App: a second copy opening, or OpenGL that fails, no longer strands Window Graphics

Two copies opened close together made the second take the first's
launch-pending marker for a crash and switch Window Graphics to OpenGL.
Where OpenGL can't open a window (NVIDIA on X11: "gl not compatible with
provided surface"), every later launch then failed, with no way to reach
the setting.

The marker is now locked while its launch runs, so a locked marker is a
launch still opening, not a crash. And a window that fails to open on
OpenGL puts Window Graphics back to Automatic.

Co-Authored-By: Claude Opus 5.5 <[email protected]>

apps/effectcraft/src/launch_guard.rs

@@ -5,8 +5,11 @@
 //! can't be caught, so each launch leaves a marker in the settings folder until its window has
 //! drawn a frame. A launch that finds the previous one's marker switches Window Graphics to
 //! OpenGL, which those drivers run, and says so once the window is up. `WGPU_BACKEND` (wgpu's
-//! own override) still wins over both.
+//! own override) still wins over both. The marker is locked while its launch runs, so a second
+//! copy opened meanwhile isn't taken for a crash; and a window OpenGL can't open either puts
+//! Window Graphics back to Automatic ([`opengl_failed`]) rather than failing every launch.
 
+use std::fs::File;
 use std::path::{Path, PathBuf};
 
 use effectcraft_engine::prefs::{PREFS_FILE, Prefs};
@@ -23,6 +26,8 @@ pub struct Launch {
     /// Switched to OpenGL because the previous launch never drew: the message to show.
     pub notice: Option<String>,
     marker: Option<PathBuf>,
+    /// Holds the marker's lock until the window draws (or the process ends).
+    lock: Option<File>,
 }
 
 impl Launch {
@@ -34,8 +39,10 @@ impl Launch {
         let prefs_path = dir.join(PREFS_FILE);
         let mut prefs = std::fs::read_to_string(&prefs_path).map(|t| Prefs::from_json(&t)).unwrap_or_default();
         let marker = dir.join(MARKER);
+        // Locked by another launch that is still opening its window: not a crash.
+        let held = File::open(&marker).is_ok_and(|f| f.try_lock().is_err());
         let mut notice = None;
-        if marker.exists() && prefs.startup.window_graphics == "auto" && cfg!(not(target_os = "macos")) {
+        if marker.exists() && !held && prefs.startup.window_graphics == "auto" && cfg!(not(target_os = "macos")) {
             prefs.startup.window_graphics = "gl".into();
             match std::fs::create_dir_all(dir).and_then(|()| std::fs::write(&prefs_path, prefs.to_json())) {
                 Ok(()) => log::warn!("the last launch never drew its window: switching Window Graphics to OpenGL"),
@@ -49,18 +56,20 @@ impl Launch {
             );
         }
         let backends = (prefs.startup.window_graphics == "gl").then_some(Backends::GL);
-        let marker = match std::fs::create_dir_all(dir).and_then(|()| std::fs::write(&marker, "")) {
-            Ok(()) => Some(marker),
+        let (marker, lock) = match std::fs::create_dir_all(dir).and_then(|()| File::create(&marker)) {
+            // Where locks aren't available the guard works as before.
+            Ok(f) => (Some(marker), f.try_lock().is_ok().then_some(f)),
             Err(e) => {
                 log::warn!("launch marker: {e}");
-                None
+                (None, None)
             }
         };
-        Launch { backends, notice, marker }
+        Launch { backends, notice, marker, lock }
     }
 
     /// The window drew its first frame: the launch made it.
     pub fn drawn(&mut self) {
+        self.lock = None;
         if let Some(m) = self.marker.take()
             && let Err(e) = std::fs::remove_file(&m)
         {
@@ -69,6 +78,18 @@ impl Launch {
     }
 }
 
+/// The window couldn't open with OpenGL (an error, not a crash): back to Automatic, and no marker.
+pub fn opengl_failed(dir: Option<&Path>) {
+    let Some(dir) = dir else { return };
+    let prefs_path = dir.join(PREFS_FILE);
+    let mut prefs = std::fs::read_to_string(&prefs_path).map(|t| Prefs::from_json(&t)).unwrap_or_default();
+    prefs.startup.window_graphics = "auto".into();
+    if let Err(e) = std::fs::write(&prefs_path, prefs.to_json()) {
+        log::warn!("saving Window Graphics failed: {e}");
+    }
+    let _ = std::fs::remove_file(dir.join(MARKER));
+}
+
 #[cfg(test)]
 mod tests {
     use super::*;
@@ -89,7 +110,7 @@ mod tests {
         l.drawn();
         assert!(!dir.join(MARKER).exists());
         // A launch that crashes before drawing (it never calls `drawn`)…
-        let _crashed = Launch::begin(Some(&dir), false);
+        drop(Launch::begin(Some(&dir), false));
         // …makes the next one use OpenGL and say so, once.
         let mut l = Launch::begin(Some(&dir), false);
         if cfg!(target_os = "macos") {
@@ -108,4 +129,23 @@ mod tests {
         assert!(!dir.join(MARKER).exists());
         let _ = std::fs::remove_dir_all(&dir);
     }
+
+    /// A second copy opened while the first is still opening isn't a crash; OpenGL that can't
+    /// open a window either goes back to Automatic instead of failing every launch.
+    #[test]
+    fn a_second_copy_or_failed_opengl_doesnt_strand_window_graphics() {
+        let dir = std::env::temp_dir().join(format!("ec-launch-guard-2-{}", std::process::id()));
+        let _ = std::fs::remove_dir_all(&dir);
+        let _first = Launch::begin(Some(&dir), false);
+        assert_eq!(Launch::begin(Some(&dir), false).backends, None);
+        assert_eq!(window_graphics(&dir), "auto");
+        drop(_first);
+        if cfg!(not(target_os = "macos")) {
+            assert_eq!(Launch::begin(Some(&dir), false).backends, Some(Backends::GL), "a real crash still switches");
+            opengl_failed(Some(&dir));
+            assert_eq!(window_graphics(&dir), "auto");
+            assert_eq!(Launch::begin(Some(&dir), false).backends, None);
+        }
+        let _ = std::fs::remove_dir_all(&dir);
+    }
 }

apps/effectcraft/src/main.rs

@@ -211,7 +211,12 @@ fn main() -> eframe::Result {
     );
     match &result {
         Ok(()) => log::info!("the window closed"),
-        Err(e) => startup_failed(e, !on_gl && cfg!(not(target_os = "macos"))),
+        Err(e) => {
+            if on_gl {
+                launch_guard::opengl_failed(effectcraft_host::config_dir().as_deref());
+            }
+            startup_failed(e, !on_gl && cfg!(not(target_os = "macos")));
+        }
     }
     result
 }