Fix #502: no startup crash on dual-GPU Linux desktops under Wayland

0.5.0 asked wgpu for the low-power adapter everywhere (#306). On a Linux
desktop whose KDE Wayland compositor runs on an NVIDIA GPU, wgpu sorted the
Ryzen's integrated RADV GPU first: it reports that it can present to the
window, but KWin can't import the buffers it allocates ("importing the
supplied dmabufs failed"), which is a fatal Wayland protocol error. The next
surface configuration then got ERROR_SURFACE_LOST_KHR and egui-wgpu panicked
("The surface isn't supported by this adapter", winit.rs:129).

- Preferences > Performance > Graphics Processor gains Automatic, the new
  default: power saving on Windows and macOS as #306 wanted, and on Linux the
  system's order, where Mesa's device-select layer puts the GPU the desktop
  runs on first. Values are now automatic / lowPower / highPerformance; 0.5.0
  saved powerSaving for everyone, which reads as automatic.
- The app picks the window's adapter itself (apps/vectorcraft/src/gpu.rs,
  eframe's native_adapter_selector): only adapters that present to the
  surface, WGPU_ADAPTER_NAME first, hardware before software, native backends
  before OpenGL, then the power preference. Every adapter and the pick are
  logged.
- run_native runs under the engine guard. When the graphics fail during the
  first frames (a wgpu error, or a panic in wgpu, egui-wgpu or naga), the app
  starts again without that adapter (exec on Unix, so an AppImage stays
  mounted; a new process on Windows), passing the failed adapters in
  VECTORCRAFT_GPU_SKIP, and says so in the status bar. When none is left it
  logs why and exits with an error, never a panic.
- docs/development.md, README and ROADMAP: the new default, the restart and
  the escape hatches (WGPU_POWER_PREF, WGPU_ADAPTER_NAME, WGPU_BACKEND,
  MESA_VK_DEVICE_SELECT, DRI_PRIME).

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

apps/vectorcraft/src/gpu.rs

@@ -0,0 +1,470 @@
+//! Which graphics adapter the window renders with (#306, #502), and what happens when it can't
+//! show the window.
+//!
+//! The canvas is rasterized on the CPU; the GPU only composites it and draws the UI, so any
+//! adapter that can present to the window will do. [`adapter_order`] ranks the adapters: those that
+//! can't present to the window's surface never, then the one `WGPU_ADAPTER_NAME` names, hardware
+//! before software, the native backends (Vulkan, Metal, DX12) before OpenGL, and the power
+//! preference ([`power_preference`]) among the rest. eframe is handed [`selector`], which takes
+//! the first of that order.
+//!
+//! An adapter can report that it presents to the window and still fail once it does: on a hybrid
+//! Linux desktop under Wayland, the compositor runs on one GPU and may refuse the frame buffers
+//! another GPU allocates, which kills the window's Wayland connection; egui-wgpu then panics while
+//! configuring the surface (#502). Nothing in the process can show a window after that, so when
+//! the graphics fail while the window is starting up, [`finish`] starts the app again without
+//! that adapter, as the next one in the order would be tried. Each restart leaves out one more
+//! adapter, so it ends when none is left.
+
+use std::sync::atomic::{AtomicBool, Ordering};
+use std::sync::{Arc, OnceLock};
+
+use eframe::egui_wgpu::NativeAdapterSelectorMethod;
+use eframe::wgpu::{self, Backend, DeviceType, PowerPreference};
+
+/// The adapters a restart leaves out, those the window failed on, as comma-separated
+/// `backend:vendor:device` keys (`Vulkan:1002:164e`). Set by the app itself when it starts again.
+pub const SKIP_ENV: &str = "VECTORCRAFT_GPU_SKIP";
+
+/// wgpu's variable for choosing an adapter by (part of) its name, any case.
+const NAME_ENV: &str = "WGPU_ADAPTER_NAME";
+
+/// Where to read how to choose another adapter, for the error that ends the app.
+const HELP: &str =
+    "to choose a graphics adapter, see https://github.com/storytold/vectorcraft/blob/main/docs/development.md#desktop-graphics-processor";
+
+/// Frames the UI ran before the failure, below which it was still starting up. The failure in
+/// #502 came on the first or second frame; later, the adapter has proved itself, and a lost
+/// window is something else (such as the compositor restarting).
+const STARTUP_FRAMES: u64 = 10;
+
+/// The preference used when Preferences say Automatic. Windows and macOS show frames from any GPU
+/// (the integrated one avoids the flicker of #306). Elsewhere, the system's own order: Mesa's
+/// device-select layer puts the GPU the desktop runs on first (the integrated one on hybrid
+/// laptops), and a Wayland compositor may not show frames from another GPU (#502).
+pub const AUTOMATIC: PowerPreference = if cfg!(any(windows, target_os = "macos")) { PowerPreference::LowPower } else { PowerPreference::None };
+
+/// The power preference that orders the adapters: the `WGPU_POWER_PREF` environment variable
+/// (`low`, `high`, `none`) when set, otherwise Preferences › Performance › Graphics Processor
+/// (`gpuPreference`). Automatic, and any value this version doesn't know (0.5.0's default
+/// `powerSaving`, a newer or damaged preference file), is [`AUTOMATIC`].
