From 152d4c3d57d537eeed310b894d5584965fbbf49a Mon Sep 17 00:00:00 2001
From: Claudio Schaad <c.schaad@pog.ch>
Date: Thu, 2 Jul 2026 20:40:55 +0200
Subject: [PATCH] PR B: atomic XML writes and per-file locking

Save<T> writes to a .tmp sibling and atomically renames, so a crash
mid-write leaves the previous file intact. Load<T>, Save<T>, and
EnsureFileExisits share a per-absolute-path lock so concurrent
Save+Save and Save+Load can't observe a half-written file.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>
---
 .../Repositories/BaseRepository.cs            | 70 ++++++++++++++-----
 1 file changed, 51 insertions(+), 19 deletions(-)

diff --git a/Schaad.Accounting.Db/Repositories/BaseRepository.cs b/Schaad.Accounting.Db/Repositories/BaseRepository.cs
index 139c9da..9f727d8 100644
--- a/Schaad.Accounting.Db/Repositories/BaseRepository.cs
+++ b/Schaad.Accounting.Db/Repositories/BaseRepository.cs
@@ -1,4 +1,6 @@
-using System.IO;
+using System;
+using System.Collections.Concurrent;
+using System.IO;
 using System.Text;
 using System.Xml;
 using System.Xml.Serialization;
@@ -8,6 +10,11 @@ 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 BaseRepository(ISettingsService settingsService)
@@ -18,35 +25,53 @@ namespace Schaad.Accounting.Repositories
         protected void EnsureFileExisits(string fileName)
         {
             string filePath = Path.Combine(settingsService.GetDbPath(), fileName);
-            if (File.Exists(filePath) == false)
+            lock (GetLock(filePath))
             {
-                var lastYearFile = Path.Combine(settingsService.GetLastYearDbPath(), fileName);
-                if (File.Exists(lastYearFile))
+                if (File.Exists(filePath) == false)
                 {
-                    File.Copy(lastYearFile, filePath);
+                    var lastYearFile = Path.Combine(settingsService.GetLastYearDbPath(), fileName);
+                    if (File.Exists(lastYearFile))
+                    {
+                        File.Copy(lastYearFile, filePath);
+                    }
                 }
             }
         }
 
         /// <summary>
-        /// Save an object to an xml file
+        /// 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.
         /// </summary>
         protected void Save<T>(T obj, string fileName)
         {
             var filePath = Path.Combine(settingsService.GetDbPath(), fileName);
-            using (var sww = new MemoryStream())
+            var tmpPath = filePath + ".tmp";
+
+            lock (GetLock(filePath))
             {
                 var settings = new XmlWriterSettings
                 {
                     Encoding = Encoding.UTF8,
                     Indent = true
                 };
-                using (var writer = XmlWriter.Create(sww, settings))
+
+                try
                 {
-                    var xsSubmit = new XmlSerializer(typeof(T));
-                    xsSubmit.Serialize(writer, obj);
-                    var xml = Encoding.UTF8.GetString(sww.ToArray());
-                    File.WriteAllText(filePath, xml);
+                    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;
                 }
             }
         }
@@ -57,16 +82,23 @@ namespace Schaad.Accounting.Repositories
         protected T Load<T>(string fileName)
         {
             var filePath = Path.Combine(settingsService.GetDbPath(), fileName);
-            if (File.Exists(filePath) == false)
-            {
-                return default(T);
-            }
 
-            using (XmlReader reader = XmlReader.Create(filePath))
+            lock (GetLock(filePath))
             {
-                var serializer = new XmlSerializer(typeof(T));
-                return (T)serializer.Deserialize(reader);
+                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());
     }
 }
\ No newline at end of file