Stream snapshot download without storing the archive

src/Nethermind/Nethermind.Init.Snapshot.Test/FlakySnapshotServer.cs

@@ -0,0 +1,179 @@
+// SPDX-FileCopyrightText: 2026 Demerzel Solutions Limited
+// SPDX-License-Identifier: LGPL-3.0-only
+
+using System.Collections.Concurrent;
+using System.Net;
+
+namespace Nethermind.Init.Snapshot.Test;
+
+internal sealed class FlakySnapshotServer : IDisposable
+{
+    private readonly HttpListener _listener;
+    private readonly ConcurrentDictionary<string, int> _attemptsPerRange = new();
+    private int _requestCount;
+    private int _switchAfterRequests = int.MaxValue;
+    private byte[] _newContent = [];
+    private string? _newETag;
+
+    public FlakySnapshotServer()
+    {
+        (_listener, int port) = StartListener();
+        Url = $"http://127.0.0.1:{port}/snapshot.tar.zst";
+        Task.Run(AcceptLoopAsync);
+    }
+
+    public string Url { get; }
+
+    public byte[] Content { get; set; } = [];
+
+    public string? ETag { get; set; } = "\"v1\"";
+
+    public bool SupportsRanges { get; set; } = true;
+
+    public int? DropFirstAttemptPerRangeAfterBytes { get; set; }
+
+    public int? FailWithNotFoundAfterRequests { get; set; }
+
+    public int RequestCount => _requestCount;
+
+    public void SwitchSourceAfterRequests(int requestCount, byte[] newContent, string? newETag)
+    {
+        _newContent = newContent;
+        _newETag = newETag;
+        _switchAfterRequests = requestCount;
+    }
+
+    private static (HttpListener Listener, int Port) StartListener()
+    {
+        for (int attempt = 0; ; attempt++)
+        {
+            HttpListener listener = new();
+            int port = Random.Shared.Next(20000, 60000);
+            listener.Prefixes.Add($"http://127.0.0.1:{port}/");
+            try
+            {
+                listener.Start();
+                return (listener, port);
+            }
+            catch (HttpListenerException) when (attempt < 5)
+            {
+                listener.Close();
+            }
+        }
+    }
+
+    public void Dispose()
+    {
+        _listener.Stop();
+        _listener.Close();
+    }
+
+    private async Task AcceptLoopAsync()
+    {
+        while (_listener.IsListening)
+        {
+            HttpListenerContext context;
+            try
+            {
+                context = await _listener.GetContextAsync();
+            }
+            catch
+            {
+                return;
+            }
+
+            _ = Task.Run(() => HandleAsync(context));
+        }
+    }
+
+    private async Task HandleAsync(HttpListenerContext context)
+    {
+        int requestNumber = Interlocked.Increment(ref _requestCount);
+        byte[] content = requestNumber > _switchAfterRequests ? _newContent : Content;
+        string? etag = requestNumber > _switchAfterRequests ? _newETag : ETag;
+        HttpListenerResponse response = context.Response;
+
+        try
+        {
+            if (FailWithNotFoundAfterRequests is int failAfter && requestNumber > failAfter)
+            {
+                response.StatusCode = 404;
+                response.Close();
+                return;
+            }
+
+            if (etag is not null)
+                response.Headers["ETag"] = etag;
+            string? rangeHeader = context.Request.Headers["Range"];
+            string? ifRange = context.Request.Headers["If-Range"];
+            long from = 0;
+            long to = content.Length - 1;
+            bool ranged = SupportsRanges
+                          && rangeHeader is not null
+                          && (ifRange is null || ifRange == etag)
+                          && TryParseRange(rangeHeader, content.Length, ref from, ref to);
+
+            if (ranged && from >= content.Length)
+            {
+                response.StatusCode = 416;
+                response.Headers["Content-Range"] = $"bytes */{content.Length}";
+                response.Close();
+                return;
+            }
+
+            if (ranged)
+            {
+                response.StatusCode = 206;
+                response.Headers["Content-Range"] = $"bytes {from}-{to}/{content.Length}";
+            }
+            else
+            {
+                response.StatusCode = 200;
+                from = 0;
+                to = content.Length - 1;
+            }
+
+            long length = to - from + 1;
+            response.ContentLength64 = length;
+
+            string rangeKey = rangeHeader ?? "full";
+            int attempt = _attemptsPerRange.AddOrUpdate(rangeKey, 1, static (_, previous) => previous + 1);
+            if (DropFirstAttemptPerRangeAfterBytes is int dropAfter && attempt == 1 && length > dropAfter)
+            {
+                await response.OutputStream.WriteAsync(content.AsMemory((int)from, dropAfter));
+                response.Abort();
+                return;
+            }
+
+            await response.OutputStream.WriteAsync(content.AsMemory((int)from, (int)length));
+            response.Close();
+        }
+        catch
+        {
+            try
+            {
+                response.Abort();
+            }
+            catch
+            {
+            }
+        }
+    }
+
+    private static bool TryParseRange(string rangeHeader, long contentLength, ref long from, ref long to)
+    {
+        if (!rangeHeader.StartsWith("bytes=", StringComparison.Ordinal))
+            return false;
+
+        string[] parts = rangeHeader["bytes=".Length..].Split('-');
+        if (parts.Length != 2 || !long.TryParse(parts[0], out long start))
+            return false;
+
+        from = start;
+        to = parts[1].Length > 0 && long.TryParse(parts[1], out long end)
+            ? Math.Min(end, contentLength - 1)
+            : contentLength - 1;
+        return true;
+    }
+
+}