Gateway: Fix repeat joins on same channel from stalling (#47)

Joining a channel returns a future which fires on receipt of two messages from discord (by locally storing a channel). However, joining this same channel again after a success returns only *one* such message, causing the command to hang until another join fires or the channel is left. This alters internal behaviour to correctly cancel an in-progress connection attempt, or return success with known data if such a connection is present.

This introduces a breaking change on `Call::update_state` to include the target `ChannelId`. The reason for this is that although the `ChannelId` of a target channel was being stored, server admins may move or kick a bot from its voice channel. This changes the true channel, and may accidentally trigger a "double join" elsewhere.

This fix was tested by using an example to have a bot join its channel twice, to do so in a channel it had been moved to, and to move from a channel it had been moved to.

src/info.rs

@@ -1,4 +1,4 @@
-use crate::id::{GuildId, UserId};
+use crate::id::{ChannelId, GuildId, UserId};
 use std::fmt;
 
 #[derive(Clone, Debug)]
@@ -8,8 +8,9 @@ pub(crate) enum ConnectionProgress {
 }
 
 impl ConnectionProgress {
-    pub fn new(guild_id: GuildId, user_id: UserId) -> Self {
+    pub(crate) fn new(guild_id: GuildId, user_id: UserId, channel_id: ChannelId) -> Self {
         ConnectionProgress::Incomplete(Partial {
+            channel_id,
             guild_id,
             user_id,
             ..Default::default()
@@ -24,7 +25,46 @@ impl ConnectionProgress {
         }
     }
 
-    pub(crate) fn apply_state_update(&mut self, session_id: String) -> bool {
+    pub(crate) fn in_progress(&self) -> bool {
+        matches!(self, ConnectionProgress::Incomplete(_))
+    }
+
+    pub(crate) fn channel_id(&self) -> ChannelId {
+        match self {
+            ConnectionProgress::Complete(conn_info) => conn_info
+                .channel_id
+                .expect("All code paths MUST set channel_id for local tracking."),
+            ConnectionProgress::Incomplete(part) => part.channel_id,
+        }
+    }
+
+    pub(crate) fn guild_id(&self) -> GuildId {
+        match self {
+            ConnectionProgress::Complete(conn_info) => conn_info.guild_id,
+            ConnectionProgress::Incomplete(part) => part.guild_id,
+        }
+    }
+
+    pub(crate) fn user_id(&self) -> UserId {
+        match self {
+            ConnectionProgress::Complete(conn_info) => conn_info.user_id,
+            ConnectionProgress::Incomplete(part) => part.user_id,
+        }
+    }
+
+    pub(crate) fn info(&self) -> Option<ConnectionInfo> {
+        match self {
+            ConnectionProgress::Complete(conn_info) => Some(conn_info.clone()),
+            _ => None,
+        }
+    }
+
+    pub(crate) fn apply_state_update(&mut self, session_id: String, channel_id: ChannelId) -> bool {
+        if self.channel_id() != channel_id {
+            // Likely that the bot was moved to a different channel by an admin.
+            *self = ConnectionProgress::new(self.guild_id(), self.user_id(), channel_id);
+        }
+
         use ConnectionProgress::*;
         match self {
             Complete(c) => {
@@ -33,7 +73,7 @@ impl ConnectionProgress {
                 should_reconn
             },
             Incomplete(i) => i
-                .apply_state_update(session_id)
+                .apply_state_update(session_id, channel_id)
                 .map(|info| {
                     *self = Complete(info);
                 })
@@ -66,6 +106,11 @@ impl ConnectionProgress {
 /// with the Songbird driver, lavalink, or other system.
 #[derive(Clone)]
 pub struct ConnectionInfo {
+    /// ID of the voice channel being joined, if it is known.
+    ///
+    /// This is not needed to establish a connection, but can be useful
+    /// for book-keeping.
+    pub channel_id: Option<ChannelId>,
     /// URL of the voice websocket gateway server assigned to this call.
     pub endpoint: String,
     /// ID of the target voice channel's parent guild.
@@ -83,6 +128,7 @@ pub struct ConnectionInfo {
 impl fmt::Debug for ConnectionInfo {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
         f.debug_struct("ConnectionInfo")
+            .field("channel_id", &self.channel_id)
             .field("endpoint", &self.endpoint)
             .field("guild_id", &self.guild_id)
             .field("session_id", &self.session_id)
@@ -94,6 +140,7 @@ impl fmt::Debug for ConnectionInfo {
 
 #[derive(Clone, Default)]
 pub(crate) struct Partial {
+    pub channel_id: ChannelId,
     pub endpoint: Option<String>,
     pub guild_id: GuildId,
     pub session_id: Option<String>,
@@ -104,6 +151,7 @@ pub(crate) struct Partial {
 impl fmt::Debug for Partial {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
         f.debug_struct("Partial")
+            .field("channel_id", &self.channel_id)
             .field("endpoint", &self.endpoint)
             .field("session_id", &self.session_id)
             .field("token_is_some", &self.token.is_some())
@@ -119,6 +167,7 @@ impl Partial {
             let token = self.token.take().unwrap();
 
             Some(ConnectionInfo {
+                channel_id: Some(self.channel_id),
                 endpoint,
                 session_id,
                 token,
@@ -130,7 +179,17 @@ impl Partial {
         }
     }
 
-    fn apply_state_update(&mut self, session_id: String) -> Option<ConnectionInfo> {
+    fn apply_state_update(
+        &mut self,
+        session_id: String,
+        channel_id: ChannelId,
+    ) -> Option<ConnectionInfo> {
+        if self.channel_id != channel_id {
+            self.endpoint = None;
+            self.token = None;
+        }
+
+        self.channel_id = channel_id;
         self.session_id = Some(session_id);
 
         self.finalise()