+pub fn power_preference(pref: Option<&str>, env: Option<PowerPreference>) -> PowerPreference {
+    match (env, pref) {
+        (Some(p), _) => p,
+        (None, Some("highPerformance")) => PowerPreference::HighPerformance,
+        (None, Some("lowPower")) => PowerPreference::LowPower,
+        (None, _) => AUTOMATIC,
+    }
+}
+
+/// One adapter, as [`adapter_order`] sees it.
+#[derive(Clone, Debug)]
+pub struct Candidate {
+    /// Identifies the adapter across a restart: `backend:vendor:device` ([`key`]).
+    pub key: String,
+    pub name: String,
+    pub backend: Backend,
+    pub device_type: DeviceType,
+    /// Reports that it can present to the window's surface.
+    pub presents: bool,
+}
+
+impl Candidate {
+    fn new(adapter: &wgpu::Adapter, surface: Option<&wgpu::Surface<'_>>) -> Self {
+        let info = adapter.get_info();
+        Self {
+            key: key(info.backend, info.vendor, info.device),
+            presents: surface.is_none_or(|s| adapter.is_surface_supported(s)),
+            name: info.name,
+            backend: info.backend,
+            device_type: info.device_type,
+        }
+    }
+}
+
+/// `backend:vendor:device`, e.g. `Vulkan:1002:164e` (PCI ids, as `MESA_VK_DEVICE_SELECT` takes them).
+fn key(backend: Backend, vendor: u32, device: u32) -> String {
+    format!("{backend:?}:{vendor:04x}:{device:04x}")
+}
+
+/// The order to try `candidates` in (their indices): only those that present to the window and
+/// aren't in `skip`; the one whose name contains `named` (any case) first; hardware before
+/// software; native backends before OpenGL; then by `power`, keeping the system's order among
+/// equals (all of it for [`PowerPreference::None`]).
+pub fn adapter_order(candidates: &[Candidate], power: PowerPreference, named: Option<&str>, skip: &[String]) -> Vec<usize> {
+    let named = named.map(str::to_lowercase).filter(|n| !n.is_empty());
+    let mut order: Vec<(usize, &Candidate)> = candidates.iter().enumerate().filter(|(_, c)| c.presents && !skip.contains(&c.key)).collect();
+    // Stable, so equals keep the system's order.
+    order.sort_by_key(|(_, c)| {
+        let unnamed = named.as_ref().is_some_and(|n| !c.name.to_lowercase().contains(n));
+        (unnamed, c.device_type == DeviceType::Cpu, c.backend == Backend::Gl, power_rank(c.device_type, power))
+    });
+    order.into_iter().map(|(i, _)| i).collect()
+}
+
+/// wgpu's own ranking for a power preference: the preferred kind of GPU, the other kind, unknown,
+/// virtual, software.
+fn power_rank(t: DeviceType, power: PowerPreference) -> u8 {
+    match (power, t) {
+        (PowerPreference::None, _) => 0,
+        (PowerPreference::LowPower, DeviceType::IntegratedGpu) | (PowerPreference::HighPerformance, DeviceType::DiscreteGpu) => 0,
+        (_, DeviceType::IntegratedGpu | DeviceType::DiscreteGpu) => 1,
+        (_, DeviceType::Other) => 2,
+        (_, DeviceType::VirtualGpu) => 3,
+        (_, DeviceType::Cpu) => 4,
+    }
+}
+
+/// The adapters [`SKIP_ENV`] lists.
+pub fn skipped() -> Vec<String> {
+    std::env::var(SKIP_ENV).map(|v| parse_skip(&v)).unwrap_or_default()
+}
+
+fn parse_skip(v: &str) -> Vec<String> {
+    v.split(',').map(str::trim).filter(|k| !k.is_empty()).map(String::from).collect()
+}
+
+/// What the window's start-up leaves for [`finish`].
+#[derive(Default)]
+pub struct Startup {
+    /// The key of the adapter [`selector`] picked.
+    adapter: OnceLock<String>,
+    /// The UI's context, which counts the frames run.
+    ui: OnceLock<egui::Context>,
+}
+
+impl Startup {
+    /// The app was created on `ctx`.
+    pub fn created(&self, ctx: &egui::Context) {
+        // Set once; the window is created once.
+        let _ = self.ui.set(ctx.clone());
+    }
+}
+
+/// eframe's adapter choice: the first of [`adapter_order`] for `power`, `WGPU_ADAPTER_NAME` and the
+/// adapters a restart left out. Every adapter and the choice are logged, for bug reports.
