AccountingNext/Schaad.Accounting.Db/Repositories/BaseRepository.cs

using System;
using System.Collections.Concurrent;
using System.Collections.Generic;
using System.IO;
using System.Text;
using System.Xml;
using System.Xml.Serialization;
using Schaad.Accounting.Interfaces;

namespace Schaad.Accounting.Repositories
{
    public abstract class BaseRepository
    {
        // One lock per absolute file path so concurrent Save+Save and Save+Load are serialized
        // and can't observe a half-written file.
        private static readonly ConcurrentDictionary<string, object> FileLocks =
            new(StringComparer.OrdinalIgnoreCase);

        protected readonly ISettingsService settingsService;
        protected readonly RepositoryCache cache;

        protected BaseRepository(ISettingsService settingsService, RepositoryCache cache)
        {
            this.settingsService = settingsService;
            this.cache = cache;
        }

        protected void EnsureFileExists(string fileName)
        {
            string filePath = Path.Combine(settingsService.GetDbPath(), fileName);
            lock (GetLock(filePath))
            {
                if (File.Exists(filePath) == false)
                {
                    var lastYearFile = Path.Combine(settingsService.GetLastYearDbPath(), fileName);
                    if (File.Exists(lastYearFile))
                    {
                        File.Copy(lastYearFile, filePath);
                    }
                }
            }
        }

        /// <summary>
        /// Load a list, cached per scope. First call reads the XML file; subsequent calls in
        /// the same scope return the cached list. <see cref="Save"/> invalidates the entry.
        /// </summary>
        protected List<T> LoadList<T>(string fileName)
        {
            var filePath = Path.Combine(settingsService.GetDbPath(), fileName);
            return cache.GetOrLoad(filePath, () => Load<List<T>>(fileName) ?? new List<T>());
        }

        /// <summary>
        /// Save an object to an xml file. Writes to a .tmp sibling and then atomically
        /// renames it, so a crash mid-write leaves the previous file intact. Invalidates
        /// the corresponding cache entry.
        /// </summary>
        protected void Save<T>(T obj, string fileName)
        {
            var filePath = Path.Combine(settingsService.GetDbPath(), fileName);
            var tmpPath = filePath + ".tmp";

            lock (GetLock(filePath))
            {
                var settings = new XmlWriterSettings
                {
                    Encoding = Encoding.UTF8,
                    Indent = true
                };

                try
                {
                    using (var writer = XmlWriter.Create(tmpPath, settings))
                    {
                        var serializer = new XmlSerializer(typeof(T));
                        serializer.Serialize(writer, obj);
                    }

                    File.Move(tmpPath, filePath, overwrite: true);
                }
                catch
                {
                    if (File.Exists(tmpPath))
                    {
                        try { File.Delete(tmpPath); } catch { /* best effort */ }
                    }
                    throw;
                }
            }

            cache.Invalidate(filePath);
        }

        /// <summary>
        /// Load an object from an xml file (uncached; used for one-off reads such as file
        /// existence bootstrapping). Prefer <see cref="LoadList{T}"/> for repository reads.
        /// </summary>
        protected T Load<T>(string fileName)
        {
            var filePath = Path.Combine(settingsService.GetDbPath(), fileName);

            lock (GetLock(filePath))
            {
                if (File.Exists(filePath) == false)
                {
                    return default(T);
                }

                using (XmlReader reader = XmlReader.Create(filePath))
                {
                    var serializer = new XmlSerializer(typeof(T));
                    return (T)serializer.Deserialize(reader);
                }
            }
        }

        private static object GetLock(string filePath)
            => FileLocks.GetOrAdd(filePath, _ => new object());
    }
}