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.

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

Author: mjcheetham

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

@@ -95,6 +95,46 @@ 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.
+        //
+        // * Remote SSH [OK]
+        //   Connection over SSH to a remote machine.
+        //
+        // * Dev Containers [OK]
+        //   Connection to a container.
+        //
+        // * Remote 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"))
+        {
+            if (Environment.Variables.ContainsKey("SSH_CONNECTION"))
+            {
+                Trace.WriteLine("VSCode (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 (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;
     }