+pub fn selector(power: PowerPreference, startup: Arc<Startup>) -> NativeAdapterSelectorMethod {
+    let named = std::env::var(NAME_ENV).ok();
+    let skip = skipped();
+    Arc::new(move |adapters, surface| {
+        let candidates: Vec<Candidate> = adapters.iter().map(|a| Candidate::new(a, surface)).collect();
+        let order = adapter_order(&candidates, power, named.as_deref(), &skip);
+        let listed: Vec<String> = candidates
+            .iter()
+            .map(|c| {
+                let note = if !c.presents {
+                    ", can't show the window"
+                } else if skip.contains(&c.key) {
+                    ", failed before"
+                } else {
+                    ""
+                };
+                format!("{} ({:?}, {:?}, {}{note})", c.name.trim(), c.backend, c.device_type, c.key)
+            })
+            .collect();
+        log::info!("graphics adapters: {}", listed.join("; "));
+        let first = order.first().and_then(|&i| adapters.get(i).zip(candidates.get(i)));
+        let Some((adapter, chosen)) = first else {
+            return Err(format!("no graphics adapter can show the window (power preference {power:?}, {} adapters)", adapters.len()));
+        };
+        // Set once; eframe picks the adapter once.
+        let _ = startup.adapter.set(chosen.key.clone());
+        Ok(adapter.clone())
+    })
+}
+
+/// The last panic came from the graphics stack (see [`watch_panics`]).
+static GRAPHICS_PANIC: AtomicBool = AtomicBool::new(false);
+
+/// Note whether each panic comes from the graphics stack, so [`finish`] can tell a failing adapter
+/// from a bug. The default hook still prints it.
+pub fn watch_panics() {
+    let default = std::panic::take_hook();
+    std::panic::set_hook(Box::new(move |info| {
+        GRAPHICS_PANIC.store(info.location().is_some_and(|l| in_graphics_stack(l.file())), Ordering::Relaxed);
+        default(info);
+    }));
+}
+
+/// A source file of wgpu (`wgpu-core-30.0.1/src/…`, `wgpu-hal-…`), egui-wgpu or naga.
+fn in_graphics_stack(file: &str) -> bool {
+    file.split(['/', '\\']).any(|dir| ["wgpu-", "egui-wgpu-", "naga-"].iter().any(|p| dir.starts_with(p)))
+}
+
+/// The adapter to start again without: the one picked, when the graphics failed while the window
+/// was starting up and it wasn't already left out.
+fn restart_without<'a>(graphics_failed: bool, frames: u64, picked: Option<&'a str>, skip: &[String]) -> Option<&'a str> {
+    picked.filter(|p| graphics_failed && frames < STARTUP_FRAMES && !skip.iter().any(|s| s == p))
+}
+
+/// After the window's run: `Ok` when it closed normally or the app started again (see the module
+/// docs), otherwise why it failed. `outcome` is eframe's result, or the message of a panic that
+/// ended it.
+pub fn finish(outcome: Result<eframe::Result, String>, startup: &Startup) -> Result<(), String> {
+    let (why, graphics_failed) = match outcome {
+        Ok(Ok(())) => return Ok(()),
+        Ok(Err(e @ eframe::Error::Wgpu(_))) => (e.to_string(), true),
+        Ok(Err(e)) => (e.to_string(), false),
+        Err(panic) => (panic, GRAPHICS_PANIC.load(Ordering::Relaxed)),
+    };
+    let frames = startup.ui.get().map_or(0, egui::Context::cumulative_frame_nr);
+    let mut skip = skipped();
+    let Some(failed) = restart_without(graphics_failed, frames, startup.adapter.get().map(String::as_str), &skip) else {
+        return Err(if graphics_failed { format!("{why} ({HELP})") } else { why });
+    };
+    log::error!("the window failed on graphics adapter {failed} ({why}): starting again without it");
+    skip.push(failed.to_string());
+    restart(&skip.join(",")).map_err(|e| format!("{why}; starting again failed: {e}"))
+}
+
+/// Start the app again with the same arguments, leaving out the adapters in `skip`. On Unix the
+/// new app replaces this process (same process id, so an AppImage keeps its files mounted), and
+/// this returns only on failure; on Windows it starts beside this one, which then exits.
+fn restart(skip: &str) -> std::io::Result<()> {
+    log::logger().flush();
+    let mut c = std::process::Command::new(std::env::current_exe()?);
+    c.args(std::env::args_os().skip(1)).env(SKIP_ENV, skip);
+    #[cfg(unix)]
+    {
+        use std::os::unix::process::CommandExt as _;
+        Err(c.exec())
+    }
+    #[cfg(not(unix))]
+    c.spawn().map(|_| ())
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    fn gpu(name: &str, backend: Backend, device_type: DeviceType) -> Candidate {
+        Candidate { key: format!("{backend:?}:{name}"), name: name.into(), backend, device_type, presents: true }
+    }
+
+    fn names(c: &[Candidate], order: &[usize]) -> Vec<String> {
+        order.iter().map(|&i| c[i].name.clone()).collect()
+    }
+
+    /// The machine of #502 as Vulkan lists it with Mesa's device-select layer: the GPU KDE's
+    /// compositor runs on (NVIDIA) first, then the Ryzen's integrated GPU, Mesa's software
+    /// renderer, and OpenGL.
