Retain deleted messages and channels temporarily, to preserve events for replay.

Previously, when a channel (message) was deleted, `hi` would send events to all _connected_ clients to inform them of the deletion, then delete all memory of the channel (message). Any disconnected client, on reconnecting, would not receive the deletion event, and would de-synch with the service. The creation events were also immediately retconned out of the event stream, as well.

With this change, `hi` keeps a record of deleted channels (messages). When replaying events, these records are used to replay the deletion event. After 7 days, the retained data is deleted, both to keep storage under control and to conform to users' expectations that deleted means gone.

To match users' likely intuitions about what deletion does, deleting a channel (message) _does_ immediately delete some of its associated data. Channels' names are blanked, and messages' bodies are also blanked. When the event stream is replayed, the original channel.created (message.sent) event is "tombstoned", with an additional `deleted_at` field to inform clients. The included client does not use this field, at least yet.

The migration is, once again, screamingingly complicated due to sqlite's limited ALTER TABLE … ALTER COLUMN support.

This change also contains capabilities that would allow the API to return 410 Gone for deleted channels or messages, instead of 404. I did experiment with this, but it's tricky to do pervasively, especially since most app-level interfaces return an `Option<Channel>` or `Option<Message>`. Redesigning these to return either `Ok(Channel)` (`Ok(Message)`) or `Err(Error::NotFound)` or `Err(Error::Deleted)` is more work than I wanted to take on for this change, and the utility of 410 Gone responses is not obvious to me. We have other, more pressing API design warts to address.

1 files changed, 134 insertions, 70 deletions

diff --git a/src/message/repo.rs b/src/message/repo.rs
index 71c6d10..14ff7bf 100644
--- a/src/message/repo.rs
+++ b/src/message/repo.rs
@@ -53,14 +53,12 @@ impl<'c> Messages<'c> {
         )
         .map(|row| History {
             message: Message {
-                sent: Instant {
-                    at: row.sent_at,
-                    sequence: row.sent_sequence,
-                },
+                sent: Instant::new(row.sent_at, row.sent_sequence),
                 channel: row.channel,
                 sender: row.sender,
                 id: row.id,
                 body: row.body,
+                deleted_at: None,
             },
             deleted: None,
         })
@@ -70,41 +68,37 @@ impl<'c> Messages<'c> {
         Ok(message)
     }
 
