Move the dup2_to_replace_stdio test into a separate child process.

This should help it run better on macos and ios, as well as on older
Linux versions and qemu.

Author: sunfishcode

examples/dup2_to_replace_stdio.rs

@@ -0,0 +1,40 @@
+//! This is an example of how to use `dup2` to replace the stdin and stdout file
+//! descriptors.
+
+#[cfg(not(windows))]
+fn main() {
+    use io_lifetimes::AsFilelike;
+    use rustix::io::{dup2, pipe};
+    use std::io::{BufRead, BufReader, Write};
+    use std::mem::forget;
+
+    let (reader, writer) = pipe().unwrap();
+    let (stdin, stdout) = unsafe { (rustix::io::take_stdin(), rustix::io::take_stdout()) };
+    dup2(&reader, &stdin).unwrap();
+    dup2(&writer, &stdout).unwrap();
+    forget(stdin);
+    forget(stdout);
+
+    drop(reader);
+    drop(writer);
+
+    // Don't use `std::io::stdout()` because in tests it's captured.
+    unsafe {
+        writeln!(
+            rustix::io::stdout().as_filelike_view::<std::fs::File>(),
+            "hello, world!"
+        )
+        .unwrap();
+
+        let mut s = String::new();
+        BufReader::new(&*rustix::io::stdin().as_filelike_view::<std::fs::File>())
+            .read_line(&mut s)
+            .unwrap();
+        assert_eq!(s, "hello, world!\n");
+    }
+}
+
+#[cfg(windows)]
+fn main() {
+    unimplemented!()
+}

tests/io/dup2_to_replace_stdio.rs

@@ -1,44 +1,18 @@
 #![cfg(not(target_os = "wasi"))]
-// This test interacts with `cargo test` in ways which causes failures on
-// darwin; disable it until we have a better option.
-#![cfg(not(any(target_os = "ios", target_os = "macos")))]
+
+use std::env;
+use std::process::Command;
 
 /// Use `dup2` to replace the stdin and stdout file descriptors.
 #[test]
 fn dup2_to_replace_stdio() {
-    use io_lifetimes::AsFilelike;
-    use rustix::io::{dup2, pipe};
-    use std::io::{BufRead, BufReader, Write};
-    use std::mem::forget;
-
-    // This test is flaky under qemu.
-    if std::env::vars().any(|var| var.0.starts_with("CARGO_TARGET_") && var.0.ends_with("_RUNNER"))
-    {
-        return;
-    }
-
-    let (reader, writer) = pipe().unwrap();
-    let (stdin, stdout) = unsafe { (rustix::io::take_stdin(), rustix::io::take_stdout()) };
-    dup2(&reader, &stdin).unwrap();
-    dup2(&writer, &stdout).unwrap();
-    forget(stdin);
-    forget(stdout);
-
-    drop(reader);
-    drop(writer);
-
-    // Don't use `std::io::stdout()` because in tests it's captured.
-    unsafe {
-        writeln!(
-            rustix::io::stdout().as_filelike_view::<std::fs::File>(),
-            "hello, world!"
-        )
-        .unwrap();
-
-        let mut s = String::new();
-        BufReader::new(&*rustix::io::stdin().as_filelike_view::<std::fs::File>())
-            .read_line(&mut s)
-            .unwrap();
-        assert_eq!(s, "hello, world!\n");
-    }
+    // This test modifies the stdio file descriptors, so we run it in a
+    // separate process so that it doesn't inferfere with the test harness.
+    assert!(Command::new(env::var("CARGO").unwrap())
+        .arg("run")
+        .arg("--example")
+        .arg("dup2_to_replace_stdio")
+        .status()
+        .unwrap()
+        .success());
 }