Notice when a shell ends, say which way it ended, and offer the way back

A terminal whose machine rebooted stayed green for ever. Not "went stale and
recovered late" — for ever: the tab claimed CONNECTED, the elapsed timer went
on counting, and the only thing that ever corrected it was closing the
application. That is the defect this fixes, and it turned out to be two
separate failures wearing one symptom.

◆ THE HANG IS IN SSH.NET, AND KEEP-ALIVE ALONE WOULD NOT HAVE FIXED IT.

ShellStream ends a blocked read from exactly two places: its channel being
closed, and the session raising Disconnected. A shell that exits gives the
first. But Session raises Disconnected from one method only — OnDisconnectReceived,
for a server that politely says goodbye — and a socket that simply fails goes
down a different path entirely: RaiseError fires the error event, sets the
disconnecting flag and disposes the socket, without ever raising Disconnected.
So the stream was never disposed, its Monitor.Wait was never pulsed, and the
read blocked until the process ended. The transport was gone, the client knew
the transport was gone, and nothing woke the one thread that was waiting.

That is why the first attempt at a test for this sat at thirty seconds and
failed. Both halves are needed and neither is sufficient:

  - Keep-alive makes the failure HAPPEN. An idle SSH session is a socket
    nobody writes to, and TCP has no way to discover that the far end has gone
    without writing to it — nothing was asked, so nothing could fail. Thirty
    seconds, matching OpenSSH's own ServerAliveInterval examples. It is the
    write that matters and not a reply: SSH_MSG_IGNORE need not be answered,
    so this is traffic rather than a heartbeat with a deadline, and there is
    no timeout invented here to be wrong about a slow link.

  - Disposing the stream from the error handler makes it ARRIVE. Queued to the
    thread pool and not done inline, which is not tidiness: this runs on
    SSH.NET's message-listener thread, and closing a channel waits for the
    server's close — processed by that same thread — so an inline dispose is
    the listener thread waiting on itself.

SFTP gets the keep-alive too. Its operations are request/response and bounded
by ConnectionInfo.Timeout already, so it needs no equivalent wake-up.

◆ exit AND reboot ARE TOLD APART BY A LATCH, BECAUSE THE OBVIOUS WAY IS A RACE.

Wait for the read to end and then ask whether the transport is still up, and
against a real server the answer is wrong about half the time: this was the
only channel on the connection, so sshd hangs up a millisecond after closing
it. By the time anything could ask, the transport is gone either way — and a
poll would report every ordinary `exit` as a dropped connection.

So ISshShellSession.TransportFailure is set at the moment SSH.NET raises the
failure, which its own ordering puts strictly before the disconnect that wakes
the read. A read that has just returned zero consults a settled value: set if
the transport failed, still null if the remote merely closed the channel. The
pump turns that into a TerminalSessionOutcome — RemoteClosed, ConnectionLost,
ClosedLocally — carried on SessionEnded, which used to hand over a session id
and nothing else. A subscriber cannot work this out afterwards; by the time it
hears, there is no transport left to interrogate.

Deliberately no fourth value for "the remote rebooted". Nothing on the wire
distinguishes a reboot from a cable being pulled, and a name this client
cannot check would be a guess dressed as a fact.

◆ WHAT IS DRAWN, AND WHY IT IS NOT DRAWN OVER THE PANE.

The status bar says DISCONNECTED or SHELL EXITED where it used to say NOT
CONNECTED for both — a word that is honest about a tab still dialling and
useless about one whose shell has stopped. Red for the first and grey for the
second: one is a machine that stopped answering and the other is work
finished, and crying wolf on the ordinary end of every session is its own bug.
The tab strip's dot gains the same fourth state, on both heads.

Under the terminal, a strip carrying the reason and two buttons. It takes its
own Auto grid row rather than overlaying the renderer, and that is forced
rather than chosen: the WebView is a native child view composited above
everything Avalonia draws, so the alternative is collapsing the terminal — and
the scrollback is precisely what the user wants to read, because the last
thing the remote said is usually why it went. terminal.js already writes the
same sentence into the pane; this is the copy that does not scroll away.

