[MM-28100]: fix getChannelByNameForTeamName permissions (#15525)

Summary:
fix getChannelByNameForTeamName to allow team members to join public channels. I'm not sure why, but the authorization logic of getChannelByName and getChannelByNameForTeamName was different. After this change they behave the same. This fixes an issue with the mobile app.

Ticket Link:
https://mattermost.atlassian.net/browse/MM-28100
Related PR: mattermost/mattermost-mobile#4810

api4/channel.go

@@ -1150,7 +1150,15 @@ func getChannelByNameForTeamName(c *Context, w http.ResponseWriter, r *http.Requ
 		return
 	}
 
-	if !c.App.SessionHasPermissionToChannel(*c.App.Session(), channel.Id, model.PERMISSION_READ_CHANNEL) {
+	teamOk := c.App.SessionHasPermissionToTeam(*c.App.Session(), channel.TeamId, model.PERMISSION_READ_PUBLIC_CHANNEL)
+	channelOk := c.App.SessionHasPermissionToChannel(*c.App.Session(), channel.Id, model.PERMISSION_READ_CHANNEL)
+
+	if channel.Type == model.CHANNEL_OPEN {
+		if !teamOk && !channelOk {
+			c.SetPermissionError(model.PERMISSION_READ_PUBLIC_CHANNEL)
+			return
+		}
+	} else if !channelOk {
 		c.Err = model.NewAppError("getChannelByNameForTeamName", "app.channel.get_by_name.missing.app_error", nil, "teamId="+channel.TeamId+", "+"name="+channel.Name+"", http.StatusNotFound)
 		return
 	}

api4/channel_test.go

@@ -1907,6 +1907,11 @@ func TestGetChannelByNameForTeamName(t *testing.T) {
 
 	_, resp = Client.GetChannelByNameForTeamName(th.BasicChannel.Name, th.BasicTeam.Name, "")
 	CheckNoError(t, resp)
+	require.Equal(t, th.BasicChannel.Name, channel.Name, "names did not match")
+
+	channel, resp = Client.GetChannelByNameForTeamName(th.BasicPrivateChannel.Name, th.BasicTeam.Name, "")
+	CheckNoError(t, resp)
+	require.Equal(t, th.BasicPrivateChannel.Name, channel.Name, "names did not match")
 
 	_, resp = Client.GetChannelByNameForTeamName(th.BasicDeletedChannel.Name, th.BasicTeam.Name, "")
 	CheckNotFoundStatus(t, resp)
@@ -1915,6 +1920,14 @@ func TestGetChannelByNameForTeamName(t *testing.T) {
 	CheckNoError(t, resp)
 	require.Equal(t, th.BasicDeletedChannel.Name, channel.Name, "names did not match")
 
+	Client.RemoveUserFromChannel(th.BasicChannel.Id, th.BasicUser.Id)
+	_, resp = Client.GetChannelByNameForTeamName(th.BasicChannel.Name, th.BasicTeam.Name, "")
+	CheckNoError(t, resp)
+
+	Client.RemoveUserFromChannel(th.BasicPrivateChannel.Id, th.BasicUser.Id)
+	_, resp = Client.GetChannelByNameForTeamName(th.BasicPrivateChannel.Name, th.BasicTeam.Name, "")
+	CheckNotFoundStatus(t, resp)
+
 	_, resp = Client.GetChannelByNameForTeamName(th.BasicChannel.Name, model.NewRandomString(15), "")
 	CheckNotFoundStatus(t, resp)
 
@@ -1928,7 +1941,7 @@ func TestGetChannelByNameForTeamName(t *testing.T) {
 	user := th.CreateUser()
 	Client.Login(user.Email, user.Password)
 	_, resp = Client.GetChannelByNameForTeamName(th.BasicChannel.Name, th.BasicTeam.Name, "")
-	CheckNotFoundStatus(t, resp)
+	CheckForbiddenStatus(t, resp)
 }
 
 func TestGetChannelMembers(t *testing.T) {