.get_pushkeys(sender)
     }
 

src/service/rooms/alias/data.rs

@@ -18,5 +18,5 @@ pub trait Data: Send + Sync {
     fn local_aliases_for_room<'a>(
         &'a self,
         room_id: &RoomId,
-    ) -> Box<dyn Iterator<Item = Result<OwnedRoomAliasId>> + 'a>;
+    ) -> Box<dyn Send + Iterator<Item = Result<OwnedRoomAliasId>> + 'a>;
 }

src/service/rooms/alias/mod.rs

@@ -95,7 +95,7 @@ impl Service {
     pub fn local_aliases_for_room<'a>(
         &'a self,
         room_id: &RoomId,
-    ) -> Box<dyn Iterator<Item = Result<OwnedRoomAliasId>> + 'a> {
+    ) -> Box<dyn Send + Iterator<Item = Result<OwnedRoomAliasId>> + 'a> {
         self.db.local_aliases_for_room(room_id)
     }
 }

src/service/rooms/directory/data.rs

@@ -12,5 +12,5 @@ pub trait Data: Send + Sync {
     fn is_public_room(&self, room_id: &RoomId) -> Result<bool>;
 
     /// Returns the unsorted public room directory
-    fn public_rooms<'a>(&'a self) -> Box<dyn Iterator<Item = Result<OwnedRoomId>> + 'a>;
+    fn public_rooms<'a>(&'a self) -> Box<dyn Send + Iterator<Item = Result<OwnedRoomId>> + 'a>;
 }

src/service/rooms/edus/read_receipt/data.rs

@@ -17,7 +17,8 @@ pub trait Data: Send + Sync {
         room_id: &RoomId,
         since: u64,
     ) -> Box<
-        dyn Iterator<
+        dyn Send
+            + Iterator<
                 Item = Result<(
                     OwnedUserId,
                     u64,

src/service/rooms/metadata/data.rs

@@ -3,7 +3,7 @@ use ruma::{OwnedRoomId, RoomId};
 
 pub trait Data: Send + Sync {
     fn exists(&self, room_id: &RoomId) -> Result<bool>;
-    fn iter_ids<'a>(&'a self) -> Box<dyn Iterator<Item = Result<OwnedRoomId>> + 'a>;
+    fn iter_ids<'a>(&'a self) -> Box<dyn Send + Iterator<Item = Result<OwnedRoomId>> + 'a>;
     fn is_disabled(&self, room_id: &RoomId) -> Result<bool>;
     fn disable_room(&self, room_id: &RoomId, disabled: bool) -> Result<()>;
 }

src/service/rooms/metadata/mod.rs

@@ -16,7 +16,7 @@ impl Service {
         self.db.exists(room_id)
     }
 
-    pub fn iter_ids<'a>(&'a self) -> Box<dyn Iterator<Item = Result<OwnedRoomId>> + 'a> {
+    pub fn iter_ids<'a>(&'a self) -> Box<dyn Send + Iterator<Item = Result<OwnedRoomId>> + 'a> {
         self.db.iter_ids()
     }
 

src/service/rooms/pdu_metadata/data.rs

@@ -12,7 +12,7 @@ pub trait Data: Send + Sync {
         room_id: u64,
         target: u64,
         until: PduCount,
-    ) -> Result<Box<dyn Iterator<Item = Result<(PduCount, PduEvent)>> + 'a>>;
+    ) -> Result<Box<dyn Send + Iterator<Item = Result<(PduCount, PduEvent)>> + 'a>>;
     fn mark_as_referenced(&self, room_id: &RoomId, event_ids: &[Arc<EventId>]) -> Result<()>;
     fn is_event_referenced(&self, room_id: &RoomId, event_id: &EventId) -> Result<bool>;
     fn mark_event_soft_failed(&self, event_id: &EventId) -> Result<()>;

src/service/rooms/search/data.rs

@@ -11,5 +11,5 @@ pub trait Data: Send + Sync {
         &'a self,
         room_id: &RoomId,
         search_string: &str,
-    ) -> Result<Option<(Box<dyn Iterator<Item = Vec<u8>> + 'a>, Vec<String>)>>;
+    ) -> Result<Option<(Box<dyn Send + Iterator<Item = Vec<u8>> + 'a>, Vec<String>)>>;
 }

src/service/rooms/state_cache/data.rs

@@ -31,7 +31,7 @@ pub trait Data: Send + Sync {
     fn room_servers<'a>(
         &'a self,
         room_id: &RoomId,
-    ) -> Box<dyn Iterator<Item = Result<OwnedServerName>> + 'a>;
+    ) -> Box<dyn Send + Iterator<Item = Result<OwnedServerName>> + 'a>;
 
     fn server_in_room(&self, server: &ServerName, room_id: &RoomId) -> Result<bool>;
 