RECONNECT reuses the tab in place, which is what the word means to somebody
who pressed it: the terminal they were working in comes back, where it was.
It dials through the vault's ordinary connect path, so every refusal — a key
that has left the keychain, an unknown host key, a group default that has
changed since — is decided in the one place that already decides them; what
this adds is which tab the attempt belongs to. The old session is closed as
part of it, because a dead shell still holds an authenticated connection this
process has stopped reading, and its pane still holds an xterm, a scrollback
and a WebGL context, of which a browser hands out about sixteen.

Two refusals it makes rather than pretending: a host reached by typing an
address has no keychain entry to redial, and one that wants a password typed
goes to the keychain with the sheet raised rather than being dialled with an
empty box. Both say so on the strip where the button was pressed, not only in
the vault's status on a screen that surface is not.

TRY AGAIN appears on a refused connection for the same reason, and it is worth
as much: the commonest refusal is a host that was not up yet.

◆ WHAT IS ACTUALLY VERIFIED.

Two tests against a real sshd in a container. `exit` is reported as the remote
closing the session — the one a polling implementation fails, and a fake shell
cannot show it because a fake has no server behind it deciding to hang up. And
a live shell losing its transport is reported as a lost connection, produced
by dialling through a plain TCP relay the test owns and then cutting it; the
container is shared by the collection, so killing sshd was never an option.

The first draft of that second test used LoopbackProxyTests' SOCKS forward and
timed out: disconnecting the carrier of a ForwardedPortDynamic does not close
the loopback socket it accepted. Recorded in the test, because it is a
property of SSH.NET's forwarder rather than of anything here, and it is not
what a dying machine looks like.

What no test reproduces is the waiting. A relay that closes sends a FIN, so
the socket reports itself at once; a machine switched off sends nothing, and
finding that out is the keep-alive's business and the OS's retransmit budget's
— neither of which a test can hurry along without a clock this code does not
own. A reboot is prompt in practice because the machine comes back and RSTs.

Beside those: three pump outcomes, three workspace announcements, and seven
shell-flow tests covering the drop, the clean exit, reuse of the tab, retry
after a refusal, and both ways a reconnect can honestly come to nothing.

Also fixed in passing: SshNetShellSession.IsOpen read ShellStream.CanRead,
which is documented as always true — the stream stays readable after disposal
so buffered output can be drained — so it had answered "yes" for every shell
that ever existed. It reads CanWrite now. And MainWindowViewModel's session-
ended handler moved from Dispatcher.UIThread.Post to the injected post
delegate, which is what makes any of the shell half testable at all; the
remark listing the three unreachable posts is one shorter.

tests/DodoSSH.Client.App.Tests/ShellFlowTests.cs

@@ -1126,6 +1126,270 @@ public sealed class ShellFlowTests : IAsyncLifetime
     // directly beside the pre-existing `RefreshConnectedHosts();` — is covered indirectly: it is the same
     // VaultViewModel.RefreshLastConnectedAsync the activation test above already exercises.
 
