fix malware blacklist compatibility with SMAPI 4.5.2 and earlier

The revamped blacklist is more compact, which triggers the bug fixed in f090df0 (where syncing a smaller new version corrupts the file). This pads the file to the largest previous blacklist size to avoid the issue.

Author: Pathoschild

src/SMAPI.Web/BackgroundJobs/MalwareBlacklistJob.cs

@@ -20,6 +20,10 @@ internal class MalwareBlacklistJob : IBackgroundJob
     /*********
     ** Fields
     *********/
+    /// <summary>The minimum size in bytes for the generated malware blacklist file.</summary>
+    /// <remarks>The generated file is padded to this length, to avoid a bug in SMAPI 4.5.2 and earlier where a smaller newer version corrupts the file (since it overwrites the previous file without truncating).</remarks>
+    private const int MinBlacklistBytes = 8589;
+
     /// <summary>The HTTP client for fetching data from GitHub.</summary>
     private readonly IGitHubClient GitHubClient;
 
@@ -64,6 +68,7 @@ public async Task RunAsync(PerformContext? context)
         MalwareBlacklistConfig config = this.Config.Value;
         string repo = config.GitHubRepo ?? throw new InvalidOperationException("Can't update the malware blacklist: no GitHub repository is configured.");
         string outputPath = Path.Combine(this.WebRootPath, Startup.BlacklistFileName);
+        UTF8Encoding encoding = new(encoderShouldEmitUTF8Identifier: false);
 
         // fetch file
         GitFile file;
@@ -93,10 +98,15 @@ await this.GitHubClient.GetFileAsync(repo, config.FilePath, config.GitRef, previ
         string publicJson = MalwareBlacklistConverter.ToPublicJson(blacklist);
         context.WriteLine($"Updated malware blacklist ({blacklist.Blacklist.Length} mod entries and {blacklist.LooseFileBlacklist.Length} loose file entries).");
 
+        // pad to minimum size
+        int byteCount = encoding.GetByteCount(publicJson);
+        if (byteCount < MalwareBlacklistJob.MinBlacklistBytes)
+            publicJson += new string(' ', MalwareBlacklistJob.MinBlacklistBytes - byteCount);
+
         // save file
         {
             string tempPath = $"{outputPath}.tmp";
-            await File.WriteAllTextAsync(tempPath, publicJson, new UTF8Encoding(encoderShouldEmitUTF8Identifier: false));
+            await File.WriteAllTextAsync(tempPath, publicJson, encoding);
             File.Move(tempPath, outputPath, overwrite: true);
 
             context.WriteLine($"Saved to {Startup.BlacklistFileName}.");