@@ -39,13 +39,13 @@ pub trait Data: Send + Sync {
     fn server_rooms<'a>(
         &'a self,
         server: &ServerName,
-    ) -> Box<dyn Iterator<Item = Result<OwnedRoomId>> + 'a>;
+    ) -> Box<dyn Send + Iterator<Item = Result<OwnedRoomId>> + 'a>;
 
     /// Returns an iterator over all joined members of a room.
     fn room_members<'a>(
         &'a self,
         room_id: &RoomId,
-    ) -> Box<dyn Iterator<Item = Result<OwnedUserId>> + 'a>;
+    ) -> Box<dyn Send + Iterator<Item = Result<OwnedUserId>> + 'a>;
 
     fn room_joined_count(&self, room_id: &RoomId) -> Result<Option<u64>>;
 
@@ -55,13 +55,13 @@ pub trait Data: Send + Sync {
     fn room_useroncejoined<'a>(
         &'a self,
         room_id: &RoomId,
-    ) -> Box<dyn Iterator<Item = Result<OwnedUserId>> + 'a>;
+    ) -> Box<dyn Send + Iterator<Item = Result<OwnedUserId>> + 'a>;
 
     /// Returns an iterator over all invited members of a room.
     fn room_members_invited<'a>(
         &'a self,
         room_id: &RoomId,
-    ) -> Box<dyn Iterator<Item = Result<OwnedUserId>> + 'a>;
+    ) -> Box<dyn Send + Iterator<Item = Result<OwnedUserId>> + 'a>;
 
     fn get_invite_count(&self, room_id: &RoomId, user_id: &UserId) -> Result<Option<u64>>;
 
@@ -71,14 +71,14 @@ pub trait Data: Send + Sync {
     fn rooms_joined<'a>(
         &'a self,
         user_id: &UserId,
-    ) -> Box<dyn Iterator<Item = Result<OwnedRoomId>> + 'a>;
+    ) -> Box<dyn Send + Iterator<Item = Result<OwnedRoomId>> + 'a>;
 
     /// Returns an iterator over all rooms a user was invited to.
     #[allow(clippy::type_complexity)]
     fn rooms_invited<'a>(
         &'a self,
         user_id: &UserId,
-    ) -> Box<dyn Iterator<Item = Result<(OwnedRoomId, Vec<Raw<AnyStrippedStateEvent>>)>> + 'a>;
+    ) -> Box<dyn Send + Iterator<Item = Result<(OwnedRoomId, Vec<Raw<AnyStrippedStateEvent>>)>> + 'a>;
 
     fn invite_state(
         &self,
@@ -97,7 +97,7 @@ pub trait Data: Send + Sync {
     fn rooms_left<'a>(
         &'a self,
         user_id: &UserId,
-    ) -> Box<dyn Iterator<Item = Result<(OwnedRoomId, Vec<Raw<AnySyncStateEvent>>)>> + 'a>;
+    ) -> Box<dyn Send + Iterator<Item = Result<(OwnedRoomId, Vec<Raw<AnySyncStateEvent>>)>> + 'a>;
 
     fn once_joined(&self, user_id: &UserId, room_id: &RoomId) -> Result<bool>;
 

src/service/rooms/threads/data.rs

@@ -9,7 +9,7 @@ pub trait Data: Send + Sync {
         room_id: &'a RoomId,
         until: u64,
         include: &'a IncludeThreads,
-    ) -> Result<Box<dyn Iterator<Item = Result<(u64, PduEvent)>> + 'a>>;
+    ) -> Result<Box<dyn Send + Iterator<Item = Result<(u64, PduEvent)>> + 'a>>;
 
     fn update_participants(&self, root_id: &[u8], participants: &[OwnedUserId]) -> Result<()>;
     fn get_participants(&self, root_id: &[u8]) -> Result<Option<Vec<OwnedUserId>>>;

src/service/rooms/timeline/data.rs

@@ -72,7 +72,7 @@ pub trait Data: Send + Sync {
         user_id: &UserId,
         room_id: &RoomId,
         until: PduCount,
-    ) -> Result<Box<dyn Iterator<Item = Result<(PduCount, PduEvent)>> + 'a>>;
+    ) -> Result<Box<dyn Send + Iterator<Item = Result<(PduCount, PduEvent)>> + 'a>>;
 