+    // ---- A session that stops: the status it gets, and the way back ----
+
+    /// <remarks>
+    /// <para>
+    /// ◆ THE REBOOT CASE, as far as this shell can see it. A machine going away is a transport failure
+    /// latched under a read that then reports the end of the shell — see <c>ISshShellSession.TransportFailure</c>
+    /// — and what has to come of it here is that the tab stops claiming to be connected and says which of the
+    /// two endings it was.
+    /// </para>
+    /// <para>
+    /// The words are asserted rather than only the flags. DISCONNECTED against SHELL EXITED is the whole
+    /// difference a user reads, and a status bar that had a bool but printed the same thing for both would
+    /// pass a test written against <c>IsLive</c> alone.
+    /// </para>
+    /// </remarks>
+    [Fact]
+    public async Task ASessionWhoseConnectionBreaks_SaysSoAndOffersTheWayBack()
+    {
+        var vault = await ReadyToConnectAsync();
+
+        await using var renderer = await FakeRenderer.AttachAsync(workspace, Token);
+        await vault.ConnectCommand.ExecuteAsync(null);
+
+        var tab = shell.Tabs.ShouldHaveSingleItem();
+        tab.IsLive.ShouldBeTrue();
+        shell.IsSessionConnected.ShouldBeTrue();
+
+        ssh.Shells.ShouldHaveSingleItem().Drop("An established connection was aborted by the server.");
+
+        await EventuallyAsync(() => !tab.IsLive, "the dropped shell should have ended the session");
+
+        tab.EndReason.ShouldBe(SshSessionEnd.ConnectionLost);
+        tab.IsDropped.ShouldBeTrue("the strip draws a dropped connection differently from a clean exit");
+
+        shell.IsSessionConnected.ShouldBeFalse();
+        shell.SessionEndedText.ShouldBe("DISCONNECTED");
+        shell.SessionEndedDetail.ShouldBe("An established connection was aborted by the server.");
+        shell.IsSessionDropped.ShouldBeTrue();
+        shell.IsSessionIdle.ShouldBeFalse("NOT CONNECTED must not be drawn beside a named ending");
+
+        shell.CanReconnectSelectedTab.ShouldBeTrue("the host is still in the keychain to dial again");
+
+        // The pane stays, which is the whole reason a dead tab is kept: the last thing the remote said is
+        // usually why it went.
+        tab.HasSession.ShouldBeTrue();
+        shell.IsTerminalShowing.ShouldBeTrue();
+    }
+
+    /// <remarks>
+    /// ◆ THE <c>exit</c> CASE, and the reason it is a separate test from the one above rather than an extra
+    /// assertion inside it: the two differ by a latched transport failure and nothing else, all the way down
+    /// to the read that ends them, and the point is that they do not report the same thing. A shell somebody
+    /// exited is work finished — grey, no alarm — and calling it a dropped connection would be crying wolf on
+    /// the ordinary end of every session.
+    /// </remarks>
+    [Fact]
+    public async Task AShellThatExits_SaysThatRatherThanClaimingTheConnectionBroke()
+    {
+        var vault = await ReadyToConnectAsync();
+
+        await using var renderer = await FakeRenderer.AttachAsync(workspace, Token);
+        await vault.ConnectCommand.ExecuteAsync(null);
+
+        var tab = shell.Tabs.ShouldHaveSingleItem();
+
+        ssh.Shells.ShouldHaveSingleItem().Exit();
+
+        await EventuallyAsync(() => !tab.IsLive, "the exiting shell should have ended the session");
+
+        tab.EndReason.ShouldBe(SshSessionEnd.RemoteClosed);
+        tab.IsDropped.ShouldBeFalse();
+
+        shell.SessionEndedText.ShouldBe("SHELL EXITED");
+        shell.IsSessionDropped.ShouldBeFalse();
+
+        // Still worth offering, because the tab and the machine are both still there — what it is not is an
+        // alarm.
+        shell.CanReconnectSelectedTab.ShouldBeTrue();
+    }
+
+    /// <remarks>
+    /// <para>
+    /// The tab is reused rather than a second one appearing beside it. That is what reconnecting means to
+    /// somebody who pressed the button: the terminal they were working in comes back, in the place it was.
+    /// </para>
+    /// <para>
+    /// The old session is released as part of it, which the assertion on the connection's disposal stands
+    /// for. A dead shell still holds an authenticated connection this process has stopped reading, and its
