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use crate::cmd::{Parse, ParseError, Unknown}; use crate::{Command, Connection, Db, Frame, Shutdown}; use bytes::Bytes; use std::pin::Pin; use tokio::select; use tokio::stream::{Stream, StreamExt, StreamMap}; use tokio::sync::broadcast; /// Subscribes the client to one or more channels. /// /// Once the client enters the subscribed state, it is not supposed to issue any /// other commands, except for additional SUBSCRIBE, PSUBSCRIBE, UNSUBSCRIBE, /// PUNSUBSCRIBE, PING and QUIT commands. #[derive(Debug)] pub struct Subscribe { channels: Vec<String>, } /// Unsubscribes the client from one or more channels. /// /// When no channels are specified, the client is unsubscribed from all the /// previously subscribed channels. #[derive(Clone, Debug)] pub struct Unsubscribe { channels: Vec<String>, } /// Stream of messages. The stream receives messages from the /// `broadcast::Receiver`. We use `stream!` to create a `Stream` that consumes /// messages. Because `stream!` values cannot be named, we box the stream using /// a trait object. type Messages = Pin<Box<dyn Stream<Item = Bytes> + Send>>; impl Subscribe { /// Creates a new `Subscribe` command to listen on the specified channels. pub(crate) fn new(channels: &[String]) -> Subscribe { Subscribe { channels: channels.to_vec(), } } /// Parse a `Subscribe` instance from a received frame. /// /// The `Parse` argument provides a cursor-like API to read fields from the /// `Frame`. At this point, the entire frame has already been received from /// the socket. /// /// The `SUBSCRIBE` string has already been consumed. /// /// # Returns /// /// On success, the `Subscribe` value is returned. If the frame is /// malformed, `Err` is returned. /// /// # Format /// /// Expects an array frame containing two or more entries. /// /// ```text /// SUBSCRIBE channel [channel ...] /// ``` pub(crate) fn parse_frames(parse: &mut Parse) -> crate::Result<Subscribe> { use ParseError::EndOfStream; // The `SUBSCRIBE` string has already been consumed. At this point, // there is one or more strings remaining in `parse`. These represent // the channels to subscribe to. // // Extract the first string. If there is none, the the frame is // malformed and the error is bubbled up. let mut channels = vec![parse.next_string()?]; // Now, the remainder of the frame is consumed. Each value must be a // string or the frame is malformed. Once all values in the frame have // been consumed, the command is fully parsed. loop { match parse.next_string() { // A string has been consumed from the `parse`, push it into the // list of channels to subscribe to. Ok(s) => channels.push(s), // The `EndOfStream` error indicates there is no further data to // parse. Err(EndOfStream) => break, // All other errors are bubbled up, resulting in the connection // being terminated. Err(err) => return Err(err.into()), } } Ok(Subscribe { channels }) } /// Apply the `Subscribe` command to the specified `Db` instance. /// /// This function is the entry point and includes the initial list of /// channels to subscribe to. Additional `subscribe` and `unsubscribe` /// commands may be received from the client and the list of subscriptions /// are updated accordingly. /// /// [here]: https://redis.io/topics/pubsub pub(crate) async fn apply( mut self, db: &Db, dst: &mut Connection, shutdown: &mut Shutdown, ) -> crate::Result<()> { // Each individual channel subscription is handled using a // `sync::broadcast` channel. Messages are then fanned out to all // clients currently subscribed to the channels. // // An individual client may subscribe to multiple channels and may // dynamically add and remove channels from its subscription set. To // handle this, a `StreamMap` is used to track active subscriptions. The // `StreamMap` merges messages from individual broadcast channels as // they are received. let mut subscriptions = StreamMap::new(); loop { // `self.channels` is used to track additional channels to subscribe // to. When new `SUBSCRIBE` commands are received during the // execution of `apply`, the new channels are pushed onto this vec. for channel_name in self.channels.drain(..) { subscribe_to_channel(channel_name, &mut subscriptions, db, dst).await?; } // Wait for one of the following to happen: // // - Receive a message from one of the subscribed channels. // - Receive a subscribe or unsubscribe command from the client. // - A server shutdown signal. select! { // Receive messages from subscribed channels Some((channel_name, msg)) = subscriptions.next() => { dst.write_frame(&make_message_frame(channel_name, msg)).await?; } res = dst.read_frame() => { let frame = match res? { Some(frame) => frame, // This happens if the remote client has disconnected. None => return Ok(()) }; handle_command( frame, &mut self.channels, &mut subscriptions, dst, ).await?; } _ = shutdown.recv() => { return Ok(()); } }; } } /// Converts the command into an equivalent `Frame`. /// /// This is called by the client when encoding a `Subscribe` command to send /// to the server. pub(crate) fn into_frame(self) -> Frame { let mut frame = Frame::array(); frame.push_bulk(Bytes::from("subscribe".as_bytes())); for channel in self.channels { frame.push_bulk(Bytes::from(channel.into_bytes())); } frame } } async