Fix issue where users may not follow links to private messages they belong to (#26001)

* Fix issue where users may not follow links to private messages they belong to

* Update tests

server/channels/api4/post_test.go

@@ -3992,6 +3992,12 @@ func TestPostReminder(t *testing.T) {
 func TestPostGetInfo(t *testing.T) {
 	th := Setup(t).InitBasic()
 	defer th.TearDown()
+
+	defer th.RestoreDefaultRolePermissions(th.SaveDefaultRolePermissions())
+	th.RemovePermissionFromRole(model.PermissionManagePrivateChannelMembers.Id, model.SystemUserRoleId)
+	th.RemovePermissionFromRole(model.PermissionManagePrivateChannelMembers.Id, model.ChannelUserRoleId)
+	th.RemovePermissionFromRole(model.PermissionManagePrivateChannelMembers.Id, model.TeamUserRoleId)
+
 	client := th.Client
 	sysadminClient := th.SystemAdminClient
 	sysadminClient.AddTeamMember(context.Background(), th.BasicTeam.Id, th.SystemAdminUser.Id)

server/channels/app/post.go

@@ -2296,20 +2296,27 @@ func (a *App) GetPostInfo(c request.CTX, postID string) (*model.PostInfo, *model
 	}
 
 	hasPermissionToAccessChannel := false
-	if channel.Type == model.ChannelTypeOpen {
+
+	_, channelMemberErr := a.GetChannelMember(c, channel.Id, userID)
+
+	if channelMemberErr == nil {
 		hasPermissionToAccessChannel = true
-	} else if channel.Type == model.ChannelTypePrivate {
-		hasPermissionToAccessChannel = a.HasPermissionToChannel(c, userID, channel.Id, model.PermissionManagePrivateChannelMembers)
-	} else if channel.Type == model.ChannelTypeDirect || channel.Type == model.ChannelTypeGroup {
-		hasPermissionToAccessChannel = a.HasPermissionToChannel(c, userID, channel.Id, model.PermissionReadChannelContent)
+	}
+
+	if !hasPermissionToAccessChannel {
+		if channel.Type == model.ChannelTypeOpen {
+			hasPermissionToAccessChannel = true
+		} else if channel.Type == model.ChannelTypePrivate {
+			hasPermissionToAccessChannel = a.HasPermissionToChannel(c, userID, channel.Id, model.PermissionManagePrivateChannelMembers)
+		} else if channel.Type == model.ChannelTypeDirect || channel.Type == model.ChannelTypeGroup {
+			hasPermissionToAccessChannel = a.HasPermissionToChannel(c, userID, channel.Id, model.PermissionReadChannelContent)
+		}
 	}
 
 	if !hasPermissionToAccessChannel {
 		return nil, notFoundError
 	}
 
-	_, channelMemberErr := a.GetChannelMember(c, channel.Id, userID)
-
 	info := model.PostInfo{
 		ChannelId:          channel.Id,
 		ChannelType:        channel.Type,