+    /// pane still holds an xterm, a scrollback and a WebGL context — of which a browser hands out about
+    /// sixteen. Leaking one of each per reconnect would be invisible until a working day's worth of them ran
+    /// the renderer out.
+    /// </para>
+    /// </remarks>
+    [Fact]
+    public async Task ReconnectingADroppedTab_DialsAgainInTheSameTab()
+    {
+        var vault = await ReadyToConnectAsync();
+
+        // Bound to a key, which is what makes this host redialable without anybody present: a host that
+        // wants a password typed is diverted to the keychain instead, and has a test of its own below.
+        await AddKeyAsync(vault, "deploy");
+        await BindKeyAsync(vault, vault.Hosts[0], vault.Keys[0].EntityId);
+
+        await using var renderer = await FakeRenderer.AttachAsync(workspace, Token);
+        await vault.ConnectCommand.ExecuteAsync(null);
+
+        var tab = shell.Tabs.ShouldHaveSingleItem();
+        var firstSession = tab.SessionId;
+
+        ssh.Shells[0].Drop("An established connection was aborted by the server.");
+
+        await EventuallyAsync(() => !tab.IsLive, "the dropped shell should have ended the session");
+
+        await shell.ReconnectTabCommand.ExecuteAsync(tab);
+
+        shell.Tabs.ShouldHaveSingleItem().ShouldBe(tab, "a reconnect reuses the tab it was asked for");
+
+        tab.IsLive.ShouldBeTrue();
+        tab.SessionId.ShouldNotBe(firstSession, "a reconnect is a new session, not the old one resumed");
+        tab.EndReason.ShouldBeNull("the ending belonged to the session that has been replaced");
+
+        shell.SessionEndedText.ShouldBeNull();
+        shell.IsSessionConnected.ShouldBeTrue();
+        shell.SelectedTab.ShouldBe(tab);
+
+        ssh.Requests.Count.ShouldBe(2, "the vault's own connect path was used, not a second one");
+        await EventuallyAsync(
+            () => !workspace.IsSessionLive(firstSession),
+            "the session that was replaced should have been released with its pane");
+    }
+
+    /// <remarks>
+    /// A connection that was refused is worth retrying for the same reason a dropped one is, and the
+    /// commonest refusal — a machine that was not up yet — is the one most likely to succeed a minute later.
+    /// The card's button is the same command, which is what makes this worth asserting here rather than
+    /// trusting to two code paths that happen to agree today.
+    /// </remarks>
+    [Fact]
+    public async Task ARefusedConnection_CanBeRetriedInItsOwnTab()
+    {
+        var vault = await ReadyToConnectAsync();
+
+        await AddKeyAsync(vault, "deploy");
+        await BindKeyAsync(vault, vault.Hosts[0], vault.Keys[0].EntityId);
+
+        await using var renderer = await FakeRenderer.AttachAsync(workspace, Token);
+
+        ssh.Failure = new IOException("The host did not answer.");
+        await vault.ConnectCommand.ExecuteAsync(null);
+
+        var tab = shell.Tabs.ShouldHaveSingleItem();
+        tab.IsFailed.ShouldBeTrue(tab.Status);
+        shell.CanReconnectSelectedTab.ShouldBeTrue();
+
+        ssh.Failure = null;
+        await shell.ReconnectTabCommand.ExecuteAsync(tab);
+
+        shell.Tabs.ShouldHaveSingleItem().ShouldBe(tab);
+        tab.IsLive.ShouldBeTrue();
+        tab.HasSession.ShouldBeTrue();
+    }
+
+    /// <remarks>
+    /// <para>
+    /// A host that wants a password typed cannot be redialled on its own: this client does not keep the
+    /// password, so dialling with an empty box would be an authentication failure dressed up as a reconnect.
+    /// It goes to the keychain with the password sheet raised instead, which is the one screen that sheet is
+    /// drawn on.
+    /// </para>
+    /// <para>
+    /// The tab is deliberately left as it was rather than put back to connecting. Nothing has been dialled
+    /// yet — it waits on somebody typing — and a tab that had thrown away its ending and its pane to sit on a
+    /// step list nothing was advancing would be the worse of the two wrongs.
+    /// </para>
+    /// </remarks>
+    [Fact]
+    public async Task ReconnectingAHostThatWantsAPassword_AsksForOneRatherThanDiallingWithout()
+    {
+        var vault = await ReadyToConnectAsync();
+