+    fn issue_502() -> Vec<Candidate> {
+        vec![
+            gpu("NVIDIA GeForce RTX 5070", Backend::Vulkan, DeviceType::DiscreteGpu),
+            gpu("AMD Ryzen 9 7900X (RADV RAPHAEL_MENDOCINO)", Backend::Vulkan, DeviceType::IntegratedGpu),
+            gpu("llvmpipe (LLVM 20.1.8, 256 bits)", Backend::Vulkan, DeviceType::Cpu),
+            gpu("NVIDIA GeForce RTX 5070/PCIe/SSE2", Backend::Gl, DeviceType::Other),
+        ]
+    }
+
+    #[test]
+    fn automatic_keeps_the_systems_order_on_linux_and_saves_power_elsewhere() {
+        let c = issue_502();
+        let order = adapter_order(&c, power_preference(None, None), None, &[]);
+        let first = &c[order[0]].name;
+        if cfg!(any(windows, target_os = "macos")) {
+            assert_eq!(first, "AMD Ryzen 9 7900X (RADV RAPHAEL_MENDOCINO)");
+        } else {
+            // #502: the integrated GPU couldn't show frames to a compositor on the NVIDIA GPU.
+            assert_eq!(first, "NVIDIA GeForce RTX 5070");
+        }
+    }
+
+    #[test]
+    fn each_preference_orders_the_adapters_then_falls_back_to_opengl_and_software() {
+        let c = issue_502();
+        let order = |power| names(&c, &adapter_order(&c, power, None, &[]));
+        assert_eq!(
+            order(PowerPreference::None),
+            [
+                "NVIDIA GeForce RTX 5070",
+                "AMD Ryzen 9 7900X (RADV RAPHAEL_MENDOCINO)",
+                "NVIDIA GeForce RTX 5070/PCIe/SSE2",
+                "llvmpipe (LLVM 20.1.8, 256 bits)"
+            ]
+        );
+        assert_eq!(
+            order(PowerPreference::LowPower),
+            [
+                "AMD Ryzen 9 7900X (RADV RAPHAEL_MENDOCINO)",
+                "NVIDIA GeForce RTX 5070",
+                "NVIDIA GeForce RTX 5070/PCIe/SSE2",
+                "llvmpipe (LLVM 20.1.8, 256 bits)"
+            ]
+        );
+        assert_eq!(
+            order(PowerPreference::HighPerformance),
+            [
+                "NVIDIA GeForce RTX 5070",
+                "AMD Ryzen 9 7900X (RADV RAPHAEL_MENDOCINO)",
+                "NVIDIA GeForce RTX 5070/PCIe/SSE2",
+                "llvmpipe (LLVM 20.1.8, 256 bits)"
+            ]
+        );
+    }
+
+    /// A restart leaves out the adapter the window failed on, so Power Saving on the machine of
+    /// #502 ends up on the NVIDIA GPU, and once every adapter failed there is nothing to try.
+    #[test]
+    fn a_restart_tries_the_next_adapter() {
+        let c = issue_502();
+        let mut skip = vec![];
+        let mut tried = vec![];
+        while let Some(&i) = adapter_order(&c, PowerPreference::LowPower, None, &skip).first() {
+            tried.push(c[i].name.clone());
+            skip.push(c[i].key.clone());
+        }
+        assert_eq!(
+            tried,
+            [
+                "AMD Ryzen 9 7900X (RADV RAPHAEL_MENDOCINO)",
+                "NVIDIA GeForce RTX 5070",
+                "NVIDIA GeForce RTX 5070/PCIe/SSE2",
+                "llvmpipe (LLVM 20.1.8, 256 bits)"
+            ]
+        );
+    }
+
+    #[test]
+    fn adapters_that_cannot_present_to_the_window_are_never_tried() {
+        let mut c = issue_502();
+        c[0].presents = false;
+        c[3].presents = false;
+        for power in [PowerPreference::None, PowerPreference::LowPower, PowerPreference::HighPerformance] {
+            let order = adapter_order(&c, power, None, &[]);
+            assert!(!order.contains(&0) && !order.contains(&3), "{power:?}: {order:?}");
+            assert_eq!(order.len(), 2);
+        }
+        for p in &mut c {
+            p.presents = false;
+        }
+        assert!(adapter_order(&c, PowerPreference::None, None, &[]).is_empty());
+        assert!(adapter_order(&[], PowerPreference::HighPerformance, None, &[]).is_empty());
+    }
+
+    #[test]
+    fn wgpu_adapter_name_picks_an_adapter_first_and_the_rest_stay_as_fallbacks() {
+        let c = issue_502();
+        let order = adapter_order(&c, PowerPreference::HighPerformance, Some("radv"), &[]);
+        assert_eq!(c[order[0]].name, "AMD Ryzen 9 7900X (RADV RAPHAEL_MENDOCINO)");
+        assert_eq!(order.len(), 4);
+        // Any case; every match comes before the rest, natively first.
+        let order = adapter_order(&c, PowerPreference::LowPower, Some("GeForce"), &[]);
+        assert_eq!(names(&c, &order[..2]), ["NVIDIA GeForce RTX 5070", "NVIDIA GeForce RTX 5070/PCIe/SSE2"]);
+        // An empty name or one nothing matches changes nothing.
