Avoid buffering file download bundles in to memory

MareSynchronosServer/MareSynchronosStaticFilesServer/Controllers/CacheController.cs

@@ -34,27 +34,22 @@ public class CacheController : ControllerBase
         _requestQueue.ActivateRequest(requestId);
 
         Response.ContentType = "application/octet-stream";
-        var memoryStream = new MemoryStream();
-        var streamWriter = new BinaryWriter(memoryStream);
 
         long requestSize = 0;
+        var streamList = new List<Stream>();
 
         foreach (var file in request.FileIds)
         {
             var fs = await _cachedFileProvider.GetAndDownloadFileStream(file);
             if (fs == null) continue;
-            streamWriter.Write(Encoding.ASCII.GetBytes("#" + file + ":" + fs.Length.ToString(CultureInfo.InvariantCulture) + "#"));
-            byte[] buffer = new byte[fs.Length];
-            _ = await fs.ReadAsync(buffer, HttpContext.RequestAborted);
-            streamWriter.Write(buffer);
+            var headerBytes = Encoding.ASCII.GetBytes("#" + file + ":" + fs.Length.ToString(CultureInfo.InvariantCulture) + "#");
+            streamList.Add(new MemoryStream(headerBytes));
+            streamList.Add(fs);
             requestSize += fs.Length;
         }
 
-        streamWriter.Flush();
-        memoryStream.Position = 0;
-
         _fileStatisticsService.LogRequest(requestSize);
 
-        return _requestFileStreamResultFactory.Create(requestId, memoryStream);
+        return _requestFileStreamResultFactory.Create(requestId, new ConcatenatedStreamReader(streamList));
     }
 }

MareSynchronosServer/MareSynchronosStaticFilesServer/Utils/RequestFileStreamResultFactory.cs

@@ -17,9 +17,9 @@ public class RequestFileStreamResultFactory
         _configurationService = configurationService;
     }
 
-    public RequestFileStreamResult Create(Guid requestId, MemoryStream ms)
+    public RequestFileStreamResult Create(Guid requestId, Stream stream)
     {
         return new RequestFileStreamResult(requestId, _requestQueueService,
-            _metrics, ms, "application/octet-stream");
+            _metrics, stream, "application/octet-stream");
     }
 }

MareSynchronosServer/MareSynchronosStaticFilesServer/Utils/StreamUtils.cs

@@ -0,0 +1,109 @@
+namespace MareSynchronosStaticFilesServer.Utils;
+
+public class CountedStream : Stream
+{
+    private readonly Stream _stream;
+    public ulong BytesRead { get; private set; }
+    public ulong BytesWritten { get; private set; }
+
+    public CountedStream(Stream underlyingStream)
+    {
+        _stream = underlyingStream;
+    }
+
+    public override bool CanRead => _stream.CanRead;
+
+    public override bool CanSeek => _stream.CanSeek;
+
+    public override bool CanWrite => _stream.CanWrite;
+
+    public override long Length => _stream.Length;
+
+    public override long Position { get => _stream.Position; set => _stream.Position = value; }
+
+    public override void Flush()
+    {
+        _stream.Flush();
+    }
+
+    public override int Read(byte[] buffer, int offset, int count)
+    {
+        int n = _stream.Read(buffer, offset, count);
+        BytesRead += (ulong)n;
+        return n;
+    }
+
+    public override long Seek(long offset, SeekOrigin origin)
+    {
+        return _stream.Seek(offset, origin);
+    }
+
+    public override void SetLength(long value)
+    {
+        _stream.SetLength(value);
+    }
+
+    public override void Write(byte[] buffer, int offset, int count)
+    {
+        BytesWritten += (ulong)count;
+        _stream.Write(buffer, offset, count);
+    }
+}
+
+public class ConcatenatedStreamReader : Stream
+{
+    private IEnumerable<Stream> _streams;
+    private IEnumerator<Stream> _iter;
+    private bool _finished;
+
+    public ConcatenatedStreamReader(IEnumerable<Stream> streams)
+    {
+        _streams = streams;
+        _iter = streams.GetEnumerator();
+        _finished = !_iter.MoveNext();
+    }
+
+    public override bool CanRead => true;
+
+    public override bool CanSeek => false;
+
+    public override bool CanWrite => false;
+
+    public override long Length => throw new NotSupportedException();
+
+    public override long Position { get => throw new NotSupportedException(); set => throw new NotSupportedException(); }
+
+    public override void Flush()
+    {
+    }
+
+    public override int Read(byte[] buffer, int offset, int count)
+    {
+        int n = 0;
+
+        while (n == 0 && !_finished)
+        {
+            n = _iter.Current.Read(buffer, offset, count);
+
+            if (n == 0)
+                _finished = !_iter.MoveNext();
+        }
+
+        return n;
+    }
+
+    public override long Seek(long offset, SeekOrigin origin)
+    {
+        throw new NotSupportedException();
+    }
+
+    public override void SetLength(long value)
+    {
+        throw new NotSupportedException();
+    }
+
+    public override void Write(byte[] buffer, int offset, int count)
+    {
+        throw new NotSupportedException();
+    }
+}