linuxsessionmgr: special case VSCode remote session

In a Visual Studio Code remote session, we may be able to launch a
browser without the `DISPLAY` environment variable set. This is because
VSCode sets the `BROWSER` environment variable in remote sessions, such
that it can forward requests to start a browser to the client machine.

However! There are several types of remote session in VSCode, and the
way they handle automatic port forwarding differs slightly. Since a very
common case in GCM for launching the user browser is the ability to
connect back to GCM via localhost:<port>, we only consider a subset of
remote sessions to be able to launch a browser:

* Remote SSH     [YES]
* Dev Containers [YES]
* Remote Tunnel  [NO]

Sadly, for whatever reason, the remote connection over Microsoft Dev
Tunnels does NOT automatically surface a forwarded port to localhost on
the client. The forwarded port is only available via the devtunnels.ms
URLs.

Detecting the different types of remote session is tricky as it's not
always as explicit as we'd like.

All types: VSCODE_IPC_HOOK_CLI is set.

Dev Containers : REMOTE_CONTAINERS is set.
Remote SSH     : SSH_CONNECTION is set.
Remote Tunnel  : absense of REMOTE_CONTAINERS and SSH_CONNECTION.

Note, when starting a tunnel server from a regular SSH connection the
SSH_CONNECTION variable gets inherited by the tunnel connections
themselves! This means we need to also check for the absence of SSH_TTY
because the tunnel server unsets this. SSH_TTY is set only in regular
SSH sessions.

Signed-off-by: Matthew John Cheetham <mjcheetham@outlook.com>

Author: mjcheetham

src/shared/Core/Interop/Linux/LinuxSessionManager.cs

@@ -95,6 +95,53 @@ private bool GetWebBrowserAvailable()
             return true;
         }
 
+        //
+        // We may also be able to launch a browser if we're inside a Visual Studio Code remote session.
+        // VSCode overrides the BROWSER environment variable to a script that allows the user to open
+        // the browser on their client machine.
+        //
+        // Even though we can start a browser, one piece of critical functionality we need is the ability
+        // to have that browser be able to connect back to GCM over localhost. There are several types
+        // of VSCode remote session, and only some of them automatically forward ports in such a way that
+        // the client browser can automatically connect back to GCM over localhost.
+        //
+        // * SSH [OK]
+        //   Connection over SSH to a remote machine.
+        //
+        // * Dev Containers [OK]
+        //   Connection to a container.
+        //
+        // * Dev Tunnels [Not OK - forwarded ports not accessible on the client via localhost]
+        //   Connection to a remote machine over the Internet using Microsoft Dev Tunnels.
+        //
+        // * WSL [Ignored - already handled above]
+        //
+        if (Environment.Variables.ContainsKey("VSCODE_IPC_HOOK_CLI") &&
+            Environment.Variables.ContainsKey("BROWSER"))
+        {
+            // Looking for SSH_CONNECTION tells us we're connected via SSH.
+            // HOWEVER, we may also see SSH_CONNECTION in a Dev Tunnel session if the tunnel server
+            // process was started within an SSH session (and the SSH_CONNECTION environment variable
+            // was inherited).
+            // We therefore check for the absence of the SSH_TTY variable, which gets unset
+            // in Dev Tunnel sessions but is always still set in regular SSH sessions.
+            if (Environment.Variables.ContainsKey("SSH_CONNECTION") &&
+                !Environment.Variables.ContainsKey("SSH_TTY"))
+            {
+                Trace.WriteLine("VSCode (Remote SSH) detected - browser is available.");
+                return true;
+            }
+
+            if (Environment.Variables.ContainsKey("REMOTE_CONTAINERS"))
+            {
+                Trace.WriteLine("VSCode (Dev Containers) detected - browser is available.");
+                return true;
+            }
+
+            Trace.WriteLine("VSCode (Remote Tunnel) detected - browser is not available.");
+            return false;
+        }
+
         // We need a desktop session to be able to launch the browser in the general case
         return IsDesktopSession;
     }