+        for named in ["", "no such gpu"] {
+            assert_eq!(adapter_order(&c, PowerPreference::LowPower, Some(named), &[]), adapter_order(&c, PowerPreference::LowPower, None, &[]));
+        }
+        // A named adapter that failed is left out like any other.
+        let skip = vec![c[1].key.clone()];
+        assert_eq!(adapter_order(&c, PowerPreference::None, Some("radv"), &skip)[0], 0);
+    }
+
+    /// A Windows machine lists every GPU twice (Vulkan and DX12): power saving still takes the
+    /// integrated GPU first, as wgpu's own choice did (#306).
+    #[test]
+    fn hybrid_laptop_on_windows_renders_on_the_integrated_gpu_with_power_saving() {
+        let c = vec![
+            gpu("Intel(R) Arc(TM) A370M", Backend::Vulkan, DeviceType::DiscreteGpu),
+            gpu("Intel(R) Iris(R) Xe Graphics", Backend::Vulkan, DeviceType::IntegratedGpu),
+            gpu("Intel(R) Arc(TM) A370M", Backend::Dx12, DeviceType::DiscreteGpu),
+            gpu("Intel(R) Iris(R) Xe Graphics", Backend::Dx12, DeviceType::IntegratedGpu),
+            gpu("Microsoft Basic Render Driver", Backend::Dx12, DeviceType::Cpu),
+        ];
+        let order = adapter_order(&c, PowerPreference::LowPower, None, &[]);
+        assert_eq!(order, [1, 3, 0, 2, 4]);
+        assert_eq!(adapter_order(&c, PowerPreference::HighPerformance, None, &[]), [0, 2, 1, 3, 4]);
+    }
+
+    #[test]
+    fn power_preference_follows_the_environment_then_the_preference() {
+        assert_eq!(power_preference(None, None), AUTOMATIC);
+        assert_eq!(power_preference(Some("automatic"), None), AUTOMATIC);
+        assert_eq!(power_preference(Some("lowPower"), None), PowerPreference::LowPower);
+        assert_eq!(power_preference(Some("highPerformance"), None), PowerPreference::HighPerformance);
+        // 0.5.0's default, and values this version doesn't know (a newer or damaged file).
+        for old in ["powerSaving", "turbo", ""] {
+            assert_eq!(power_preference(Some(old), None), AUTOMATIC, "{old}");
+        }
+        // WGPU_POWER_PREF wins over the preference, either way.
+        assert_eq!(power_preference(Some("lowPower"), Some(PowerPreference::HighPerformance)), PowerPreference::HighPerformance);
+        assert_eq!(power_preference(Some("highPerformance"), Some(PowerPreference::LowPower)), PowerPreference::LowPower);
+        assert_eq!(power_preference(Some("lowPower"), Some(PowerPreference::None)), PowerPreference::None);
+        assert_eq!(AUTOMATIC, if cfg!(any(windows, target_os = "macos")) { PowerPreference::LowPower } else { PowerPreference::None });
+    }
+
+    /// The preference the engine saves is the one the app reads back before the window opens.
+    #[test]
+    fn the_saved_engine_preference_is_found() {
+        let mut prefs = vectorcraft_engine::Prefs::default();
+        let saved = prefs.to_json();
+        assert_eq!(saved.get("gpuPreference").and_then(serde_json::Value::as_str), Some("automatic"));
+        for (value, power) in [("lowPower", PowerPreference::LowPower), ("highPerformance", PowerPreference::HighPerformance)] {
+            prefs.gpu_preference = value.into();
+            let saved = prefs.to_json();
+            assert_eq!(power_preference(saved.get("gpuPreference").and_then(serde_json::Value::as_str), None), power);
+        }
+        // Every choice Preferences offer means something here.
+        for (value, _) in vectorcraft_engine::cmd::prefscmds::GPU_PREFERENCES {
+            assert!(["automatic", "lowPower", "highPerformance"].contains(value), "{value}");
+        }
+    }
+
+    #[test]
+    fn skipped_adapters_parse_from_a_comma_separated_list() {
+        assert_eq!(parse_skip("Vulkan:1002:164e, Gl:10de:2f04,,"), ["Vulkan:1002:164e", "Gl:10de:2f04"]);
+        assert!(parse_skip("").is_empty());
+        assert_eq!(key(Backend::Vulkan, 0x1002, 0x164e), "Vulkan:1002:164e");
+        assert_eq!(key(Backend::Gl, 0x10de, 0x2f04), "Gl:10de:2f04");
+    }
+
+    #[test]
+    fn panics_in_wgpu_egui_wgpu_and_naga_are_graphics_failures() {
+        for file in [
+            "/home/u/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/egui-wgpu-0.36.2/src/winit.rs",
+            r"C:\Users\u\.cargo\registry\src\index.crates.io-1949cf8c6b5b557f\wgpu-core-30.0.1\src\device\mod.rs",
+            "/rustc/deps/wgpu-hal-30.0.1/src/vulkan/adapter.rs",
+            "naga-30.0.0/src/back/spv/writer.rs",
+        ] {
+            assert!(in_graphics_stack(file), "{file}");
+        }
+        for file in [
+            "apps/vectorcraft/src/main.rs",
+            "/home/wgpu/vectorcraft/crates/ui-egui/src/lib.rs",
+            "egui-0.36.2/src/context.rs",
+            "winit-0.30.12/src/x.rs",
+        ] {
+            assert!(!in_graphics_stack(file), "{file}");
+        }
+    }
+
+    #[test]
+    fn only_a_graphics_failure_while_starting_up_restarts_without_the_adapter() {
+        let amd = Some("Vulkan:1002:164e");
+        assert_eq!(restart_without(true, 0, amd, &[]), amd);
+        assert_eq!(restart_without(true, STARTUP_FRAMES - 1, amd, &[]), amd);
+        // After start-up the adapter has shown frames; a lost window is something else.
