MM-41349: CRT, fix LastUpdated semantics (#19523)

* deadcode: remove UpdateChannelLastViewedAt

* deadcode: remove ThreadStore.(Save(Multiple)|Update|Delete)

* deadcode: followThead in App.MarkChannelAsUnreadFromPost

* document ThreadMembership, Thread structs

* maintain LastUpdated consistently

Whenever we touch a `ThreadMembership` record, we should be setting `LastUpdated` to the current timestamp. The mobile client relies on this to detect changes to these records.

* simplify: never updateThreads from `App.MarkChannelAsUnreadFromPost`

Change all invocations of `ChannelStore.UpdateLastViewedAtPost` from `App.MarkChannelAsUnreadFromPost` to pass `updateThreads` as `false`. When `ChannelStore.UpdateLastViewedAtPost` was invoked with `updateThreads` as `true`, it would in turn call `ThreadStore.UpdateUnreadsByChannel` but pass `updateViewedTimestamp` as `false`. This effectively updated the `LastUpdated` field of the corresponding thread memberships but never touched any of the actual data (such as `LastViewed`).

The overall CRT feature continued to work, because `App.MarkChannelAsUnreadFromPost` directly updates the relevant thread memberships via `ThreadStore.MaintainMembership`.

* deadcode: updateThreads in ChannelStore.UpdateLastViewedAtPost

* simplify: never updateThreads from App.SendNotifications

Change all invocations of `ChannelStore.IncrementMentionCount` from
`App.SendNotifications` to pass `updateThreads` as `false`. When `ChannelStore.IncrementMentionCount` was invoked with `updateThreads` as `true`, it would in turn call `ThreadStore.UpdateUnreadsByChannel` but pass `updateViewedTimestamp` as `false`. This effectively updated the `LastUpdated` field of the corresponding thread memberships but never touched any of the actual data (such as `UnreadMentions`).

The overall CRT feature continued to work, because `App.SendNotifications` directly updates the relevant thread memberships mention counts via `ThreadStore.MaintainMembership`.

* deadcode: updateThreads in ChannelStore.IncrementMentionCount

* fix & rename ThreadStore.UpdateUnreadsByChannel

Rename `ThreadStore.UpdateUnreadsByChannel` to `ThreadStore.UpdateLastViewedByThreadIds`, making it unconditionally set the `LastViewed` for the given threads (as well as `LastUpdated`).

All previous invocations of this method that passed `updateViewedTimestamp` have been previously removed.

* unrelated gofmt -w -s changes to satisfy linter

* always set LastUpdated to model.GetMillis()

* deadcode: ThreadStore.SaveMembership

* fix TestMarkUnreadWithThreads

* GetMasterX

model/thread.go

@@ -3,11 +3,24 @@
 
 package model
 
+// Thread tracks the metadata associated with a root post and its reply posts.
+//
+// Note that Thread metadata does not exist until the first reply to a root post.
 type Thread struct {
-	PostId       string      `json:"id"`
-	ChannelId    string      `json:"channel_id"`
-	ReplyCount   int64       `json:"reply_count"`
-	LastReplyAt  int64       `json:"last_reply_at"`
+	// PostId is the root post of the thread.
+	PostId string `json:"id"`
+
+	// ChannelId is the channel in which the thread was posted.
+	ChannelId string `json:"channel_id"`
+
+	// ReplyCount is the number of replies to the thread (excluding deleted posts).
+	ReplyCount int64 `json:"reply_count"`
+
+	// LastReplyAt is the timestamp of the most recent post to the thread.
+	LastReplyAt int64 `json:"last_reply_at"`
+
+	// Participants is a list of user ids that have replied to the thread, sorted by the oldest
+	// to newest. Note that the root post author is not included in this list until they reply.
 	Participants StringArray `json:"participants"`
 }
 
@@ -62,11 +75,37 @@ func (o *Thread) Etag() string {
 	return Etag(o.PostId, o.LastReplyAt)
 }
 
+// ThreadMembership models the relationship between a user and a thread of posts, with a similar
+// data structure as ChannelMembership.
 type ThreadMembership struct {
-	PostId         string `json:"post_id"`
-	UserId         string `json:"user_id"`
-	Following      bool   `json:"following"`
-	LastViewed     int64  `json:"last_view_at"`
-	LastUpdated    int64  `json:"last_update_at"`
-	UnreadMentions int64  `json:"unread_mentions"`
+	// PostId is the root post id of the thread in question.
+	PostId string `json:"post_id"`
+
+	// UserId is the user whose membership in the thread is being tracked.
+	UserId string `json:"user_id"`
+
+	// Following tracks whether the user is following the given thread. This defaults to true
+	// when a ThreadMembership record is created (a record doesn't exist until the user first
+	// starts following the thread), but the user can stop following or resume following at
+	// will.
+	Following bool `json:"following"`
+
+	// LastUpdated is either the creation time of the membership record, or the last time the
+	// membership record was changed (e.g. started/stopped following, viewed thread, mention
+	// count change).
+	//
+	// This field is used to constrain queries of thread memberships to those updated after
+	// a given timestamp (e.g. on websocket reconnect). It's also used as the time column for
+	// deletion decisions during any configured retention policy.
+	LastUpdated int64 `json:"last_update_at"`
+
+	// LastViewed is the last time the user viewed this thread. It is the thread analogue to
+	// the ChannelMembership's LastViewedAt and is used to decide when there are new replies
+	// for the user and where the user should start reading.
+	LastViewed int64 `json:"last_view_at"`
+
+	// UnreadMentions is the number of unseen at-mentions for the user in the given thread. It
+	// is the thread analogue to the ChannelMembership's MentionCount, and is used to highlight
+	// threads with the mention count.
+	UnreadMentions int64 `json:"unread_mentions"`
 }