3 files changed, 259 insertions, 0 deletions
diff --git a/src/events/app/broadcaster.rs b/src/events/app/broadcaster.rs
new file mode 100644
index 0000000..6a1219a
--- /dev/null
+++ b/src/events/app/broadcaster.rs
@@ -0,0 +1,116 @@
+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 crate::{
+    events::repo::broadcast,
+    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.clone());
+    }
+
+    // panic: if ``channel`` has not been previously registered, and was not
+    // part of the initial set of channels.
+    pub fn subscribe(
+        &self,
+        channel: &channel::Id,
+    ) -> impl Stream<Item = broadcast::Message> + std::fmt::Debug {
+        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
+    }
+}
diff --git a/src/events/app/events.rs b/src/events/app/events.rs
new file mode 100644
index 0000000..a8814c9
--- /dev/null
+++ b/src/events/app/events.rs
@@ -0,0 +1,138 @@
+use chrono::TimeDelta;
+use futures::{
+    future,
+    stream::{self, StreamExt as _},
+    Stream,
+};
+use sqlx::sqlite::SqlitePool;
+
+use super::Broadcaster;
+use crate::{
+    clock::DateTime,
+    events::repo::broadcast::{self, Provider as _},
+    repo::{
+        channel::{self, Provider as _},
+        error::NotFound as _,
+        login::Login,
+    },
+};
+
+pub struct Events<'a> {
+    db: &'a SqlitePool,
+    broadcaster: &'a Broadcaster,
+}
+
+impl<'a> Events<'a> {
+    pub const fn new(db: &'a SqlitePool, broadcaster: &'a Broadcaster) -> Self {
+        Self { db, broadcaster }
+    }
+
+    pub async fn send(
+        &self,
+        login: &Login,
+        channel: &channel::Id,
+        body: &str,
+        sent_at: &DateTime,
+    ) -> Result<broadcast::Message, EventsError> {
+        let mut tx = self.db.begin().await?;
+        let channel = tx
+            .channels()
+            .by_id(channel)
+            .await
+            .not_found(|| EventsError::ChannelNotFound(channel.clone()))?;
+        let message = tx
+            .broadcast()
+            .create(login, &channel, body, sent_at)
+            .await?;
+        tx.commit().await?;
+
+        self.broadcaster.broadcast(&channel.id, &message);
+        Ok(message)
+    }
+
+    pub async fn subscribe(
+        &self,
+        channel: &channel::Id,
+        subscribed_at: &DateTime,
+        resume_at: Option<broadcast::Sequence>,
+    ) -> Result<impl Stream<Item = broadcast::Message> + std::fmt::Debug, EventsError> {
+        // Somewhat arbitrarily, expire after 90 days.
+        let expire_at = subscribed_at.to_owned() - TimeDelta::days(90);
+
+        let mut tx = self.db.begin().await?;
+        let channel = tx
+            .channels()
+            .by_id(channel)
+            .await
+            .not_found(|| EventsError::ChannelNotFound(channel.clone()))?;
+
+        // Subscribe before retrieving, to catch messages broadcast while we're
+        // querying the DB. We'll prune out duplicates later.
+        let live_messages = self.broadcaster.subscribe(&channel.id);
+
+        tx.broadcast().expire(&expire_at).await?;
+        let stored_messages = tx.broadcast().replay(&channel, resume_at).await?;
+        tx.commit().await?;
+
+        let resume_broadcast_at = stored_messages
+            .last()
+            .map(|message| message.sequence)
+            .or(resume_at);
+
+        // This should always be the case, up to integer rollover, primarily
+        // because every message in stored_messages has a sequence not less
+        // than `resume_at`, or `resume_at` is None. We use the last message
+        // (if any) to decide when to resume the `live_messages` stream.
+        //
+        // It probably simplifies to assert!(resume_at <= resume_broadcast_at), but
+        // this form captures more of the reasoning.
+        assert!(
+            (resume_at.is_none() && resume_broadcast_at.is_none())
+                || (stored_messages.is_empty() && resume_at == resume_broadcast_at)
+                || resume_at < resume_broadcast_at
+        );
+
+        // no skip_expired or resume transforms for stored_messages, as it's
+        // constructed not to contain messages meeting either criterion.
+        //
+        // * skip_expired is redundant with the `tx.broadcasts().expire(…)` call;
+        // * resume is redundant with the resume_at argument to
+        //   `tx.broadcasts().replay(…)`.
+        let stored_messages = stream::iter(stored_messages);
+        let live_messages = live_messages
+            // Sure, it's temporally improbable that we'll ever skip a message
+            // that's 90 days old, but there's no reason not to be thorough.
+            .filter(Self::skip_expired(&expire_at))
+            // Filtering on the broadcast resume point filters out messages
+            // before resume_at, and filters out messages duplicated from
+            // stored_messages.
+            .filter(Self::resume(resume_broadcast_at));
+
+        Ok(stored_messages.chain(live_messages))
+    }
+
+    fn resume(
+        resume_at: Option<broadcast::Sequence>,
+    ) -> impl for<'m> FnMut(&'m broadcast::Message) -> future::Ready<bool> {
+        move |msg| {
+            future::ready(match resume_at {
+                None => true,
+                Some(resume_at) => msg.sequence > resume_at,
+            })
+        }
+    }
+    fn skip_expired(
+        expire_at: &DateTime,
+    ) -> impl for<'m> FnMut(&'m broadcast::Message) -> future::Ready<bool> {
+        let expire_at = expire_at.to_owned();
+        move |msg| future::ready(msg.sent_at > expire_at)
+    }
+}
+
+#[derive(Debug, thiserror::Error)]
+pub enum EventsError {
+    #[error("channel {0} not found")]
+    ChannelNotFound(channel::Id),
+    #[error(transparent)]
+    DatabaseError(#[from] sqlx::Error),
+}
diff --git a/src/events/app/mod.rs b/src/events/app/mod.rs
new file mode 100644
index 0000000..03a7da2
--- /dev/null
+++ b/src/events/app/mod.rs
@@ -0,0 +1,5 @@
+mod broadcaster;
+mod events;
+
+pub use self::broadcaster::Broadcaster;
+pub use self::events::{Events, EventsError};