Does this work?

LightlessSyncServer/LightlessSyncStaticFilesServer/Controllers/ShardServerFilesController.cs

@@ -20,13 +20,14 @@ public class ShardServerFilesController : ControllerBase
     [AllowAnonymous]
     public async Task<IActionResult> DownloadFileDirect(string hash, [FromQuery] long expires, [FromQuery] string signature)
     {
-        var result = await _cdnDownloadsService.GetDownloadAsync(hash, expires, signature).ConfigureAwait(false);
+        var result = _cdnDownloadsService.GetDownloadWithCacheCheck(hash, expires, signature);
 
         return result.Status switch
         {
             CDNDownloadsService.ResultStatus.Disabled => NotFound(),
             CDNDownloadsService.ResultStatus.Unauthorized => Unauthorized(),
             CDNDownloadsService.ResultStatus.NotFound => NotFound(),
+            CDNDownloadsService.ResultStatus.Downloading => StatusCode(503),
             CDNDownloadsService.ResultStatus.Success => PhysicalFile(result.File!.FullName, "application/octet-stream"),
             _ => NotFound()
         };

LightlessSyncServer/LightlessSyncStaticFilesServer/Services/CDNDownloadsService.cs

@@ -10,7 +10,8 @@ public class CDNDownloadsService
         Disabled,
         Unauthorized,
         NotFound,
-        Success
+        Success,
+        Downloading
     }
 
     public readonly record struct Result(ResultStatus Status, FileInfo? File);
@@ -53,4 +54,32 @@ public class CDNDownloadsService
 
         return new Result(ResultStatus.Success, fileInfo);
     }
+
+    public Result GetDownloadWithCacheCheck(string hash, long expiresUnixSeconds, string signature)
+    {
+        if (!_cdnDownloadUrlService.DirectDownloadsEnabled)
+        {
+            return new Result(ResultStatus.Disabled, null);
+        }
+
+        if (string.IsNullOrEmpty(signature) || string.IsNullOrEmpty(hash))
+        {
+            return new Result(ResultStatus.Unauthorized, null);
+        }
+
+        hash = hash.ToUpperInvariant();
+
+        if (!_cdnDownloadUrlService.TryValidateSignature(hash, expiresUnixSeconds, signature))
+        {
+            return new Result(ResultStatus.Unauthorized, null);
+        }
+
+        var fileInfo = _cachedFileProvider.TryGetLocalFileInfo(hash);
+        if (fileInfo == null)
+        {
+            return new Result(ResultStatus.Downloading, null);
+        }
+
+        return new Result(ResultStatus.Success, fileInfo);
+    }
 }

LightlessSyncServer/LightlessSyncStaticFilesServer/Services/CachedFileProvider.cs

@@ -219,4 +219,27 @@ public sealed class CachedFileProvider : IDisposable
     {
         return hashes.Exists(_currentTransfers.Keys.Contains);
     }
+
+    public FileInfo? TryGetLocalFileInfo(string hash)
+    {
+        var fi = FilePathUtil.GetFileInfoForHash(_hotStoragePath, hash);
+        if (fi != null)
+        {
+            return GetLocalFilePath(hash);
+        }
+
+        _ = Task.Run(async () =>
+        {
+            try
+            {
+                await DownloadFileWhenRequired(hash).ConfigureAwait(false);
+            }
+            catch (Exception ex)
+            {
+                _logger.LogWarning(ex, "Download failed for {hash}", hash);
+            }
+        });
+
+        return null;
+    }
 }