+        assert_eq!(restart_without(true, STARTUP_FRAMES, amd, &[]), None);
+        // A bug outside the graphics, or no adapter picked: nothing another adapter would change.
+        assert_eq!(restart_without(false, 0, amd, &[]), None);
+        assert_eq!(restart_without(true, 0, None, &[]), None);
+        // Never the same adapter twice, so restarts end.
+        assert_eq!(restart_without(true, 0, amd, &["Vulkan:1002:164e".into()]), None);
+    }
+
+    #[test]
+    fn a_normal_exit_or_another_failure_does_not_restart() {
+        let startup = Startup::default();
+        assert_eq!(finish(Ok(Ok(())), &startup), Ok(()));
+        // No adapter was picked (none can show the window): the error is returned, nothing restarts.
+        let none =
+            eframe::Error::Wgpu(eframe::egui_wgpu::WgpuError::CustomNativeAdapterSelectionError("no graphics adapter can show the window".into()));
+        assert_eq!(
+            finish(Ok(Err(none)), &startup),
+            Err(format!("WGPU error: Adapter selection failed: no graphics adapter can show the window ({HELP})"))
+        );
+        let _ = startup.adapter.set("Vulkan:1002:164e".into());
+        GRAPHICS_PANIC.store(false, Ordering::Relaxed);
+        assert_eq!(finish(Err("a bug".into()), &startup), Err("a bug".into()));
+    }
+}

apps/vectorcraft/src/main.rs

@@ -16,6 +16,8 @@ compile_error!("the win7 target requires --no-default-features --features window
 
 mod clipboard;
 mod control_server;
+#[cfg(feature = "wgpu")]
+mod gpu;
 mod logging;
 #[cfg(target_os = "macos")]
 mod native_menu;
@@ -297,23 +299,6 @@ fn app_icon() -> egui::IconData {
     eframe::icon_data::from_png_bytes(png).unwrap_or_default()
 }
 
-/// The power preference wgpu picks the window's graphics adapter with (#306): the
-/// `WGPU_POWER_PREF` environment variable (`low`, `high`, `none`) when set, otherwise Preferences ›
-/// Performance › Graphics Processor (`gpuPreference`). Power saving unless the preference asks for
-/// high performance: the canvas is rasterized on the CPU and the GPU only composites it, which the
-/// integrated GPU of a hybrid-graphics laptop does easily, while presenting frames rendered on the
-/// discrete GPU through the integrated one made the window flicker on some laptops. With a single
-/// GPU both preferences pick it.
-#[cfg(feature = "wgpu")]
-fn power_preference(pref: Option<&str>, env: Option<eframe::wgpu::PowerPreference>) -> eframe::wgpu::PowerPreference {
-    use eframe::wgpu::PowerPreference;
-    match (env, pref) {
-        (Some(p), _) => p,
-        (None, Some("highPerformance")) => PowerPreference::HighPerformance,
-        (None, _) => PowerPreference::LowPower,
-    }
-}
-
 /// "name (backend, kind)" of the adapter the window renders with, for Help › About and bug reports.
 #[cfg(feature = "wgpu")]
 fn adapter_summary(info: &eframe::wgpu::AdapterInfo) -> String {
@@ -324,7 +309,7 @@ fn adapter_summary(info: &eframe::wgpu::AdapterInfo) -> String {
 /// macOS keeps its traffic lights over the integrated title strip.
 const CUSTOM_TITLEBAR: bool = !cfg!(target_os = "macos");
 
-fn main() -> eframe::Result {
+fn main() -> std::process::ExitCode {
     // First, so every start-up warning is recorded (`logging`).
