1 files changed, 111 insertions, 0 deletions
diff --git a/src/events/app.rs b/src/events/app.rs
new file mode 100644
index 0000000..dfb23d7
--- /dev/null
+++ b/src/events/app.rs
@@ -0,0 +1,111 @@
+use std::collections::{hash_map::Entry, HashMap};
+use std::sync::{Arc, Mutex, MutexGuard};
+
+use futures::{future, stream::StreamExt as _, Stream};
+use sqlx::sqlite::SqlitePool;
+use tokio::sync::broadcast::{channel, Sender};
+use tokio_stream::wrappers::{errors::BroadcastStreamRecvError, BroadcastStream};
+
+use super::repo::broadcast;
+use crate::repo::channel::{self, Provider as _};
+
+// Clones will share the same senders collection.
+#[derive(Clone)]
+pub struct Broadcaster {
+    // The use of std::sync::Mutex, and not tokio::sync::Mutex, follows Tokio's
+    // own advice: <https://tokio.rs/tokio/tutorial/shared-state>. Methods that
+    // lock it must be sync.
+    senders: Arc<Mutex<HashMap<channel::Id, Sender<broadcast::Message>>>>,
+}
+
+impl Broadcaster {
+    pub async fn from_database(db: &SqlitePool) -> Result<Self, sqlx::Error> {
+        let mut tx = db.begin().await?;
+        let channels = tx.channels().all().await?;
+        tx.commit().await?;
+
+        let channels = channels.iter().map(|c| &c.id);
+        let broadcaster = Self::new(channels);
+        Ok(broadcaster)
+    }
+
+    fn new<'i>(channels: impl IntoIterator<Item = &'i channel::Id>) -> Self {
+        let senders: HashMap<_, _> = channels
+            .into_iter()
+            .cloned()
+            .map(|id| (id, Self::make_sender()))
+            .collect();
+
+        Self {
+            senders: Arc::new(Mutex::new(senders)),
+        }
+    }
+
+    // panic: if ``channel`` is already registered.
+    pub fn register_channel(&self, channel: &channel::Id) {
+        match self.senders().entry(channel.clone()) {
+            // This ever happening indicates a serious logic error.
+            Entry::Occupied(_) => panic!("duplicate channel registration for channel {channel}"),
+            Entry::Vacant(entry) => {
+                entry.insert(Self::make_sender());
+            }
+        }
+    }
+
+    // panic: if ``channel`` has not been previously registered, and was not
+    // part of the initial set of channels.
+    pub fn broadcast(&self, channel: &channel::Id, message: broadcast::Message) {
+        let tx = self.sender(channel);
+
+        // Per the Tokio docs, the returned error is only used to indicate that
+        // there are no receivers. In this use case, that's fine; a lack of
+        // listening consumers (chat clients) when a message is sent isn't an
+        // error.
+        //
+        // The successful return value, which includes the number of active
+        // receivers, also isn't that interesting to us.
+        let _ = tx.send(message);
+    }
+
+    // panic: if ``channel`` has not been previously registered, and was not
+    // part of the initial set of channels.
+    pub fn listen(&self, channel: &channel::Id) -> impl Stream<Item = broadcast::Message> {
+        let rx = self.sender(channel).subscribe();
+
+        BroadcastStream::from(rx)
+            .take_while(|r| {
+                future::ready(match r {
+                    Ok(_) => true,
+                    // Stop the stream here. This will disconnect SSE clients
+                    // (see `routes.rs`), who will then resume from
+                    // `Last-Event-ID`, allowing them to catch up by reading
+                    // the skipped messages from the database.
+                    Err(BroadcastStreamRecvError::Lagged(_)) => false,
+                })
+            })
+            .map(|r| {
+                // Since the previous transform stops at the first error, this
+                // should always hold.
+                //
+                // See also <https://users.rust-lang.org/t/taking-from-stream-while-ok/48854>.
+                r.expect("after filtering, only `Ok` messages should remain")
+            })
+    }
+
+    // panic: if ``channel`` has not been previously registered, and was not
+    // part of the initial set of channels.
+    fn sender(&self, channel: &channel::Id) -> Sender<broadcast::Message> {
+        self.senders()[channel].clone()
+    }
+
+    fn senders(&self) -> MutexGuard<HashMap<channel::Id, Sender<broadcast::Message>>> {
+        self.senders.lock().unwrap() // propagate panics when mutex is poisoned
+    }
+
+    fn make_sender() -> Sender<broadcast::Message> {
+        // Queue depth of 16 chosen entirely arbitrarily. Don't read too much
+        // into it.
+        let (tx, _) = channel(16);
+        tx
+    }
+}