Windows: force Mesa/Software mode when started in an RDP session

refs: #265

docs/changelog.markdown

@@ -61,6 +61,9 @@ brief notes about them may accumulate here.
 * The multiplexer server has been moved into its own
   `wezterm-mux-server` executable.  You will need to revise
   your `serve_command` configuration.
+* Windows: when started in an RDP session, force the use
+  of the Mesa software renderer to work around problems with
+  RDP GPU emulation.
 
 ### 20200909-002054-4c9af461
 

wezterm/src/gui/mod.rs

@@ -53,6 +53,15 @@ impl GuiFrontEnd {
                 Connection::disable_wayland();
             }
         }
+        #[cfg(windows)]
+        {
+            if is_running_in_rdp_session() {
+                // Using OpenGL in RDP has problematic behavior upon
+                // disconnect, so we force the use of software rendering.
+                log::trace!("Running in an RDP session, use SWRAST");
+                prefer_swrast();
+            }
+        }
 
         let connection = Connection::init()?;
         let front_end = Rc::new(GuiFrontEnd { connection });

window/src/os/windows/mod.rs

@@ -17,3 +17,38 @@ fn wide_string(s: &str) -> Vec<u16> {
         .chain(std::iter::once(0))
         .collect()
 }
+
+/// Returns true if we are running in an RDP session.
+/// See <https://docs.microsoft.com/en-us/windows/win32/termserv/detecting-the-terminal-services-environment>
+pub fn is_running_in_rdp_session() -> bool {
+    use winapi::shared::minwindef::DWORD;
+    use winapi::um::processthreadsapi::{GetCurrentProcessId, ProcessIdToSessionId};
+    use winapi::um::winuser::{GetSystemMetrics, SM_REMOTESESSION};
+    use winreg::{enums::HKEY_LOCAL_MACHINE, RegKey};
+
+    if unsafe { GetSystemMetrics(SM_REMOTESESSION) } != 0 {
+        return true;
+    }
+
+    let hklm = RegKey::predef(HKEY_LOCAL_MACHINE);
+    let terminal_server =
+        match hklm.open_subkey("SYSTEM\\CurrentControlSet\\Control\\Terminal Server\\") {
+            Ok(k) => k,
+            Err(_) => return false,
+        };
+
+    let glass_session_id: DWORD = match terminal_server.get_value("GlassSessionId") {
+        Ok(sess) => sess,
+        Err(_) => return false,
+    };
+
+    unsafe {
+        let mut current_session = 0;
+        if ProcessIdToSessionId(GetCurrentProcessId(), &mut current_session) != 0 {
+            // If we're not the glass session then we're a remote session
+            current_session != glass_session_id
+        } else {
+            false
+        }
+    }
+}