[MM-18946] [MM-26721] [MM-6842] Cross team search+private channel autocomplete (#18468)

* Show private channels in autocomplete

This is supported in all Engines:
MySQL, Postgres, Bleve, Elasticsearch.

https://mattermost.atlassian.net/browse/MM-18496

```release-note
Private channels will now appear in channel autocomplete.
If you are using Bleve or ElasticSearch, you will have to reindex
the channels again to populate them with the new attributes.
```

A large chunk of this work has been based on the earlier
effort at https://github.com/mattermost/mattermost-server/pull/17804.

Full credit goes to https://github.com/arvinDarmawan.

* Add comment

```release-note
NONE
```

* Adding more tests

```release-note
NONE
```

* fix more tests

```release-note
NONE
```

* tmp

```release-note
NONE
```

* more fixes

```release-note
NONE
```

* add tests

```release-note
NONE
```

* Add review comments from previous PR

```release-note
NONE
```

* Add API to return all channels from all team

```release-note
NONE
```

* Added support for bleve and ES

```release-note
NONE
```

* Streaming response for GetAllChannels

```release-note
NONE
```

* Fix tests

```release-note
NONE
```

* Trigger CI
```release-note
NONE
```

* fix tests

```release-note
NONE
```

* Addressing review comments

```release-note
NONE
```

* Fix lint

```release-note
NONE
```

* Removing flaky test

```release-note
NONE
```

* Address comments

```release-note
NONE
```

* Trigger CI
```release-note
NONE
```

* Added /users/<userid>/channel_members endpoint

```release-note
NONE
```

* Minor edit

```release-note
NONE
```

* Improve embedding

```release-note
NONE
```

* Fix lint error

```release-note
NONE
```

Co-authored-by: Mattermod <mattermod@users.noreply.github.com>

api4/channel.go

@@ -35,6 +35,7 @@ func (api *API) InitChannel() {
 	api.BaseRoutes.ChannelsForTeam.Handle("/autocomplete", api.APISessionRequired(autocompleteChannelsForTeam)).Methods("GET")
 	api.BaseRoutes.ChannelsForTeam.Handle("/search_autocomplete", api.APISessionRequired(autocompleteChannelsForTeamForSearch)).Methods("GET")
 	api.BaseRoutes.User.Handle("/teams/{team_id:[A-Za-z0-9]+}/channels", api.APISessionRequired(getChannelsForTeamForUser)).Methods("GET")
+	api.BaseRoutes.User.Handle("/channels", api.APISessionRequired(getChannelsForUser)).Methods("GET")
 
 	api.BaseRoutes.ChannelCategories.Handle("", api.APISessionRequired(getCategoriesForTeamForUser)).Methods("GET")
 	api.BaseRoutes.ChannelCategories.Handle("", api.APISessionRequired(createCategoryForTeamForUser)).Methods("POST")
@@ -66,7 +67,7 @@ func (api *API) InitChannel() {
 	api.BaseRoutes.ChannelMembers.Handle("", api.APISessionRequired(getChannelMembers)).Methods("GET")
 	api.BaseRoutes.ChannelMembers.Handle("/ids", api.APISessionRequired(getChannelMembersByIds)).Methods("POST")
 	api.BaseRoutes.ChannelMembers.Handle("", api.APISessionRequired(addChannelMember)).Methods("POST")
-	api.BaseRoutes.ChannelMembersForUser.Handle("", api.APISessionRequired(getChannelMembersForUser)).Methods("GET")
+	api.BaseRoutes.ChannelMembersForUser.Handle("", api.APISessionRequired(getChannelMembersForTeamForUser)).Methods("GET")
 	api.BaseRoutes.ChannelMember.Handle("", api.APISessionRequired(getChannelMember)).Methods("GET")
 	api.BaseRoutes.ChannelMember.Handle("", api.APISessionRequired(removeChannelMember)).Methods("DELETE")
 	api.BaseRoutes.ChannelMember.Handle("/roles", api.APISessionRequired(updateChannelMemberRoles)).Methods("PUT")
@@ -877,7 +878,7 @@ func getChannelsForTeamForUser(c *Context, w http.ResponseWriter, r *http.Reques
 		return
 	}
 
-	channels, err := c.App.GetChannelsForUser(c.Params.TeamId, c.Params.UserId, c.Params.IncludeDeleted, lastDeleteAt)
+	channels, err := c.App.GetChannelsForTeamForUser(c.Params.TeamId, c.Params.UserId, c.Params.IncludeDeleted, lastDeleteAt)
 	if err != nil {
 		c.Err = err
 		return
@@ -899,6 +900,79 @@ func getChannelsForTeamForUser(c *Context, w http.ResponseWriter, r *http.Reques
 	}
 }
 