fn subscribe_to_channel( channel_name: String, subscriptions: &mut StreamMap<String, Messages>, db: &Db, dst: &mut Connection, ) -> crate::Result<()> { let mut rx = db.subscribe(channel_name.clone()); // Subscribe to the channel. let rx = Box::pin(async_stream::stream! { loop { match rx.recv().await { Ok(msg) => yield msg, // If we lagged in consuming messages, just resume. Err(broadcast::error::RecvError::Lagged(_)) => {} Err(_) => break, } } }); // Track subscription in this client's subscription set. subscriptions.insert(channel_name.clone(), rx); // Respond with the successful subscription let response = make_subscribe_frame(channel_name, subscriptions.len()); dst.write_frame(&response).await?; Ok(()) } /// Handle a command received while inside `Subscribe::apply`. Only subscribe /// and unsubscribe commands are permitted in this context. /// /// Any new subscriptions are appended to `subscribe_to` instead of modifying /// `subscriptions`. async fn handle_command( frame: Frame, subscribe_to: &mut Vec<String>, subscriptions: &mut StreamMap<String, Messages>, dst: &mut Connection, ) -> crate::Result<()> { // A command has been received from the client. // // Only `SUBSCRIBE` and `UNSUBSCRIBE` commands are permitted // in this context. match Command::from_frame(frame)? { Command::Subscribe(subscribe) => { // The `apply` method will subscribe to the channels we add to this // vector. subscribe_to.extend(subscribe.channels.into_iter()); } Command::Unsubscribe(mut unsubscribe) => { // If no channels are specified, this requests unsubscribing from // **all** channels. To implement this, the `unsubscribe.channels` // vec is populated with the list of channels currently subscribed // to. if unsubscribe.channels.is_empty() { unsubscribe.channels = subscriptions .keys() .map(|channel_name| channel_name.to_string()) .collect(); } for channel_name in unsubscribe.channels { subscriptions.remove(&channel_name); let response = make_unsubscribe_frame(channel_name, subscriptions.len()); dst.write_frame(&response).await?; } } command => { let cmd = Unknown::new(command.get_name()); cmd.apply(dst).await?; } } Ok(()) } /// Creates the response to a subcribe request. /// /// All of these functions take the `channel_name` as a `String` instead of /// a `&str` since `Bytes::from` can reuse the allocation in the `String`, and /// taking a `&str` would require copying the data. This allows the caller to /// decide whether to clone the channel name or not. fn make_subscribe_frame(channel_name: String, num_subs: usize) -> Frame { let mut response = Frame::array(); response.push_bulk(Bytes::from_static(b"subscribe")); response.push_bulk(Bytes::from(channel_name)); response.push_int(num_subs as u64); response } /// Creates the response to an unsubcribe request. fn make_unsubscribe_frame(channel_name: String, num_subs: usize) -> Frame { let mut response = Frame::array(); response.push_bulk(Bytes::from_static(b"unsubscribe")); response.push_bulk(Bytes::from(channel_name)); response.push_int(num_subs as u64); response } /// Creates a message informing the client about a new message on a channel that /// the client subscribes to. fn make_message_frame(channel_name: String, msg: Bytes) -> Frame { let mut response = Frame::array(); response.push_bulk(Bytes::from_static(b"message")); response.push_bulk(Bytes::from(channel_name)); response.push_bulk(msg); response } impl Unsubscribe { /// Create a new `Unsubscribe` command with the given `channels`. pub(crate) fn new(channels: &[String]) -> Unsubscribe { Unsubscribe { channels: channels.to_vec(), } } /// Parse a `Unsubscribe` instance from a received frame. /// /// The `Parse` argument provides a cursor-like API to read fields from the /// `Frame`. At this point, the entire frame has already been received from /// the socket. /// /// The `UNSUBSCRIBE` string has already been consumed. /// /// # Returns /// /// On success, the `Unsubscribe` value is returned. If the frame is /// malformed, `Err` is returned. /// /// # Format /// /// Expects an array frame containing at least one entry. /// /// ```text /// UNSUBSCRIBE [channel [channel ...]] /// ``` pub(crate) fn parse_frames(parse: &mut Parse) -> Result<Unsubscribe, ParseError> { use ParseError::EndOfStream; // There may be no channels listed, so start with an empty vec. let mut channels = vec![]; // Each entry in the frame must be a string or the frame is malformed. // Once all values in the frame have been consumed, the command is fully // parsed. loop { match parse.next_string() { // A string has been consumed from the `parse`, push it into the // list of channels to unsubscribe from. Ok(s) => channels.push(s), // The `EndOfStream` error indicates there is no further data to // parse. Err(EndOfStream) => break, // All other errors are bubbled up, resulting in the connection // being terminated. Err(err) => return Err(err), } } Ok(Unsubscribe { channels }) } /// Converts the command into an equivalent `Frame`. /// /// This is called by the client when encoding an `Unsubscribe` command to /// send to the server. pub(crate) fn into_frame(self) -> Frame { let mut frame = Frame::array(); frame.push_bulk(Bytes::from("unsubscribe".as_bytes())); for channel in self.channels { frame.push_bulk(Bytes::from(channel.into_bytes())); } frame } }