Add ResumeDownloadToFileAsync

StabilityMatrix.Core/Services/DownloadService.cs

@@ -101,6 +101,105 @@ public class DownloadService : IDownloadService
         progress?.Report(new ProgressReport(1f, message: "Download complete!"));
     }
 
+    /// <inheritdoc />
+    public async Task ResumeDownloadToFileAsync(
+        string downloadUrl, 
+        string downloadPath, 
+        long existingFileSize,
+        IProgress<ProgressReport>? progress = null, 
+        string? httpClientName = null,
+        CancellationToken cancellationToken = default)
+    {
+        using var client = string.IsNullOrWhiteSpace(httpClientName)
+            ? httpClientFactory.CreateClient()
+            : httpClientFactory.CreateClient(httpClientName);
+
+        client.Timeout = TimeSpan.FromMinutes(10);
+        client.DefaultRequestHeaders.UserAgent.Add(
+            new ProductInfoHeaderValue("StabilityMatrix", "2.0")
+        );
+        
+        // Create file if it doesn't exist
+        if (!File.Exists(downloadPath))
+        {
+            logger.LogInformation("Resume file doesn't exist, creating file {DownloadPath}", downloadPath);
+            File.Create(downloadPath).Close();
+        }
+        
+        await using var file = new FileStream(
+            downloadPath,
+            FileMode.Append,
+            FileAccess.Write,
+            FileShare.None
+        );
+
+        long contentLength = 0;
+
+        using var request = new HttpRequestMessage();
+        request.Method = HttpMethod.Get;
+        request.RequestUri = new Uri(downloadUrl);
+        request.Headers.Range = new RangeHeaderValue(existingFileSize, null);
+
+        HttpResponseMessage? response = null;
+        foreach (var delay in Backoff.DecorrelatedJitterBackoffV2(
+                     TimeSpan.FromMilliseconds(50),
+                     retryCount: 4
+                 ))
+        {
+            response = await client.SendAsync(request, HttpCompletionOption.ResponseHeadersRead,
+                cancellationToken).ConfigureAwait(false);
+            contentLength = response.Content.Headers.ContentLength ?? 0;
+            
+            if (contentLength > 0)
+                break;
+            
+            logger.LogDebug("Retrying get-headers for content-length");
+            await Task.Delay(delay, cancellationToken).ConfigureAwait(false);
+        }
+        
+        if (response == null)
+        {
+            throw new ApplicationException("Response is null");
+        }
+        
+        var isIndeterminate = contentLength == 0;
+
+        await using var stream = await response.Content
+            .ReadAsStreamAsync(cancellationToken)
+            .ConfigureAwait(false);
+        var totalBytesRead = 0L;
+        var buffer = new byte[BufferSize];
+        while (true)
+        {
+            var bytesRead = await stream.ReadAsync(buffer, cancellationToken).ConfigureAwait(false);
+            if (bytesRead == 0)
+                break;
+            await file.WriteAsync(buffer.AsMemory(0, bytesRead), cancellationToken)
+                .ConfigureAwait(false);
+
+            totalBytesRead += bytesRead;
+
+            if (isIndeterminate)
+            {
+                progress?.Report(new ProgressReport(-1, isIndeterminate: true));
+            }
+            else
+            {
+                progress?.Report(
+                    new ProgressReport(
+                        current: Convert.ToUInt64(totalBytesRead),
+                        total: Convert.ToUInt64(contentLength),
+                        message: "Downloading..."
+                    )
+                );
+            }
+        }
+
+        await file.FlushAsync(cancellationToken).ConfigureAwait(false);
+
+        progress?.Report(new ProgressReport(1f, message: "Download complete!"));
+    }
+
     public async Task<Stream> GetImageStreamFromUrl(string url)
     {
         using var client = httpClientFactory.CreateClient();

StabilityMatrix.Core/Services/IDownloadService.cs

@@ -11,6 +11,15 @@ public interface IDownloadService
         string? httpClientName = null,
         CancellationToken cancellationToken = default
     );
+    
+    Task ResumeDownloadToFileAsync(
+        string downloadUrl,
+        string downloadPath,
+        long existingFileSize,
+        IProgress<ProgressReport>? progress = null,
+        string? httpClientName = null,
+        CancellationToken cancellationToken = default
+    );
 
     Task<Stream> GetImageStreamFromUrl(string url);
 }