-    pub async fn in_channel(
-        &mut self,
-        channel: &channel::History,
-        resume_at: ResumePoint,
-    ) -> Result<Vec<History>, sqlx::Error> {
+    pub async fn live(&mut self, channel: &channel::History) -> Result<Vec<History>, sqlx::Error> {
         let channel_id = channel.id();
         let messages = sqlx::query!(
             r#"
                 select
-                    channel as "channel: channel::Id",
-                    sender as "sender: login::Id",
+                    message.channel as "channel: channel::Id",
+                    message.sender as "sender: login::Id",
                     id as "id: Id",
-                    body,
-                    sent_at as "sent_at: DateTime",
-                    sent_sequence as "sent_sequence: Sequence"
+                    message.body,
+                    message.sent_at as "sent_at: DateTime",
+                    message.sent_sequence as "sent_sequence: Sequence",
+                    deleted.deleted_at as "deleted_at: DateTime",
+                    deleted.deleted_sequence as "deleted_sequence: Sequence"
                 from message
-                where channel = $1
-                    and coalesce(sent_sequence <= $2, true)
-                order by sent_sequence
+                left join message_deleted as deleted
+                    using (id)
+                where message.channel = $1
+                    and deleted.id is null
             "#,
             channel_id,
-            resume_at,
         )
         .map(|row| History {
             message: Message {
-                sent: Instant {
-                    at: row.sent_at,
-                    sequence: row.sent_sequence,
-                },
+                sent: Instant::new(row.sent_at, row.sent_sequence),
                 channel: row.channel,
                 sender: row.sender,
                 id: row.id,
                 body: row.body,
+                deleted_at: row.deleted_at,
             },
-            deleted: None,
+            deleted: Instant::optional(row.deleted_at, row.deleted_sequence),
         })
         .fetch_all(&mut *self.0)
         .await?;
@@ -116,30 +110,32 @@ impl<'c> Messages<'c> {
         let messages = sqlx::query!(
             r#"
                 select
-                    channel as "channel: channel::Id",
-                    sender as "sender: login::Id",
+                    message.channel as "channel: channel::Id",
+                    message.sender as "sender: login::Id",
                     id as "id: Id",
-                    body,
-                    sent_at as "sent_at: DateTime",
-                    sent_sequence as "sent_sequence: Sequence"
+                    message.body,
+                    message.sent_at as "sent_at: DateTime",
+                    message.sent_sequence as "sent_sequence: Sequence",
+                    deleted.deleted_at as "deleted_at: DateTime",
+                    deleted.deleted_sequence as "deleted_sequence: Sequence"
                 from message
-                where coalesce(sent_sequence <= $2, true)
-                order by sent_sequence
+                left join message_deleted as deleted
+                    using (id)
+                where coalesce(message.sent_sequence <= $2, true)
+                order by message.sent_sequence
             "#,
             resume_at,
         )
         .map(|row| History {
             message: Message {
-                sent: Instant {
-                    at: row.sent_at,
-                    sequence: row.sent_sequence,
-                },
+                sent: Instant::new(row.sent_at, row.sent_sequence),
                 channel: row.channel,
                 sender: row.sender,
                 id: row.id,
                 body: row.body,
+                deleted_at: row.deleted_at,
             },
-            deleted: None,
+            deleted: Instant::optional(row.deleted_at, row.deleted_sequence),
         })
         .fetch_all(&mut *self.0)
         .await?;
@@ -147,33 +143,35 @@ impl<'c> Messages<'c> {
         Ok(messages)
     }
 
-    async fn by_id(&mut self, message: &Id) -> Result<History, sqlx::Error> {
+    pub async fn by_id(&mut self, message: &Id) -> Result<History, sqlx::Error> {
         let message = sqlx::query!(
             r#"
                 select
-                    channel as "channel: channel::Id",
-                    sender as "sender: login::Id",
+                    message.channel as "channel: channel::Id",
+                    message.sender as "sender: login::Id",
                     id as "id: Id",
-                    body,
-                    sent_at as "sent_at: DateTime",
-                    sent_sequence as "sent_sequence: Sequence"
+                    message.body,
+                    message.sent_at as "sent_at: DateTime",
+                    message.sent_sequence as "sent_sequence: Sequence",
+                    deleted.deleted_at as "deleted_at: DateTime",
+                    deleted.deleted_sequence as "deleted_sequence: Sequence"
                 from message
+                left join message_deleted as deleted
+                    using (id)
                 where id = $1
             "#,
             message,
         )
         .map(|row| History {
             message: Message {
-                sent: Instant {
-                    at: row.sent_at,
-                    sequence: row.sent_sequence,
-                },
+                sent: Instant::new(row.sent_at, row.sent_sequence),
                 channel: row.channel,
                 sender: row.sender,
                 id: row.id,
                 body: row.body,
+                deleted_at: row.deleted_at,
             },
-            deleted: None,
+            deleted: Instant::optional(row.deleted_at, row.deleted_sequence),
         })
         .fetch_one(&mut *self.0)
         .await?;
@@ -183,39 +181,103 @@ impl<'c> Messages<'c> {
 
     pub async fn delete(
         &mut self,
-        message: &Id,
+        message: &History,
         deleted: &Instant,
     ) -> Result<History, sqlx::Error> {
-        let history = self.by_id(message).await?;
+        let id = message.id();
 
         sqlx::query_scalar!(
             r#"
-                delete from message
-                where
-                    id = $1
-                returning 1 as "deleted: i64"
+                insert into message_deleted (id, deleted_at, deleted_sequence)
+                values ($1, $2, $3)
+                returning 1 as "deleted: bool"
             "#,
-            history.message.id,
+            id,
+            deleted.at,
+            deleted.sequence,
         )
         .fetch_one(&mut *self.0)
         .await?;
 
