feat: honor async-backend with epoll/io_uring, fix last RV13 items

Resolves the 4 P3 items in docs/rewrite/notes/review-rv13-codex.md
(final-Z16.md):
- async-backend now takes effect like Ghostty: ghostty_os::xev ports
  libxev's backend detection (io_uring, then epoll); global init detects
  and logs it, the GTK startup applies the config with Zig's messages,
  and +version reports the real backend. The termio IO loop runs on a
  new reactor with epoll, io_uring (io-uring crate) and poll(2) (kept
  for macOS, where libxev uses kqueue). The 21 exec integration tests
  run under every backend; io_uring was exercised here.
- auto-update-channel defaults to the build's release channel.
- Windows error names: sharing violation -> AccessDenied, plus
  ERROR_FILE_EXISTS -> PathAlreadyExists and virus errors ->
  AntivirusInterference, mirroring std.os.windows.OpenFile's NTSTATUS
  mapping in Zig 0.15.2 (verified against its source by the
  orchestrator).
- Exact ports of std.fs.path.basenameWindows/basenamePosix for theme
  name validation (drive, UNC and device roots).

Test Plan:
- cargo test --workspace --no-fail-fast (3555 passed, exit 0)
- just check-macos (both), just check-windows, just check-wasm
- +show-config --default --docs vs /usr/bin/ghostty (only the
  build-specific gtk-opengl-debug line differs)

crates/ghostty-os/src/lib.rs

@@ -32,6 +32,7 @@ pub mod file_type;
 #[cfg(unix)]
 pub mod flatpak;
 pub mod osbytes;
+pub mod xev;
 pub use ghostty_zigfmt as zig_fmt;
 #[cfg(windows)]
 pub mod flatpak {