     let logger = logging::install();
     vectorcraft_ui_egui::i18n::detect_system_lang_in_background();
@@ -336,7 +321,7 @@ fn main() -> eframe::Result {
             "--control" => control_port = args.next().and_then(|p| p.parse().ok()),
             "--version" => {
                 println!("vectorcraft {}", env!("CARGO_PKG_VERSION"));
-                return Ok(());
+                return std::process::ExitCode::SUCCESS;
             }
             _ => files.push(a),
         }
@@ -360,7 +345,9 @@ fn main() -> eframe::Result {
     #[cfg(feature = "wgpu")]
     let gpu_pref = saved.as_ref().and_then(|ui| ui.engine_prefs.get("gpuPreference")).and_then(serde_json::Value::as_str);
     #[cfg(feature = "wgpu")]
-    let power = power_preference(gpu_pref, eframe::wgpu::PowerPreference::from_env());
+    let power = gpu::power_preference(gpu_pref, eframe::wgpu::PowerPreference::from_env());
+    #[cfg(feature = "wgpu")]
+    let startup = std::sync::Arc::new(gpu::Startup::default());
     let options = eframe::NativeOptions {
         viewport: egui::ViewportBuilder::default()
             .with_title("VectorCraft")
@@ -383,86 +370,94 @@ fn main() -> eframe::Result {
         // One frame queued, not two: the canvas is rasterized on the CPU and the GPU only
         // composites it, so the window answers the pointer a frame sooner (#444).
         options.wgpu_options.surface = eframe::egui_wgpu::SurfaceConfig::LOW_LATENCY;
+        // Only adapters that can show the window, in the order `gpu` gives (#306, #502).
         if let eframe::egui_wgpu::WgpuSetup::CreateNew(create) = &mut options.wgpu_options.wgpu_setup {
-            create.power_preference = power;
+            create.native_adapter_selector = Some(gpu::selector(power, startup.clone()));
         }
         options
     };
+    #[cfg(feature = "wgpu")]
+    gpu::watch_panics();
     // Files opened from Finder and the Dock arrive as events, not arguments.
     #[cfg(target_os = "macos")]
     open_documents::install();
-    eframe::run_native(
-        "VectorCraft",
-        options,
-        Box::new(move |cc| {
-            let mut app = VectorcraftApp::new(Session::new(), services());
-            load_prefs(&mut app, saved);
-            // Fit the window to its monitor, or put it back where it was (still hidden).
-            if let Some(w) = cc.winit_window() {
-                app.ui.window = Some(window::restore(w, saved_window));
-            }
-            // User Defined swatch and graphic style libraries live next to the preferences.
-            let swatches = prefs_path().and_then(|p| Some(p.parent()?.join("Swatches").to_string_lossy().to_string()));
-            app.session.swatch_libraries.set_user_dir(swatches);
-            let styles = prefs_path().and_then(|p| Some(p.parent()?.join("Graphic Styles").to_string_lossy().to_string()));
-            app.session.style_libraries.set_user_dir(styles);
-            // Data Recovery copies live next to the preferences too (none for runs without
-            // preferences, such as agents' test runs, unless the recoveryFolder preference is set).
-            if std::env::var_os("VECTORCRAFT_NO_PREFS").is_none() {
-                let recovery = prefs_path().and_then(|p| Some(p.parent()?.join("Data Recovery").to_string_lossy().to_string()));
-                app.session.recovery.set_default_folder(recovery);
-            }
-            let graphics_loss = GraphicsLoss::default();
-            #[cfg(feature = "wgpu")]
-            if let Some(rs) = &cc.wgpu_render_state {
-                let summary = adapter_summary(&rs.adapter.get_info());
-                log::info!("rendering with {summary} (power preference {power:?})");
-                app.graphics_adapter = Some(summary);
-                let (loss, ctx) = (graphics_loss.clone(), cc.egui_ctx.clone());
-                rs.device.set_device_lost_callback(move |reason, msg| loss.report(&ctx, format!("{reason:?}: {msg}")));
-            }
-            #[cfg(feature = "windows7")]
-            {
-                app.graphics_adapter = Some("OpenGL (Windows 7 compatibility)".into());
-            }
-            app.integrated_titlebar = cfg!(target_os = "macos");
-            app.custom_titlebar = CUSTOM_TITLEBAR;
-            if let Some(port) = control_port {
-                let rx = control_server::start(port, cc.egui_ctx.clone());
-                app = app.with_control(rx);
-            }
-            #[cfg(target_os = "macos")]
-            open_documents::set_ui(&cc.egui_ctx);
-            open_files(&mut app, files);
-            Ok(Box::new(App {
-                app,
-                graphics_loss,
-                graphics_lost: false,
+    #[cfg(feature = "wgpu")]
+    let created = startup.clone();
+    // A panic that ends the window (egui-wgpu's own, #502) is caught here: never a crash.
+    let outcome = vectorcraft_engine::guard::catch_panic(|| {
+        eframe::run_native(
+            "VectorCraft",
+            options,
+            Box::new(move |cc| {
+                let mut app = VectorcraftApp::new(Session::new(), services());
+                load_prefs(&mut app, saved);
+                // Fit the window to its monitor, or put it back where it was (still hidden).
+                if let Some(w) = cc.winit_window() {
+                    app.ui.window = Some(window::restore(w, saved_window));
+                }
+                // User Defined swatch and graphic style libraries live next to the preferences.