-        Ok(History {
-            deleted: Some(*deleted),
-            ..history
-        })
+        // Small social responsibility hack here: when a message is deleted, its body is
+        // retconned to have been the empty string. Someone reading the event stream
+        // afterwards, or looking at messages in the channel, cannot retrieve the
+        // "deleted" message by ignoring the deletion event.
+        sqlx::query_scalar!(
+            r#"
+                update message
+                set body = ""
+                where id = $1
+                returning 1 as "blanked: bool"
+            "#,
+            id,
+        )
+        .fetch_one(&mut *self.0)
+        .await?;
+
+        let message = self.by_id(id).await?;
+
+        Ok(message)
     }
 
-    pub async fn expired(&mut self, expire_at: &DateTime) -> Result<Vec<Id>, sqlx::Error> {
+    pub async fn purge(&mut self, purge_at: &DateTime) -> Result<(), sqlx::Error> {
         let messages = sqlx::query_scalar!(
             r#"
+                delete from message_deleted
+                where deleted_at < $1
+                returning id as "id: Id"
+            "#,
+            purge_at,
+        )
+        .fetch_all(&mut *self.0)
+        .await?;
+
+        for message in messages {
+            sqlx::query!(
+                r#"
+                    delete from message
+                    where id = $1
+                "#,
+                message,
+            )
+            .execute(&mut *self.0)
+            .await?;
+        }
+
+        Ok(())
+    }
+
+    pub async fn expired(&mut self, expire_at: &DateTime) -> Result<Vec<History>, sqlx::Error> {
+        let messages = sqlx::query!(
+            r#"
                 select
-                    id as "message: Id"
+                    id as "id: Id",
+                    message.channel as "channel: channel::Id",
+                    message.sender as "sender: login::Id",
+                    message.sent_at as "sent_at: DateTime",
+                    message.sent_sequence as "sent_sequence: Sequence",
+                    message.body,
+                    deleted.deleted_at as "deleted_at: DateTime",
+                    deleted.deleted_sequence as "deleted_sequence: Sequence"
                 from message
-                where sent_at < $1
+                left join message_deleted as deleted
+                    using (id)
+                where message.sent_at < $1
+                    and deleted.id is null
             "#,
             expire_at,
         )
+        .map(|row| History {
+            message: Message {
+                sent: Instant::new(row.sent_at, row.sent_sequence),
+                id: row.id,
+                channel: row.channel,
+                sender: row.sender,
+                body: row.body,
+                deleted_at: row.deleted_at,
+            },
+            deleted: Instant::optional(row.deleted_at, row.deleted_sequence),
+        })
         .fetch_all(&mut *self.0)
         .await?;
 
@@ -226,29 +288,31 @@ impl<'c> Messages<'c> {
         let messages = sqlx::query!(
             r#"
                 select
-                    channel as "channel: channel::Id",
-                    sender as "sender: login::Id",
                     id as "id: Id",
-                    body,
-                    sent_at as "sent_at: DateTime",
-                    sent_sequence as "sent_sequence: Sequence"
+                    message.channel as "channel: channel::Id",
+                    message.sender as "sender: login::Id",
+                    message.sent_at as "sent_at: DateTime",
+                    message.sent_sequence as "sent_sequence: Sequence",
+                    message.body,
+                    deleted.deleted_at as "deleted_at: DateTime",
+                    deleted.deleted_sequence as "deleted_sequence: Sequence"
                 from message
+                left join message_deleted as deleted
+                    using (id)
                 where coalesce(message.sent_sequence > $1, true)
             "#,
             resume_at,
         )
         .map(|row| History {
             message: Message {
-                sent: Instant {
-                    at: row.sent_at,
-                    sequence: row.sent_sequence,
-                },
+                sent: Instant::new(row.sent_at, row.sent_sequence),
                 channel: row.channel,
                 sender: row.sender,
                 id: row.id,
                 body: row.body,
+                deleted_at: row.deleted_at,
             },
-            deleted: None,
+            deleted: Instant::optional(row.deleted_at, row.deleted_sequence),
         })
         .fetch_all(&mut *self.0)
         .await?;