crates/ghostty-os/src/xev.rs

@@ -0,0 +1,214 @@
+//! Runtime selection of the async event-loop backend (libxev's
+//! `Dynamic` API: `detect`, `prefer`, `backend`; `Backend.candidates`).
+//!
+//! On Linux the candidates are `io_uring` then `epoll`; every other platform
+//! has exactly one backend (kqueue / IOCP / wasi poll), for which libxev's
+//! dynamic API collapses to the static one: nothing can be selected.
+//!
+//! The selection is process-global, like the `pub var backend` in libxev's
+//! `dynamic.zig`. It defaults to the *last* candidate (`epoll`) without
+//! testing availability; call [`detect`] at startup (`global.zig`) and
+//! optionally [`prefer`] (GTK `Application.startupXev`) before any loop is
+//! created.
+
+use std::sync::atomic::{AtomicU8, Ordering};
+
+/// Zig: `xev.Backend`.
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+pub enum Backend {
+    IoUring,
+    Epoll,
+    Kqueue,
+    WasiPoll,
+    Iocp,
+}
+
+impl Backend {
+    /// `@tagName`.
+    pub fn name(self) -> &'static str {
+        match self {
+            Backend::IoUring => "io_uring",
+            Backend::Epoll => "epoll",
+            Backend::Kqueue => "kqueue",
+            Backend::WasiPoll => "wasi_poll",
+            Backend::Iocp => "iocp",
+        }
+    }
+
+    fn to_u8(self) -> u8 {
+        self as u8
+    }
+
+    fn from_u8(v: u8) -> Backend {
+        match v {
+            0 => Backend::IoUring,
+            1 => Backend::Epoll,
+            2 => Backend::Kqueue,
+            3 => Backend::WasiPoll,
+            _ => Backend::Iocp,
+        }
+    }
+}
+
+impl std::fmt::Display for Backend {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        f.write_str(self.name())
+    }
+}
+
+/// Zig: `Backend.candidates()` in priority order.
+pub fn candidates() -> &'static [Backend] {
+    #[cfg(target_os = "linux")]
+    {
+        &[Backend::IoUring, Backend::Epoll]
+    }
+    #[cfg(target_os = "windows")]
+    {
+        &[Backend::Iocp]
+    }
+    #[cfg(target_arch = "wasm32")]
+    {
+        &[Backend::WasiPoll]
+    }
+    #[cfg(not(any(target_os = "linux", target_os = "windows", target_arch = "wasm32")))]
+    {
+        &[Backend::Kqueue]
+    }
+}
+
+/// Zig: `xev.dynamic` (more than one candidate).
+pub fn is_dynamic() -> bool {
+    candidates().len() > 1
+}
+
+static BACKEND: AtomicU8 = AtomicU8::new(0);
+static INIT: std::sync::Once = std::sync::Once::new();
+
+fn init_default() {
+    INIT.call_once(|| {
+        let last = *candidates().last().unwrap_or(&Backend::Epoll);
+        BACKEND.store(last.to_u8(), Ordering::SeqCst);
+    });
+}
+
+/// The backend in use (Zig: `xev.backend`).
+pub fn backend() -> Backend {
+    init_default();
+    Backend::from_u8(BACKEND.load(Ordering::SeqCst))
+}
+
+/// Zig: `Api().available()`.
+pub fn available(be: Backend) -> bool {
+    match be {
+        Backend::Epoll => cfg!(target_os = "linux"),
+        Backend::IoUring => io_uring_available(),
+        Backend::Kqueue => cfg!(any(target_vendor = "apple", target_os = "freebsd")),
+        Backend::WasiPoll => cfg!(target_arch = "wasm32"),
+        Backend::Iocp => cfg!(windows),
+    }
+}
+
+/// libxev performs a harmless `io_uring_setup(256)` to see if we have
+/// io_uring support. We additionally require what our loop needs
+/// (`IORING_OP_POLL_ADD`, `IORING_OP_POLL_REMOVE` and the `EXT_ARG` timeout),
+/// so that "available" never selects a ring that cannot run the IO loop.
+#[cfg(target_os = "linux")]
+fn io_uring_available() -> bool {
+    let Ok(ring) = io_uring::IoUring::new(256) else {
+        return false;
+    };
+    if !ring.params().is_feature_ext_arg() {
+        return false;
+    }
+    let mut probe = io_uring::Probe::new();
+    if ring.submitter().register_probe(&mut probe).is_err() {
+        return false;
+    }
+    probe.is_supported(io_uring::opcode::PollAdd::CODE)
+        && probe.is_supported(io_uring::opcode::PollRemove::CODE)
+}
+
+#[cfg(not(target_os = "linux"))]
+fn io_uring_available() -> bool {
+    false
+}
+
+/// There is no available backend (Zig: `error.NoAvailableBackends`).
+#[derive(Debug, Clone, Copy, PartialEq, Eq, thiserror::Error)]
+#[error("NoAvailableBackends")]
+pub struct NoAvailableBackends;
+
+/// Detect the preferred backend: the first available candidate.
+pub fn detect() -> Result<(), NoAvailableBackends> {
+    init_default();
+    for &be in candidates() {
+        if available(be) {
+            BACKEND.store(be.to_u8(), Ordering::SeqCst);
+            return Ok(());
+        }
+    }
+    Err(NoAvailableBackends)
+}
+
+/// Manually set the backend to use. If the backend is not a candidate or not
+/// available the backend in use is unchanged and false is returned.
+pub fn prefer(be: Backend) -> bool {
+    init_default();
+    if !candidates().contains(&be) || !available(be) {
+        return false;
+    }
+    BACKEND.store(be.to_u8(), Ordering::SeqCst);
+    true
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    /// The process-global is shared between tests.
+    static LOCK: std::sync::Mutex<()> = std::sync::Mutex::new(());
+
+    #[test]
+    fn names_match_zig_tags() {
+        assert_eq!(Backend::IoUring.name(), "io_uring");
+        assert_eq!(Backend::Epoll.to_string(), "epoll");
+    }
+
+    #[test]
+    fn detect_picks_first_available_candidate() {
+        let _g = LOCK.lock().unwrap();
+        detect().unwrap();
+        let expected = candidates()
+            .iter()
+            .copied()
+            .find(|b| available(*b))
+            .unwrap();
+        assert_eq!(backend(), expected);
+    }
+
+    #[test]
+    fn prefer_rejects_non_candidates() {
+        let _g = LOCK.lock().unwrap();
+        detect().unwrap();
+        let before = backend();
+        assert!(!prefer(Backend::Iocp));
+        assert_eq!(backend(), before);
+    }
+
+    #[cfg(target_os = "linux")]
+    #[test]
+    fn prefer_epoll_then_detect() {
+        let _g = LOCK.lock().unwrap();
+        assert!(prefer(Backend::Epoll));
+        assert_eq!(backend(), Backend::Epoll);
+        if available(Backend::IoUring) {
+            assert!(prefer(Backend::IoUring));
+            assert_eq!(backend(), Backend::IoUring);
+        } else {
+            let before = backend();
+            assert!(!prefer(Backend::IoUring));
+            assert_eq!(backend(), before);
+        }
+        detect().unwrap();
+    }
+}

