fix(launcher): harden updater crash recovery

src/AcDream.Launcher.Core/Updates/VerifiedArtifactDownloader.cs

@@ -1,4 +1,5 @@
 using System.Buffers;
+using System.Net;
 using System.Security.Cryptography;
 
 namespace AcDream.Launcher.Core.Updates;
@@ -55,15 +56,11 @@ public sealed class VerifiedArtifactDownloader
         bool ownsDestination = false;
         try
         {
-            using HttpResponseMessage response = await _httpClient.GetAsync(
+            using HttpResponseMessage response = await SendWithValidatedRedirectsAsync(
                     artifact.Url,
-                    HttpCompletionOption.ResponseHeadersRead,
                     cancellationToken)
                 .ConfigureAwait(false);
             response.EnsureSuccessStatusCode();
-            ReleaseManifestClient.RequireSecureOrLoopback(
-                response.RequestMessage?.RequestUri ?? artifact.Url,
-                "artifact redirect");
             if (response.Content.Headers.ContentLength is long contentLength
                 && contentLength != artifact.Size)
             {
@@ -183,6 +180,86 @@ public sealed class VerifiedArtifactDownloader
         }
     }
 
+    private async Task<HttpResponseMessage> SendWithValidatedRedirectsAsync(
+        Uri initialUri,
+        CancellationToken cancellationToken)
+    {
+        bool allowLoopbackHttp = initialUri.Scheme == Uri.UriSchemeHttp
+            && initialUri.IsLoopback;
+        Uri current = initialUri;
+        var visited = new HashSet<string>(StringComparer.Ordinal);
+        for (int redirectCount = 0;;)
+        {
+            ReleaseManifestClient.RequireTransport(
+                current,
+                "artifact redirect",
+                allowLoopbackHttp);
+            if (!visited.Add(current.AbsoluteUri))
+            {
+                throw new LauncherUpdateException(
+                    "The release artifact redirect chain contains a loop.");
+            }
+
+            using var request = new HttpRequestMessage(HttpMethod.Get, current);
+            HttpResponseMessage response = await _httpClient.SendAsync(
+                    request,
+                    HttpCompletionOption.ResponseHeadersRead,
+                    cancellationToken)
+                .ConfigureAwait(false);
+            Uri effectiveUri = response.RequestMessage?.RequestUri ?? current;
+            if (!Uri.Equals(effectiveUri, current))
+            {
+                response.Dispose();
+                throw new LauncherUpdateException(
+                    "The artifact HTTP transport followed an automatic redirect; "
+                    + "every redirect must be validated before it is requested.");
+            }
+
+            if (!IsRedirect(response.StatusCode))
+            {
+                return response;
+            }
+
+            try
+            {
+                if (redirectCount >= ReleaseManifestClient.MaximumRedirects)
+                {
+                    throw new LauncherUpdateException(
+                        $"The release artifact exceeded "
+                        + $"{ReleaseManifestClient.MaximumRedirects} redirects.");
+                }
+
+                Uri? location = response.Headers.Location;
+                if (location is null)
+                {
+                    throw new LauncherUpdateException(
+                        "The release artifact redirect has no Location header.");
+                }
+
+                Uri next = location.IsAbsoluteUri
+                    ? location
+                    : new Uri(current, location);
+                ReleaseManifestClient.RequireTransport(
+                    next,
+                    "artifact redirect",
+                    allowLoopbackHttp);
+                current = next;
+                redirectCount++;
+            }
+            finally
+            {
+                response.Dispose();
+            }
+        }
+    }
+
+    private static bool IsRedirect(HttpStatusCode statusCode) => statusCode is
+        HttpStatusCode.MovedPermanently
+        or HttpStatusCode.Found
+        or HttpStatusCode.SeeOther
+        or HttpStatusCode.TemporaryRedirect
+        or HttpStatusCode.PermanentRedirect;
+
     internal static void TryDelete(string path)
     {
         try