Actually fix concurrency in tests this time

DiscordChatExporter.Cli.Tests/Infra/ExportWrapper.cs

@@ -1,9 +1,11 @@
 using System;
+using System.Collections.Concurrent;
 using System.Collections.Generic;
 using System.IO;
 using System.Linq;
 using System.Reflection;
 using System.Text.Json;
+using System.Threading;
 using System.Threading.Tasks;
 using AngleSharp.Dom;
 using AngleSharp.Html.Dom;
@@ -18,6 +20,8 @@ namespace DiscordChatExporter.Cli.Tests.Infra;
 
 public static class ExportWrapper
 {
+    private static readonly ConcurrentDictionary<string, SemaphoreSlim> Locks = new();
+
     private static readonly string DirPath = Path.Combine(
         Path.GetDirectoryName(Assembly.GetExecutingAssembly().Location) ?? Directory.GetCurrentDirectory(),
         "ExportCache"
@@ -38,22 +42,33 @@ public static class ExportWrapper
 
     private static async ValueTask<string> ExportAsync(Snowflake channelId, ExportFormat format)
     {
-        var fileName = channelId.ToString() + '.' + format.GetFileExtension();
-        var filePath = Path.Combine(DirPath, fileName);
+        // Lock separately for each channel and format
+        var channelLock = Locks.GetOrAdd($"{channelId}_{format}", _ => new SemaphoreSlim(1, 1));
+        await channelLock.WaitAsync();
 
-        // Perform export only if it hasn't been done before
-        if (!File.Exists(filePath))
+        try
         {
-            await new ExportChannelsCommand
+            var fileName = channelId.ToString() + '.' + format.GetFileExtension();
+            var filePath = Path.Combine(DirPath, fileName);
+
+            // Perform export only if it hasn't been done before
+            if (!File.Exists(filePath))
             {
-                Token = Secrets.DiscordToken,
-                ChannelIds = new[] { channelId },
-                ExportFormat = format,
-                OutputPath = filePath
-            }.ExecuteAsync(new FakeConsole());
+                await new ExportChannelsCommand
+                {
+                    Token = Secrets.DiscordToken,
+                    ChannelIds = new[] { channelId },
+                    ExportFormat = format,
+                    OutputPath = filePath
+                }.ExecuteAsync(new FakeConsole());
+            }
+
+            return await File.ReadAllTextAsync(filePath);
+        }
+        finally
+        {
+            channelLock.Release();
         }
-
-        return await File.ReadAllTextAsync(filePath);
     }
 
     public static async ValueTask<IHtmlDocument> ExportAsHtmlAsync(Snowflake channelId) => Html.Parse(