crates/ghostty-os/src/zig_error.rs

@@ -61,63 +61,67 @@ pub fn errno_name(errno: i32) -> &'static str {
 }
 
 /// See the unix variant; this is the Windows (Win32 error code) table.
+///
+/// Zig's Windows file opens go through `std.os.windows.OpenFile`
+/// (`NtCreateFile`), which switches on the NTSTATUS. Rust opens with
+/// `CreateFileW`, whose error is `RtlNtStatusToDosError(status)`, so each arm
+/// below is the Win32 code of the NTSTATUS arm named in its comment (codes
+/// that `OpenFile` does not name are `error.Unexpected`):
+///
+/// | NTSTATUS | Win32 | Zig error |
+/// |---|---|---|
+/// | `OBJECT_NAME_INVALID` | `ERROR_INVALID_NAME` | `BadPathName` |
+/// | `OBJECT_NAME_NOT_FOUND` / `OBJECT_PATH_NOT_FOUND` | `ERROR_FILE_NOT_FOUND` / `ERROR_PATH_NOT_FOUND` | `FileNotFound` |
+/// | `BAD_NETWORK_PATH` / `BAD_NETWORK_NAME` | `ERROR_BAD_NETPATH` / `ERROR_BAD_NET_NAME` | `NetworkNotFound` |
+/// | `NO_MEDIA_IN_DEVICE` | `ERROR_NOT_READY` | `NoDevice` |
+/// | `SHARING_VIOLATION`, `ACCESS_DENIED`, `USER_MAPPED_FILE`, `FILE_IS_A_DIRECTORY` | `ERROR_SHARING_VIOLATION`, `ERROR_ACCESS_DENIED`, `ERROR_USER_MAPPED_FILE` | `AccessDenied` |
+/// | `PIPE_BUSY` / `PIPE_NOT_AVAILABLE` | `ERROR_PIPE_BUSY` | `PipeBusy` |
+/// | `OBJECT_NAME_COLLISION` | `ERROR_ALREADY_EXISTS` | `PathAlreadyExists` |
+/// | `NOT_A_DIRECTORY` | `ERROR_DIRECTORY` | `NotDir` |
+///
+/// plus `wToPrefixedFileW`'s `NameTooLong` (`ERROR_FILENAME_EXCED_RANGE`).
 #[must_use]
 #[cfg(windows)]
-#[allow(non_upper_case_globals)] // windows-sys spells two constants `..._64k`
 pub fn errno_name(code: i32) -> &'static str {
-    use windows_sys::Win32::Foundation::{
-        ERROR_ACCESS_DENIED,
-        ERROR_AUTODATASEG_EXCEEDS_64k,
-        ERROR_BAD_EXE_FORMAT,
-        ERROR_BAD_FORMAT,
-        ERROR_COMMITMENT_LIMIT,
-        ERROR_DIRECTORY,
-        ERROR_DISK_FULL,
-        ERROR_DYNLINK_FROM_INVALID_RING,
-        ERROR_EXE_MACHINE_TYPE_MISMATCH,
-        ERROR_EXE_MARKED_INVALID,
-        ERROR_FILE_NOT_FOUND,
-        ERROR_FILENAME_EXCED_RANGE,
-        ERROR_INFLOOP_IN_RELOC_CHAIN,
-        ERROR_INVALID_EXE_SIGNATURE,
-        ERROR_INVALID_MINALLOCSIZE,
-        ERROR_INVALID_MODULETYPE,
-        ERROR_INVALID_NAME,
-        ERROR_INVALID_SEGDPL,
-        ERROR_INVALID_STACKSEG,
-        ERROR_INVALID_STARTING_CODESEG,
-        ERROR_IOPL_NOT_ENABLED,
-        ERROR_ITERATED_DATA_EXCEEDS_64k,
-        ERROR_PATH_NOT_FOUND,
-        ERROR_RELOC_CHAIN_XEEDS_SEGLIM,
-        ERROR_RING2SEG_MUST_BE_MOVABLE,
-        ERROR_SHARING_VIOLATION,
-    };
-    match code as u32 {
-        ERROR_FILE_NOT_FOUND | ERROR_PATH_NOT_FOUND | ERROR_DIRECTORY => "FileNotFound",
-        ERROR_ACCESS_DENIED => "AccessDenied",
-        ERROR_INVALID_NAME => "InvalidName",
+    windows_errno_name(code as u32)
+}
+
+/// The Windows table on plain numbers (so it is testable on every host).
+#[must_use]
+#[cfg_attr(not(windows), allow(dead_code))]
+fn windows_errno_name(code: u32) -> &'static str {
+    // Win32 error codes (winerror.h). Mirrors the NTSTATUS mapping of
+    // `std.os.windows.OpenFile` in Zig 0.15.2 (verified against its source).
+    const ERROR_FILE_NOT_FOUND: u32 = 2;
+    const ERROR_PATH_NOT_FOUND: u32 = 3;
+    const ERROR_ACCESS_DENIED: u32 = 5;
+    const ERROR_NOT_READY: u32 = 21;
+    const ERROR_SHARING_VIOLATION: u32 = 32;
+    const ERROR_BAD_NETPATH: u32 = 53;
+    const ERROR_BAD_NET_NAME: u32 = 67;
+    const ERROR_FILE_EXISTS: u32 = 80;
+    const ERROR_INVALID_NAME: u32 = 123;
+    const ERROR_ALREADY_EXISTS: u32 = 183;
+    const ERROR_FILENAME_EXCED_RANGE: u32 = 206;
+    const ERROR_VIRUS_INFECTED: u32 = 225;
+    const ERROR_VIRUS_DELETED: u32 = 226;
+    const ERROR_PIPE_BUSY: u32 = 231;
+    const ERROR_DIRECTORY: u32 = 267;
+    const ERROR_USER_MAPPED_FILE: u32 = 1224;
+    match code {
+        ERROR_INVALID_NAME => "BadPathName",
+        ERROR_FILE_NOT_FOUND | ERROR_PATH_NOT_FOUND => "FileNotFound",
+        ERROR_BAD_NETPATH | ERROR_BAD_NET_NAME => "NetworkNotFound",
+        ERROR_NOT_READY => "NoDevice",
+        // A sharing violation is `AccessDenied` in `OpenFile` (the
+        // running-executable `FileBusy` retry logic is only in Zig 0.16).
+        ERROR_SHARING_VIOLATION | ERROR_ACCESS_DENIED | ERROR_USER_MAPPED_FILE => "AccessDenied",
+        ERROR_PIPE_BUSY => "PipeBusy",
+        // STATUS_OBJECT_NAME_COLLISION surfaces as either code.
+        ERROR_FILE_EXISTS | ERROR_ALREADY_EXISTS => "PathAlreadyExists",
+        ERROR_VIRUS_INFECTED | ERROR_VIRUS_DELETED => "AntivirusInterference",
+        ERROR_DIRECTORY => "NotDir",
         ERROR_FILENAME_EXCED_RANGE => "NameTooLong",
-        ERROR_SHARING_VIOLATION => "FileBusy",
-        ERROR_COMMITMENT_LIMIT => "SystemResources",
-        ERROR_DISK_FULL => "NoSpaceLeft",
-        ERROR_BAD_FORMAT
-        | ERROR_INVALID_STARTING_CODESEG
-        | ERROR_INVALID_STACKSEG
-        | ERROR_INVALID_MODULETYPE
-        | ERROR_INVALID_EXE_SIGNATURE
-        | ERROR_EXE_MARKED_INVALID
-        | ERROR_BAD_EXE_FORMAT
-        | ERROR_ITERATED_DATA_EXCEEDS_64k
-        | ERROR_INVALID_MINALLOCSIZE
-        | ERROR_DYNLINK_FROM_INVALID_RING
-        | ERROR_IOPL_NOT_ENABLED
-        | ERROR_INVALID_SEGDPL
-        | ERROR_AUTODATASEG_EXCEEDS_64k
-        | ERROR_RING2SEG_MUST_BE_MOVABLE
-        | ERROR_RELOC_CHAIN_XEEDS_SEGLIM
-        | ERROR_INFLOOP_IN_RELOC_CHAIN
-        | ERROR_EXE_MACHINE_TYPE_MISMATCH => "InvalidExe",
         _ => "Unexpected",
     }
 }
@@ -209,25 +213,38 @@ mod tests {
         assert_eq!(errno_name(40), "Unexpected");
     }
 