+        await using var renderer = await FakeRenderer.AttachAsync(workspace, Token);
+        await vault.ConnectCommand.ExecuteAsync(null);
+
+        var tab = shell.Tabs.ShouldHaveSingleItem();
+
+        ssh.Shells[0].Drop("An established connection was aborted by the server.");
+        await EventuallyAsync(() => !tab.IsLive, "the dropped shell should have ended the session");
+
+        await shell.ReconnectTabCommand.ExecuteAsync(tab);
+
+        vault.IsAskingForConnectPassword.ShouldBeTrue();
+        shell.Screen.ShouldBe(ShellScreen.Hosts, "the password sheet is drawn there and nowhere else");
+
+        ssh.Requests.ShouldHaveSingleItem("nothing should have been dialled without the password");
+        tab.HasSession.ShouldBeTrue("the tab keeps its pane while it waits on somebody typing");
+    }
+
+    /// <remarks>
+    /// A machine reached by typing an address has no keychain entry, so there is nothing to dial again with —
+    /// this client would need the password, which it does not keep. The button is drawn disabled rather than
+    /// hidden, so the refusal has to be a fact the shell states rather than an absence.
+    /// </remarks>
+    [Fact]
+    public async Task ATabWhoseHostIsNotInTheKeychain_IsNotOfferedAReconnect()
+    {
+        var vault = await ReadyToConnectAsync();
+
+        await using var renderer = await FakeRenderer.AttachAsync(workspace, Token);
+        await vault.ConnectCommand.ExecuteAsync(null);
+
+        var tab = shell.Tabs.ShouldHaveSingleItem();
+
+        ssh.Shells[0].Drop("An established connection was aborted by the server.");
+        await EventuallyAsync(() => !tab.IsLive, "the dropped shell should have ended the session");
+
+        // The host leaves, as deleting it or locking the vault would take it. The tab outlives the vault
+        // that opened it, which is why this state is reachable at all.
+        vault.Hosts.Clear();
+
+        shell.CanReconnectSelectedTab.ShouldBeFalse();
+
+        await shell.ReconnectTabCommand.ExecuteAsync(tab);
+
+        ssh.Requests.ShouldHaveSingleItem("nothing should have been dialled");
+
+        // And it says so where the button that was pressed is, rather than only in the vault's own status on
+        // a screen this surface is not.
+        shell.SessionEndedDetail.ShouldNotBeNull().ShouldContain("not in this keychain");
+        shell.SessionEndedText.ShouldBe("DISCONNECTED", "the ending it is still in is unchanged");
+    }
+
+    /// <remarks>
+    /// The other way a reconnect can come to nothing: the host is still there, but what it authenticates
+    /// with is not. The vault refuses before it dials — which is the whole reason a reconnect goes through
+    /// the vault's own connect path rather than reaching past it — and the refusal has to arrive where the
+    /// button was pressed, because the vault draws its own status on the keychain screen.
+    /// </remarks>
+    [Fact]
+    public async Task AReconnectTheVaultRefusesBeforeDialling_SaysWhyOnTheTab()
+    {
+        var vault = await ReadyToConnectAsync();
+
+        await AddKeyAsync(vault, "deploy");
+        await BindKeyAsync(vault, vault.Hosts[0], vault.Keys[0].EntityId);
+
+        await using var renderer = await FakeRenderer.AttachAsync(workspace, Token);
+        await vault.ConnectCommand.ExecuteAsync(null);
+
+        var tab = shell.Tabs.ShouldHaveSingleItem();
+
+        ssh.Shells[0].Drop("An established connection was aborted by the server.");
+        await EventuallyAsync(() => !tab.IsLive, "the dropped shell should have ended the session");
+
+        // The key the host is bound to leaves, which is a binding that no longer resolves.
+        vault.Keys.Clear();
+
+        await shell.ReconnectTabCommand.ExecuteAsync(tab);
+
+        ssh.Requests.ShouldHaveSingleItem("a binding that cannot be resolved must not be dialled with");
+
+        shell.SessionEndedDetail.ShouldNotBeNull().ShouldContain("SSH key");
+        tab.HasSession.ShouldBeTrue("the tab keeps the pane it had; there was nothing to replace it with");
+    }
+
     /// <remarks>
     /// The phone's connect menu is drawn over the terminal's own rectangle, so it obeys the rule the palette
     /// does: whatever covers the renderer collapses it instead. The surface stays, because the bar the menu