+                let swatches = prefs_path().and_then(|p| Some(p.parent()?.join("Swatches").to_string_lossy().to_string()));
+                app.session.swatch_libraries.set_user_dir(swatches);
+                let styles = prefs_path().and_then(|p| Some(p.parent()?.join("Graphic Styles").to_string_lossy().to_string()));
+                app.session.style_libraries.set_user_dir(styles);
+                // Data Recovery copies live next to the preferences too (none for runs without
+                // preferences, such as agents' test runs, unless the recoveryFolder preference is set).
+                if std::env::var_os("VECTORCRAFT_NO_PREFS").is_none() {
+                    let recovery = prefs_path().and_then(|p| Some(p.parent()?.join("Data Recovery").to_string_lossy().to_string()));
+                    app.session.recovery.set_default_folder(recovery);
+                }
+                let graphics_loss = GraphicsLoss::default();
+                #[cfg(feature = "wgpu")]
+                if let Some(rs) = &cc.wgpu_render_state {
+                    let summary = adapter_summary(&rs.adapter.get_info());
+                    log::info!("rendering with {summary} (power preference {power:?})");
+                    created.created(&cc.egui_ctx);
+                    if !gpu::skipped().is_empty() {
+                        app.status(format!("The graphics processor tried first couldn't show the window, so VectorCraft started again on {summary}"));
+                    }
+                    app.graphics_adapter = Some(summary);
+                    let (loss, ctx) = (graphics_loss.clone(), cc.egui_ctx.clone());
+                    rs.device.set_device_lost_callback(move |reason, msg| loss.report(&ctx, format!("{reason:?}: {msg}")));
+                }
+                #[cfg(feature = "windows7")]
+                {
+                    app.graphics_adapter = Some("OpenGL (Windows 7 compatibility)".into());
+                }
+                app.integrated_titlebar = cfg!(target_os = "macos");
+                app.custom_titlebar = CUSTOM_TITLEBAR;
+                if let Some(port) = control_port {
+                    let rx = control_server::start(port, cc.egui_ctx.clone());
+                    app = app.with_control(rx);
+                }
                 #[cfg(target_os = "macos")]
-                menu: None,
-            }))
-        }),
-    )
+                open_documents::set_ui(&cc.egui_ctx);
+                open_files(&mut app, files);
+                Ok(Box::new(App {
+                    app,
+                    graphics_loss,
+                    graphics_lost: false,
+                    #[cfg(target_os = "macos")]
+                    menu: None,
+                }))
+            }),
+        )
+    });
+    #[cfg(feature = "wgpu")]
+    let outcome = gpu::finish(outcome, &startup);
+    #[cfg(not(feature = "wgpu"))]
+    let outcome = outcome.and_then(|r| r.map_err(|e| e.to_string()));
+    match outcome {
+        Ok(()) => std::process::ExitCode::SUCCESS,
+        Err(e) => {
+            log::error!("VectorCraft stopped: {e}");
+            std::process::ExitCode::FAILURE
+        }
+    }
 }
 
 #[cfg(all(test, feature = "wgpu"))]
 mod tests {
     use super::*;
-    use eframe::wgpu::PowerPreference;
-
-    #[test]
-    fn power_saving_unless_the_preference_or_the_environment_says_otherwise() {
-        assert_eq!(power_preference(None, None), PowerPreference::LowPower);
-        assert_eq!(power_preference(Some("powerSaving"), None), PowerPreference::LowPower);
-        assert_eq!(power_preference(Some("highPerformance"), None), PowerPreference::HighPerformance);
-        // A value this version doesn't know (a newer or damaged preference file) is the default.
-        assert_eq!(power_preference(Some("turbo"), None), PowerPreference::LowPower);
-        assert_eq!(power_preference(Some(""), None), PowerPreference::LowPower);
-        // WGPU_POWER_PREF wins over the preference, either way.
-        assert_eq!(power_preference(Some("powerSaving"), Some(PowerPreference::HighPerformance)), PowerPreference::HighPerformance);
-        assert_eq!(power_preference(Some("highPerformance"), Some(PowerPreference::LowPower)), PowerPreference::LowPower);
-        assert_eq!(power_preference(None, Some(PowerPreference::None)), PowerPreference::None);
-    }
 
     /// The file extensions the macOS bundle declares: its document types and its own exported type.
     fn plist_extensions(plist: &str) -> Vec<&str> {
@@ -492,16 +487,4 @@ mod tests {
         assert!(types > 1 && plist.matches("<key>LSHandlerRank</key>").count() == types, "every document type has a rank");
         assert_eq!(plist.matches("<string>Owner</string>").count(), 2, "only VectorCraft documents and templates are owned");
     }
-
-    /// The preference the engine saves is the one the app reads back before the window opens.
-    #[test]
-    fn the_saved_engine_preference_is_found() {
-        let mut prefs = vectorcraft_engine::Prefs::default();
-        let saved = prefs.to_json();
-        assert_eq!(saved.get("gpuPreference").and_then(serde_json::Value::as_str), Some("powerSaving"));
-        prefs.gpu_preference = "highPerformance".into();
-        let saved = prefs.to_json();
-        let pref = saved.get("gpuPreference").and_then(serde_json::Value::as_str);
-        assert_eq!(power_preference(pref, None), PowerPreference::HighPerformance);
-    }
 }