-    #[cfg(windows)]
     #[test]
     fn windows_table() {
         for (code, name) in [
-            (2, "FileNotFound"),
+            (2u32, "FileNotFound"),
             (3, "FileNotFound"),
             (5, "AccessDenied"),
-            (32, "FileBusy"),
-            (112, "NoSpaceLeft"),
-            (123, "InvalidName"),
-            (193, "InvalidExe"),
+            // ERROR_SHARING_VIOLATION: AccessDenied like OpenFile.
+            (32, "AccessDenied"),
+            (1224, "AccessDenied"),
+            (21, "NoDevice"),
+            (53, "NetworkNotFound"),
+            (67, "NetworkNotFound"),
+            (123, "BadPathName"),
+            (183, "PathAlreadyExists"),
+            (80, "PathAlreadyExists"),
+            (225, "AntivirusInterference"),
+            (226, "AntivirusInterference"),
             (206, "NameTooLong"),
-            (267, "FileNotFound"),
-            (1455, "SystemResources"),
+            (231, "PipeBusy"),
+            (267, "NotDir"),
             (87, "Unexpected"),
+            (112, "Unexpected"),
+            (193, "Unexpected"),
         ] {
-            assert_eq!(errno_name(code), name);
+            assert_eq!(windows_errno_name(code), name, "code {code}");
         }
-        let e = std::io::Error::from_raw_os_error(5);
+    }
+
+    #[cfg(windows)]
+    #[test]
+    fn windows_io_error_name() {
+        let e = std::io::Error::from_raw_os_error(32);
         assert_eq!(io_error_name(&e), "error.AccessDenied");
     }
 }