+func getChannelsForUser(c *Context, w http.ResponseWriter, r *http.Request) {
+	c.RequireUserId()
+	if c.Err != nil {
+		return
+	}
+
+	if !c.App.SessionHasPermissionToUser(*c.AppContext.Session(), c.Params.UserId) {
+		c.SetPermissionError(model.PermissionEditOtherUsers)
+		return
+	}
+
+	query := r.URL.Query()
+	lastDeleteAt, nErr := strconv.Atoi(query.Get("last_delete_at"))
+	if nErr != nil {
+		lastDeleteAt = 0
+	}
+	if lastDeleteAt < 0 {
+		c.SetInvalidURLParam("last_delete_at")
+		return
+	}
+
+	pageSize := 100
+	fromChannelID := ""
+	// We have to write `[` and `]` separately because we want to stream the response.
+	// The internal API is paginated, but the client always needs to get the full data.
+	// Therefore, to avoid forcing the client to go through all the pages,
+	// we stream the full data from server side itself.
+	//
+	// Note that this means if an error occurs in mid-stream, the response won't be
+	// fully JSON.
+	w.Write([]byte(`[`))
+	enc := json.NewEncoder(w)
+	for {
+		channels, err := c.App.GetChannelsForUser(c.Params.UserId, c.Params.IncludeDeleted, lastDeleteAt, pageSize, fromChannelID)
+		if err != nil {
+			// If the page size was a perfect multiple of the total number of results,
+			// then the last query will always return zero results.
+			if fromChannelID != "" && err.Id == "app.channel.get_channels.not_found.app_error" {
+				break
+			}
+			c.Err = err
+			return
+		}
+
+		err = c.App.FillInChannelsProps(channels)
+		if err != nil {
+			c.Err = err
+			return
+		}
+
+		// intermediary comma between sets
+		if fromChannelID != "" {
+			w.Write([]byte(`,`))
+		}
+
+		for i, ch := range channels {
+			if err := enc.Encode(ch); err != nil {
+				mlog.Warn("Error while writing response", mlog.Err(err))
+			}
+			if i < len(channels)-1 {
+				w.Write([]byte(`,`))
+			}
+		}
+
+		if len(channels) < pageSize {
+			break
+		}
+
+		fromChannelID = channels[len(channels)-1].Id
+	}
+	w.Write([]byte(`]`))
+}
+
 func autocompleteChannelsForTeam(c *Context, w http.ResponseWriter, r *http.Request) {
 	c.RequireTeamId()
 	if c.Err != nil {
@@ -912,7 +986,7 @@ func autocompleteChannelsForTeam(c *Context, w http.ResponseWriter, r *http.Requ
 
 	name := r.URL.Query().Get("name")
 
-	channels, err := c.App.AutocompleteChannels(c.Params.TeamId, name)
+	channels, err := c.App.AutocompleteChannelsForTeam(c.Params.TeamId, c.AppContext.Session().UserId, name)
 	if err != nil {
 		c.Err = err
 		return
@@ -1029,6 +1103,31 @@ func searchAllChannels(c *Context, w http.ResponseWriter, r *http.Request) {
 		c.SetInvalidParam("channel_search")
 		return
 	}
+
+	fromSysConsole := true
+	if val := r.URL.Query().Get("system_console"); val != "" {
+		fromSysConsole, err = strconv.ParseBool(val)
+		if err != nil {
+			c.SetInvalidParam("system_console")
+			return
+		}
+	}
+
+	if !fromSysConsole {
+		// If the request is not coming from system_console, only show the user level channels
+		// from all teams.
+		channels, err := c.App.AutocompleteChannels(c.AppContext.Session().UserId, props.Term)
+		if err != nil {
+			c.Err = err
+			return
+		}
+
+		if err := json.NewEncoder(w).Encode(channels); err != nil {
+			mlog.Warn("Error while writing response", mlog.Err(err))
+		}
+		return
+	}
+
 	// Only system managers may use the ExcludePolicyConstrained field
 	if props.ExcludePolicyConstrained && !c.App.SessionHasPermissionTo(*c.AppContext.Session(), model.PermissionSysconsoleReadComplianceDataRetentionPolicy) {
 		c.SetPermissionError(model.PermissionSysconsoleReadComplianceDataRetentionPolicy)
@@ -1039,6 +1138,7 @@ func searchAllChannels(c *Context, w http.ResponseWriter, r *http.Request) {
 		c.SetPermissionError(model.PermissionSysconsoleReadUserManagementChannels)
 		return
 	}
+
 	includeDeleted, _ := strconv.ParseBool(r.URL.Query().Get("include_deleted"))
 	includeDeleted = includeDeleted || props.IncludeDeleted
 
@@ -1297,7 +1397,7 @@ func getChannelMember(c *Context, w http.ResponseWriter, r *http.Request) {
 	}
 }
 
-func getChannelMembersForUser(c *Context, w http.ResponseWriter, r *http.Request) {
+func getChannelMembersForTeamForUser(c *Context, w http.ResponseWriter, r *http.Request) {
 	c.RequireUserId().RequireTeamId()
 	if c.Err != nil {
 		return