     /// Returns an iterator over all events in a room that happened after the event with id `from`
     /// in chronological order.
@@ -82,7 +82,7 @@ pub trait Data: Send + Sync {
         user_id: &UserId,
         room_id: &RoomId,
         from: PduCount,
-    ) -> Result<Box<dyn Iterator<Item = Result<(PduCount, PduEvent)>> + 'a>>;
+    ) -> Result<Box<dyn Send + Iterator<Item = Result<(PduCount, PduEvent)>> + 'a>>;
 
     fn increment_notification_counts(
         &self,

src/service/rooms/user/data.rs

@@ -23,5 +23,5 @@ pub trait Data: Send + Sync {
     fn get_shared_rooms<'a>(
         &'a self,
         users: Vec<OwnedUserId>,
-    ) -> Result<Box<dyn Iterator<Item = Result<OwnedRoomId>> + 'a>>;
+    ) -> Result<Box<dyn Send + Iterator<Item = Result<OwnedRoomId>> + 'a>>;
 }

src/service/sending/data.rs

@@ -8,11 +8,11 @@ pub trait Data: Send + Sync {
     #[allow(clippy::type_complexity)]
     fn active_requests<'a>(
         &'a self,
-    ) -> Box<dyn Iterator<Item = Result<(Vec<u8>, OutgoingKind, SendingEventType)>> + 'a>;
+    ) -> Box<dyn Send + Iterator<Item = Result<(Vec<u8>, OutgoingKind, SendingEventType)>> + 'a>;
     fn active_requests_for<'a>(
         &'a self,
         outgoing_kind: &OutgoingKind,
-    ) -> Box<dyn Iterator<Item = Result<(Vec<u8>, SendingEventType)>> + 'a>;
+    ) -> Box<dyn Send + Iterator<Item = Result<(Vec<u8>, SendingEventType)>> + 'a>;
     fn delete_active_request(&self, key: Vec<u8>) -> Result<()>;
     fn delete_all_active_requests_for(&self, outgoing_kind: &OutgoingKind) -> Result<()>;
     fn delete_all_requests_for(&self, outgoing_kind: &OutgoingKind) -> Result<()>;
@@ -23,7 +23,7 @@ pub trait Data: Send + Sync {
     fn queued_requests<'a>(
         &'a self,
         outgoing_kind: &OutgoingKind,
-    ) -> Box<dyn Iterator<Item = Result<(SendingEventType, Vec<u8>)>> + 'a>;
+    ) -> Box<dyn Send + Iterator<Item = Result<(SendingEventType, Vec<u8>)>> + 'a>;
     fn mark_as_active(&self, events: &[(SendingEventType, Vec<u8>)]) -> Result<()>;
     fn set_latest_educount(&self, server_name: &ServerName, educount: u64) -> Result<()>;
     fn get_latest_educount(&self, server_name: &ServerName) -> Result<u64>;

src/service/users/data.rs

@@ -23,7 +23,7 @@ pub trait Data: Send + Sync {
     fn find_from_token(&self, token: &str) -> Result<Option<(OwnedUserId, String)>>;
 
     /// Returns an iterator over all users on this homeserver.
-    fn iter<'a>(&'a self) -> Box<dyn Iterator<Item = Result<OwnedUserId>> + 'a>;
+    fn iter<'a>(&'a self) -> Box<dyn Send + Iterator<Item = Result<OwnedUserId>> + 'a>;
 
     /// Returns a list of local users as list of usernames.
     ///
@@ -70,7 +70,7 @@ pub trait Data: Send + Sync {
     fn all_device_ids<'a>(
         &'a self,
         user_id: &UserId,
-    ) -> Box<dyn Iterator<Item = Result<OwnedDeviceId>> + 'a>;
+    ) -> Box<dyn Send + Iterator<Item = Result<OwnedDeviceId>> + 'a>;
 
     /// Replaces the access token of one device.
     fn set_token(&self, user_id: &UserId, device_id: &DeviceId, token: &str) -> Result<()>;
@@ -127,7 +127,7 @@ pub trait Data: Send + Sync {
         user_or_room_id: &str,
         from: u64,
         to: Option<u64>,
-    ) -> Box<dyn Iterator<Item = Result<OwnedUserId>> + 'a>;
+    ) -> Box<dyn Send + Iterator<Item = Result<OwnedUserId>> + 'a>;
 
     fn mark_device_key_update(&self, user_id: &UserId) -> Result<()>;
 
@@ -205,7 +205,7 @@ pub trait Data: Send + Sync {
     fn all_devices_metadata<'a>(
         &'a self,
         user_id: &UserId,
-    ) -> Box<dyn Iterator<Item = Result<Device>> + 'a>;
+    ) -> Box<dyn Send + Iterator<Item = Result<Device>> + 'a>;
 
     /// Creates a new sync filter. Returns the filter id.
     fn create_filter(&self, user_id: &UserId, filter: &FilterDefinition) -> Result<String>;