SocialPub/PrivaPub/Infrastructure/Backup/RestoreMarker.cs

using System.Text.Json;

namespace PrivaPub.Infrastructure.Backup
{
	// A restore asked for (by the administrator's page or the CLI) and carried out at the next boot, before anything
	// else (Program, MaintenanceGate): restore.json in the backups' directory, written whole or not at all.
	public sealed class RestoreMarker
	{
		public const string FileName = "restore.json";
		public const int MaxAttempts = 3;

		public string Backup { get; set; }//the backup restored
		public string PreRestore { get; set; }//the backup taken just before, what the protective merge reads
		public string State { get; set; } = "pending";//pending, then running
		public string RequestedBy { get; set; }//who asked: a root's name, or "cli"
		public int Attempts { get; set; }
		public DateTime RequestedAt { get; set; } = DateTime.UtcNow;
		public string Error { get; set; }

		public static string PathIn(string backupsRoot) => Path.Combine(backupsRoot, FileName);

		public static RestoreMarker Read(string backupsRoot)
		{
			var path = PathIn(backupsRoot);
			if (!File.Exists(path))
				return default;
			try
			{
				return JsonSerializer.Deserialize<RestoreMarker>(File.ReadAllText(path));
			}
			catch (JsonException)
			{
				//never written half (Write is atomic): someone's hand; the server won't guess what was meant
				return new RestoreMarker { State = "unreadable", Attempts = MaxAttempts, Error = $"{FileName} can't be read" };
			}
		}

		public void Write(string backupsRoot)
		{
			Directory.CreateDirectory(backupsRoot);
			var path = PathIn(backupsRoot);
			var part = path + ".part";
			using (var file = new FileStream(part, FileMode.Create, FileAccess.Write))
			{
				JsonSerializer.Serialize(file, this);
				file.Flush(flushToDisk: true);
			}
			File.Move(part, path, overwrite: true);
		}

		public static void Clear(string backupsRoot)
		{
			var path = PathIn(backupsRoot);
			if (File.Exists(path))
				File.Delete(path);
		}
	}
}