tests/DodoSSH.Client.Terminal.Tests/TerminalDataPlaneTests.cs

@@ -416,6 +416,36 @@ public sealed class TerminalDataPlaneTests : IAsyncDisposable
             TimeProvider.System,
             new TerminalPumpOptions { FlushInterval = TimeSpan.FromMilliseconds(10) });
 
+    /// <summary>
+    /// Opens a renderer's socket and returns once the plane is actually holding it.
+    /// </summary>
+    /// <remarks>
+    /// <para>
+    /// <b>Connected and attached are two different moments, and the gap between them is where this used to
+    /// flake.</b> <c>ClientWebSocket.ConnectAsync</c> completes on the 101, which
+    /// <see cref="TerminalDataPlane.UpgradeAsync"/> writes before it has a <see cref="WebSocket"/> to
+    /// attach — it builds one from the stream and swaps it in a few instructions later, on the accept
+    /// thread. A frame sent in between is dropped, by design rather than by accident: the transport has
+    /// nowhere to put a frame for a renderer that is not there, and queueing it is the unbounded growth the
+    /// credit window exists to prevent.
+    /// </para>
+    /// <para>
+    /// So a test that connected and immediately expected a frame was racing that window on every run. It
+    /// lost one on CI — <c>Output_ReachesTheRenderer</c> read the output frame first and asked why it was
+    /// not the session's opening one, the opening one having been dropped a moment earlier — which is a
+    /// scheduling accident on a loaded machine and says nothing whatever about the transport.
+    /// </para>
+    /// <para>
+    /// Production does not race it and needs no change: everything that opens a session waits on
+    /// <c>TerminalWorkspace.WaitForRendererAsync</c> first, and that resolves from the same few lines this
+    /// event is raised from.
+    /// </para>
+    /// <para>
+    /// Subscribed before the connection rather than after it, because the event is raised on the accept
+    /// thread and can be over before <c>ConnectAsync</c> has returned here. Bounded, so that a socket that
+    /// never attaches fails this helper rather than hanging the suite in a later receive.
+    /// </para>
+    /// </remarks>
     private async Task<ClientWebSocket> ConnectAsync(
         string? token = "",
         string? origin = null)
@@ -433,17 +463,31 @@ public sealed class TerminalDataPlaneTests : IAsyncDisposable
             "Origin",
             origin ?? string.Create(CultureInfo.InvariantCulture, $"http://127.0.0.1:{plane.Port}"));
 
+        var attached = new TaskCompletionSource(TaskCreationOptions.RunContinuationsAsynchronously);
+
+        void OnAttached(object? sender, EventArgs e) => attached.TrySetResult();
+
+        plane.SocketAttached += OnAttached;
+
         try
         {
             await socket.ConnectAsync(
                 new Uri($"ws://127.0.0.1:{plane.Port}{TerminalDataPlane.SocketPath}"),
                 TestContext.Current.CancellationToken);
+
+            await attached.Task.WaitAsync(
+                TimeSpan.FromSeconds(5),
+                TestContext.Current.CancellationToken);
         }
         catch
         {
             socket.Dispose();
             throw;
         }
+        finally
+        {
+            plane.SocketAttached -= OnAttached;
+        }
 
         return socket;
     }

tests/DodoSSH.Client.Terminal.Tests/TerminalWorkspaceTests.cs

@@ -538,10 +538,26 @@ public sealed class TerminalWorkspaceTests
         new("host.invalid", 22, "dodo", new SshPasswordCredential("irrelevant"));
 
     /// <remarks>
+    /// <para>
     /// Attaches the way the real page does: by fetching the served page, reading the token and socket URL
     /// back out of it, and presenting them on the upgrade — rather than reaching into the workspace for a
     /// token it does not expose. A shortcut here would prove only that a socket can be opened, not that the
     /// workspace serves a page a renderer could actually attach with.
+    /// </para>
+    /// <para>
+    /// And it waits for the attach rather than only for the handshake, for the reason
+    /// <c>TerminalDataPlaneTests.ConnectAsync</c> sets out at length: the 101 is written before the socket
+    /// is attachable, and a frame sent in between is dropped. A caller that opens a session on the socket
+    /// this returns and then reads its opening frame is exactly the shape that loses that race.
+    /// </para>
+    /// <para>
+    /// <c>WaitForRendererAsync</c> answers only for the first renderer ever to attach, so a second call
+    /// returns immediately without proving anything about the second socket. That is enough here and is
+    /// not luck: the only frames a second socket is given are the replay, and the replay is *caused* by the
+    /// attach — <see cref="TerminalWorkspace.RendererReattached"/> and the frames before it cannot be sent
+    /// early. The day a test sends something else down a reattached socket, this needs the data plane's own
+    /// <c>SocketAttached</c>, which the workspace does not forward today.
+    /// </para>
     /// </remarks>
     private static async Task<ClientWebSocket> ConnectRendererAsync(TerminalWorkspace workspace)
     {
@@ -561,6 +577,8 @@ public sealed class TerminalWorkspaceTests
         try
         {
             await client.ConnectAsync(new Uri(socketUrl), TestContext.Current.CancellationToken);
+
+            await workspace.WaitForRendererAsync(TestContext.Current.CancellationToken);
         }
         catch
         {