feat(server): add enable_datagrams, off by default

This adds a configuration option to enable quic DATAGRAM support, and disables
it by default. Datagrams are better and the protocol is built around them, but
they're less robust in the face of packet loss, particularly without any FEC
mechanism.

We'll turn on datagram support by default once we have that.

mm-server/src/config.rs

@@ -87,6 +87,7 @@ mod parsed {
         pub(super) tls_key: Option<PathBuf>,
         pub(super) worker_threads: Option<NonZeroU32>,
         pub(super) max_connections: Option<MaxConnections>,
+        pub(super) enable_datagrams: Option<bool>,
     }
 
     #[derive(Debug, Clone, PartialEq, Deserialize, Converge)]
@@ -121,6 +122,7 @@ pub struct ServerConfig {
     pub tls_key: Option<PathBuf>,
     pub worker_threads: NonZeroU32,
     pub max_connections: Option<NonZeroU32>,
+    pub enable_datagrams: bool,
 }
 
 #[derive(Debug, Clone, PartialEq)]
@@ -182,6 +184,7 @@ impl Config {
                     parsed::MaxConnections::Value(n) => Some(n),
                     parsed::MaxConnections::Infinity => None,
                 },
+                enable_datagrams: server.enable_datagrams.unwrap(),
             },
             apps: HashMap::new(), // Handled below.
             bug_report_dir: None, // This is only set from the command line.

mm-server/src/server/handlers.rs

@@ -259,6 +259,8 @@ fn attach(
         }
     };
 
+    let use_datagrams = state.lock().unwrap().cfg.server.enable_datagrams;
+
     let (handle, display_params, bug_report_dir) = {
         let mut state = state.lock().unwrap();
 
@@ -361,6 +363,7 @@ fn attach(
     // Five varints at max 5 bytes, plus a header, works out to around 32
     // bytes. Double for safety.
     let dgram_chunk_size = max_dgram_len - 64;
+    let stream_chunk_size = protocol::MAX_MESSAGE_SIZE - 64;
 
     loop {
         select! {
@@ -542,7 +545,13 @@ fn attach(
 
                         // TODO FEC
 
-                        for (chunk, num_chunks, data) in iter_chunks(frame, dgram_chunk_size) {
+                        let chunk_size = if use_datagrams {
+                            dgram_chunk_size
+                        } else {
+                            stream_chunk_size
+                        };
+
+                        for (chunk, num_chunks, data) in iter_chunks(frame, chunk_size) {
                             let msg = protocol::VideoChunk {
                                 session_id,
                                 attachment_id: handle.attachment_id,
@@ -555,7 +564,11 @@ fn attach(
                                 timestamp: ts,
                             };
 
-                            outgoing_dgrams.send(msg.into()).ok();
+                            if use_datagrams {
+                                outgoing_dgrams.send(msg.into()).ok();
+                            } else {
+                                outgoing.send(msg.into()).ok();
+                            }
                         }
                     }
                     Ok(CompositorEvent::AudioFrame{ stream_seq, seq, ts, frame }) => {
@@ -567,7 +580,13 @@ fn attach(
                             last_audio_frame_recv = time::Instant::now();
 
 
-                            for (chunk, num_chunks, data) in iter_chunks(frame, dgram_chunk_size) {
+                            let chunk_size = if use_datagrams {
+                                dgram_chunk_size
+                            } else {
+                                stream_chunk_size
+                            };
+
+                            for (chunk, num_chunks, data) in iter_chunks(frame, chunk_size) {
                                 let msg = protocol::AudioChunk {
                                     session_id,
                                     attachment_id: handle.attachment_id,
@@ -580,8 +599,14 @@ fn attach(
                                     timestamp: ts,
                                 };
 
-                                outgoing_dgrams.send(msg.into()).ok();
+                                if use_datagrams {
+                                    outgoing_dgrams.send(msg.into()).ok();
+                                } else {
+                                    outgoing.send(msg.into()).ok();
+                                }
                             }
+
+
                     }
                     Ok(CompositorEvent::CursorUpdate{ image, icon, hotspot_x, hotspot_y }) => {
                         use protocol::update_cursor::CursorIcon;

mmserver.default.toml

@@ -49,6 +49,14 @@ worker_threads = 8
 # to specify no limit.
 max_connections = 4
 
+# Whether to enable the use of QUIC datagrams (RFC 9221) to transmit audio and video
+# packets. Datagrams offer lower latency but decreased robustness in the face of
+# packet loss, so this option can be used to decrease latency if you're on a
+# local network where you know there won't be any loss.
+#
+# This option will likely default to true in a future release.
+enable_datagrams = false
+
 # *** Configured Applications *** 
 #
 # Each application you want to stream must be configured in advance. An example