fix(engine): retry a leased file with a plain open; the FIFO test uses its own folder

crates/engine/src/io.rs

@@ -924,17 +924,56 @@ fn script_sandbox_check(path: &str) -> Result<()> {
 #[cfg(not(target_arch = "wasm32"))]
 const MAX_READ_BYTES: u64 = 2 * 1024 * 1024 * 1024;
 
+/// `O_NONBLOCK` as the platform defines it (std offers no constant and the crate takes no libc
+/// dependency); `None` on a Unix not listed, where the pathname check alone applies.
+#[cfg(all(unix, not(target_arch = "wasm32")))]
+const O_NONBLOCK: Option<i32> =
+    if cfg!(any(target_os = "macos", target_os = "ios", target_os = "freebsd", target_os = "netbsd", target_os = "openbsd", target_os = "dragonfly"))
+    {
+        Some(0x4)
+    } else if cfg!(all(
+        any(target_os = "linux", target_os = "android"),
+        any(target_arch = "mips", target_arch = "mips64", target_arch = "mips32r6", target_arch = "mips64r6")
+    )) {
+        Some(0x80)
+    } else if cfg!(all(any(target_os = "linux", target_os = "android"), any(target_arch = "sparc", target_arch = "sparc64"))) {
+        Some(0x4000)
+    } else if cfg!(any(target_os = "linux", target_os = "android")) {
+        Some(0o4000)
+    } else {
+        None
+    };
+
+/// Opens `path` for reading without blocking on a FIFO or device that has no writer.
+#[cfg(not(target_arch = "wasm32"))]
+fn open_nonblocking(path: &std::path::Path) -> std::io::Result<std::fs::File> {
+    let mut opts = std::fs::OpenOptions::new();
+    opts.read(true);
+    #[cfg(unix)]
+    if let Some(flag) = O_NONBLOCK {
+        use std::os::unix::fs::OpenOptionsExt;
+        opts.custom_flags(flag);
+    }
+    opts.open(path)
+}
+
 #[cfg(not(target_arch = "wasm32"))]
 pub fn read_file(path: &str) -> Result<Vec<u8>> {
     use std::io::Read;
     script_sandbox_check(path)?;
-    // Checked before opening: opening a FIFO nobody writes to blocks the calling (GUI) thread.
-    // shortcut: a FIFO swapped in between this check and the open still blocks; close that with
-    // O_NONBLOCK when the crate takes a libc dependency.
+    // Checked before opening: opening a FIFO nobody writes to blocks the calling (GUI) thread. The
+    // open itself is non-blocking where the platform's flag is known, so a FIFO swapped in after
+    // this check cannot block it either; the opened handle is checked again below.
     if !std::fs::metadata(path).map_err(|e| EngineError::Other(format!("{path}: {e}")))?.is_file() {
         return Err(EngineError::Other(format!("{path}: only regular files can be opened")));
     }
-    let f = std::fs::File::open(path).map_err(|e| EngineError::Other(format!("{path}: {e}")))?;
+    let f = match open_nonblocking(std::path::Path::new(path)) {
+        Ok(f) => f,
+        // A conflicting lease (Samba, NFS) makes a non-blocking open of a regular file wait to be retried; the
+        // path was just checked to be a regular file, so open it the plain way, as before.
+        Err(e) if e.kind() == std::io::ErrorKind::WouldBlock => std::fs::File::open(path).map_err(|e| EngineError::Other(format!("{path}: {e}")))?,
+        Err(e) => return Err(EngineError::Other(format!("{path}: {e}"))),
+    };
     // A symlink to an endless device reports length 0 but streams forever, so metadata
     // alone never bounds the read: require a real file and cap the bytes actually taken.
     if !f.metadata().map_err(|e| EngineError::Other(format!("{path}: {e}")))?.is_file() {

crates/engine/src/io/tests.rs

@@ -810,3 +810,44 @@ fn reading_a_fifo_is_refused_without_blocking() {
     assert!(err.contains("only regular files"), "{err}");
     let _ = std::fs::remove_dir_all(&dir);
 }
+
+/// Opening a FIFO nobody writes to returns at once (it is not a regular file), where a plain open
+/// blocks the calling thread for good.
+#[cfg(unix)]
+#[test]
+fn a_fifo_without_a_writer_does_not_block_the_open() {
+    if O_NONBLOCK.is_none() {
+        return;
+    }
+    /// Removes the folder when the test ends, however it ends.
+    struct Cleanup(std::path::PathBuf);
+    impl Drop for Cleanup {
+        fn drop(&mut self) {
+            let _ = std::fs::remove_dir_all(&self.0);
+        }
+    }
+    // Created exclusively (a fresh name per try), so no other process shares it.
+    let dir = (0..100)
+        .map(|n| {
+            std::env::temp_dir().join(format!(
+                "gridcraft-fifo-{}-{n}-{}",
+                std::process::id(),
+                std::time::UNIX_EPOCH.elapsed().map_or(0, |t| t.as_nanos())
+            ))
+        })
+        .find(|d| std::fs::create_dir(d).is_ok())
+        .expect("a fresh temporary folder");
+    let _cleanup = Cleanup(dir.clone());
+    let fifo = dir.join("pipe");
+    if !std::process::Command::new("mkfifo").arg(&fifo).status().is_ok_and(|s| s.success()) {
+        return; // no mkfifo here
+    }
+    let (tx, rx) = std::sync::mpsc::channel();
+    let path = fifo.clone();
+    std::thread::spawn(move || {
+        let _ = tx.send(open_nonblocking(&path).and_then(|f| f.metadata()).map(|m| m.is_file()));
+    });
+    let opened = rx.recv_timeout(std::time::Duration::from_secs(10)).expect("the open blocked");
+    assert!(!opened.unwrap());
+    assert!(read_file(&fifo.to_string_lossy()).is_err());
+}