The proxy's shared download is started once

ConcurrentDictionary.GetOrAdd may run its factory more than once when asked at the same time, and each run started a
download: five clients asking at once for a new file made three upstream requests in one run of
Clients_asking_at_once_share_one_download. The factory now makes a Lazy, of which only the one kept is ever started,
and a download that begins just after another cached the file uses it.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01LsXgEaXee4GCU1hwYgPJXw

PrivaPub/Domain/Media/MediaProxy.cs

@@ -55,7 +55,8 @@ namespace PrivaPub.Domain.Media
 		readonly IMediaService _media;
 		readonly IOptionsMonitor<MediaOptions> _options;
 		readonly Federation.Moderation.IDomainBlocks _domainBlocks;
-		readonly ConcurrentDictionary<string, Task<(ProxyOutcome, string, string)>> _inFlight = new();
+		// Lazy: GetOrAdd may run its factory more than once when asked at the same time, but only one Lazy is ever evaluated
+		readonly ConcurrentDictionary<string, Lazy<Task<(ProxyOutcome, string, string)>>> _inFlight = new();
 		readonly ConcurrentDictionary<string, (ProxyOutcome Outcome, DateTime Until)> _refused = new();
 		readonly SemaphoreSlim _downloads = new(Downloads);
 		readonly object _keyLock = new();
@@ -153,15 +154,15 @@ namespace PrivaPub.Domain.Media
 				_refused.TryRemove(url, out _);
 			}
 			// shared by everyone asking for it now; the download itself isn't cancelled when one of them leaves
-			var download = _inFlight.GetOrAdd(url, key => DownloadOnce(key));
+			var shared = _inFlight.GetOrAdd(url, key => new Lazy<Task<(ProxyOutcome, string, string)>>(() => DownloadOnce(key)));
 			try
 			{
-				return await download.WaitAsync(token);
+				return await shared.Value.WaitAsync(token);
 			}
 			finally
 			{
-				if (download.IsCompleted)
+				if (shared.Value.IsCompleted)
-					_inFlight.TryRemove(new KeyValuePair<string, Task<(ProxyOutcome, string, string)>>(url, download));
+					_inFlight.TryRemove(new KeyValuePair<string, Lazy<Task<(ProxyOutcome, string, string)>>>(url, shared));
 			}
 		}
 
@@ -170,6 +171,12 @@ namespace PrivaPub.Domain.Media
 			await Task.Yield();
 			if (!await _downloads.WaitAsync(TimeSpan.FromSeconds(30)))
 				return (ProxyOutcome.Failed, default, default);//too busy: the client tries again
+			// cached by a download that ended just before this one began
+			if (Cached(url) is { Path: not null } done)
+			{
+				_downloads.Release();
+				return (ProxyOutcome.Cached, done.Path, done.ContentType);
+			}
 			var (path, typePath) = CachePaths(url);
 			var part = $"{path}.{Guid.NewGuid():N}.part";
 			try