MM-38164: Paginate the GetPostThread API (#19485)

We implement a cursor based pagination model
to page through the posts in a given thread.

The cursor is a combination of the post.CreateAt+
post.Id to differentiate multiple posts in a given
timestamp.

Some additional parameters like direction, fromPost,
fromCreateAt and perPage were introduced to implement
this.

```release-note
NONE
```

api4/post.go

@@ -13,6 +13,7 @@ import (
 	"github.com/mattermost/mattermost-server/v6/audit"
 	"github.com/mattermost/mattermost-server/v6/model"
 	"github.com/mattermost/mattermost-server/v6/shared/mlog"
+	"github.com/mattermost/mattermost-server/v6/web"
 )
 
 func (api *API) InitPost() {
@@ -503,10 +504,56 @@ func getPostThread(c *Context, w http.ResponseWriter, r *http.Request) {
 	if c.Err != nil {
 		return
 	}
-	skipFetchThreads := r.URL.Query().Get("skipFetchThreads") == "true"
-	collapsedThreads := r.URL.Query().Get("collapsedThreads") == "true"
-	collapsedThreadsExtended := r.URL.Query().Get("collapsedThreadsExtended") == "true"
-	list, err := c.App.GetPostThread(c.Params.PostId, skipFetchThreads, collapsedThreads, collapsedThreadsExtended, c.AppContext.Session().UserId)
+
+	// For now, by default we return all items unless it's set to maintain
+	// backwards compatibility with mobile. But when the next ESR passes, we need to
+	// change this to web.PerPageDefault.
+	perPage := 0
+	if perPageStr := r.URL.Query().Get("perPage"); perPageStr != "" {
+		var err error
+		perPage, err = strconv.Atoi(perPageStr)
+		if err != nil || perPage > web.PerPageMaximum {
+			c.SetInvalidParam("perPage")
+			return
+		}
+	}
+
+	var fromCreateAt int64
+	if fromCreateAtStr := r.URL.Query().Get("fromCreateAt"); fromCreateAtStr != "" {
+		var err error
+		fromCreateAt, err = strconv.ParseInt(fromCreateAtStr, 10, 64)
+		if err != nil {
+			c.SetInvalidParam("fromCreateAt")
+			return
+		}
+	}
+
+	fromPost := r.URL.Query().Get("fromPost")
+	// Either both have to be set, or none have to be set.
+	// Setting one and not setting the other is an error.
+	if (fromPost == "" && fromCreateAt != 0) || (fromPost != "" && fromCreateAt == 0) {
+		c.SetInvalidParam("fromPost/fromCreateAt")
+		return
+	}
+
+	direction := ""
+	if dir := r.URL.Query().Get("direction"); dir != "" {
+		if dir != "up" && dir != "down" {
+			c.SetInvalidParam("direction")
+			return
+		}
+		direction = dir
+	}
+	opts := model.GetPostsOptions{
+		SkipFetchThreads:         r.URL.Query().Get("skipFetchThreads") == "true",
+		CollapsedThreads:         r.URL.Query().Get("collapsedThreads") == "true",
+		CollapsedThreadsExtended: r.URL.Query().Get("collapsedThreadsExtended") == "true",
+		PerPage:                  perPage,
+		Direction:                direction,
+		FromPost:                 fromPost,
+		FromCreateAt:             fromCreateAt,
+	}
+	list, err := c.App.GetPostThread(c.Params.PostId, opts, c.AppContext.Session().UserId)
 	if err != nil {
 		c.Err = err
 		return

api4/post_test.go

@@ -2184,6 +2184,22 @@ func TestGetPostThread(t *testing.T) {
 	require.Error(t, err)
 	CheckForbiddenStatus(t, resp)
 
+	// Sending some bad params
+	_, resp, err = client.GetPostThreadWithOpts(th.BasicPost.Id, "", model.GetPostsOptions{
+		CollapsedThreads: true,
+		FromPost:         "something",
+		PerPage:          10,
+	})
+	require.Error(t, err)
+	CheckBadRequestStatus(t, resp)
+
+	_, resp, err = client.GetPostThreadWithOpts(th.BasicPost.Id, "", model.GetPostsOptions{
+		CollapsedThreads: true,
+		Direction:        "sideways",
+	})
+	require.Error(t, err)
+	CheckBadRequestStatus(t, resp)
+
 	client.Logout()
 	_, resp, err = client.GetPostThread(model.NewId(), "", false)
 	require.Error(t, err)