Tell a refused inline apart from an absent owner

src/DiffEngine.Tests/ViewerProtocolTests.cs

@@ -733,6 +733,64 @@ public async Task AnUnresponsiveOwnerTimesOutRatherThanHanging()
             listener.Stop();
         }
     }
+    /// <summary>
+    /// An owner that answers with an error is not an absent one. Collapsing the two into false
+    /// meant a refused inline was read as "nobody is there", so a second viewer was launched, it
+    /// could not bind the port, and the snapshot was reported as Queued while being held by
+    /// nothing at all.
+    /// </summary>
+    [Test]
+    public async Task ARefusedExchangeIsToldApartFromAnAbsentOwner()
+    {
+        await Assert.That(ViewerServer.TryBind(0, out var bound)).IsTrue();
+        using var server = bound!;
+        using var cancel = new CancelSource();
+        var listening = server.Listen(_ => ViewerResponse.Error("no thanks"), cancel.Token);
+
+        try
+        {
+            var refused = await ViewerClient.SendAsync(new(ViewerVerb.List), default, server.Port);
+
+            await Assert.That(refused).IsEqualTo(SendOutcome.Refused);
+        }
+        finally
+        {
+            await cancel.CancelAsync();
+            await Wait(listening);
+        }
+    }
+
+    [Test]
+    public async Task AnAcceptedExchangeReportsAccepted()
+    {
+        await Assert.That(ViewerServer.TryBind(0, out var bound)).IsTrue();
+        using var server = bound!;
+        using var cancel = new CancelSource();
+        var listening = server.Listen(_ => ViewerResponse.Success("fine"), cancel.Token);
+
+        try
+        {
+            await Assert.That(await ViewerClient.SendAsync(new(ViewerVerb.List), default, server.Port))
+                .IsEqualTo(SendOutcome.Accepted);
+        }
+        finally
+        {
+            await cancel.CancelAsync();
+            await Wait(listening);
+        }
+    }
+
+    [Test]
+    public async Task AnAbsentOwnerReportsNoOwner()
+    {
+        ViewerServer.TryBind(0, out var server);
+        var port = server!.Port;
+        server.Dispose();
+
+        await Assert.That(await ViewerClient.SendAsync(new(ViewerVerb.List), default, port, TimeSpan.FromSeconds(2)))
+            .IsEqualTo(SendOutcome.NoOwner);
+    }
+
     [Test]
     public async Task AnAbsentOwnerIsNotAnError()
     {

src/DiffEngine/DiffRunner_Inline.cs

@@ -14,7 +14,9 @@ public enum InlineResult
     Disabled,
 
     /// <summary>
-    /// No DiffEngineViewer could be resolved. Callers that want a fallback should use it here.
+    /// Nothing has the snapshot. No DiffEngineViewer could be resolved, or the owner of the queue
+    /// declined the payload - which is the same thing from the caller's side, since in both cases
+    /// the snapshot is pending nowhere. Callers that want a fallback should use it here.
     /// </summary>
     NoViewerFound
 }
@@ -61,11 +63,22 @@ public static async Task<InlineResult> AddInlineAsync(InlinePatch patch, Cancel
         // Onto the payload rather than onto the patch, which belongs to the caller and may be
         // held or sent again
         var payload = InlinePatchFile.Build(patch, patch.Framework ?? RuntimeMoniker.Current);
-        if (await ViewerClient.TrySendAsync(new(ViewerVerb.Inline, Body: payload), cancel))
+        var outcome = await ViewerClient.SendAsync(new(ViewerVerb.Inline, Body: payload), cancel);
+        if (outcome == SendOutcome.Accepted)
         {
             return InlineResult.Queued;
         }
 
+        if (outcome == SendOutcome.Refused)
+        {
+            // An owner that is there and said no - an older one that does not understand the
+            // payload, or a handler that threw. Launching a second viewer cannot change that
+            // answer, and it would bind nothing, so reporting Queued would say the snapshot is
+            // somewhere when it is nowhere. Report it as no viewer, which is the answer that
+            // makes the caller stage the files instead
+            return InlineResult.NoViewerFound;
+        }
+
         var launched = await ViewerLauncher.LaunchAsync(patch, payload, cancel);
         return launched ? InlineResult.Queued : InlineResult.NoViewerFound;
     }

src/DiffEngine/Protocol/ViewerClient.cs

@@ -5,6 +5,31 @@ namespace DiffEngine;
 /// caller turns into a launch (DiffEngine), "nothing pending" (the tray), or "the owner has gone"
 /// (an attached viewer).
 /// </summary>
+/// <summary>
+/// What came back from an exchange with the queue owner. Three outcomes rather than two, because
+/// "nobody is there" and "the owner said no" call for opposite responses: the first is fixed by
+/// launching a viewer, and the second is not.
+/// </summary>
+enum SendOutcome
+{
+    /// <summary>
+    /// Nobody answered. No owner, or one present but unresponsive - the caller cannot tell, and
+    /// for its purposes they are the same.
+    /// </summary>
+    NoOwner,
+
+    /// <summary>
+    /// The owner answered and took it.
+    /// </summary>
+    Accepted,
+
+    /// <summary>
+    /// The owner answered and declined it: a version it does not understand, or a handler that
+    /// threw. Launching another viewer will not change that answer.
+    /// </summary>
+    Refused
+}
+
 static class ViewerClient
 {
     public const int DefaultPort = 3493;
@@ -114,6 +139,18 @@ public static bool TrySend(
     /// </para>
     /// </summary>
     public static async Task<bool> TrySendAsync(
+        ViewerMessage message,
+        Cancel cancel,
+        int? port = null,
+        TimeSpan? wait = null) =>
+        await SendAsync(message, cancel, port, wait) == SendOutcome.Accepted;
+
+    /// <summary>
+    /// As <see cref="TrySendAsync" />, but says which of the two failures happened. A caller that
+    /// would launch a viewer on absence needs that: launching one because the owner refused the
+    /// payload leaves two processes and still no snapshot.
+    /// </summary>
+    public static async Task<SendOutcome> SendAsync(
         ViewerMessage message,
         Cancel cancel,
         int? port = null,
@@ -154,8 +191,12 @@ public static async Task<bool> TrySendAsync(
 #else
             var text = await reader.ReadToEndAsync();
 #endif
-            return ViewerResponse.TryParse(text, out var response) &&
-                   response.Ok;
+            if (!ViewerResponse.TryParse(text, out var response))
+            {
+                return SendOutcome.NoOwner;
+            }
+
+            return response.Ok ? SendOutcome.Accepted : SendOutcome.Refused;
         }
         // The deadline, rather than the caller cancelling. Whatever the abort surfaced as - a
         // cancellation, a closed socket, a torn down stream - the owner is present but not
@@ -169,13 +210,13 @@ public static async Task<bool> TrySendAsync(
             Trace.WriteLine(
                 $"Timed out after {timeToWait} waiting for the inline queue owner on port {endpointPort}. " +
                 $"Verb: {message.Verb}. The owner is present but unresponsive. {exception.GetType().Name}");
-            return false;
+            return SendOutcome.NoOwner;
         }
         // Cancellation is the caller's business; a missing owner is not.
         catch (Exception exception)
             when (exception is not OperationCanceledException && Ignorable(exception))
         {
-            return false;
+            return SendOutcome.NoOwner;
         }
     }