Auto merge of #427 - oconnor663:pipe2, r=fiveop

call pipe2 directly on Linux

A first shot at fixing #414. This approach keeps the old implementation for platforms other than `notbsd`, because `libc` only exposes `pipe2` in the `notbsd` module.

I've tested this by hand on my Linux machine in a couple ways:

- Create a toy program that opens a pipe and passes it to `cat`, which hags if `O_CLOEXEC` doesn't get set properly. Confirm that it doesn't hang, but that it does if I pass `0` as the flags to `libc::pipe2`.
- Delete the new implementation entirely, and delete the `cfg` guards on the old implementation, and confirm that above is still true.

I haven't actually tested a cross compilation build though. Is there a standard way to do that?

CHANGELOG.md

@@ -5,6 +5,11 @@ This project adheres to [Semantic Versioning](http://semver.org/).
 
 ## [Unreleased]
 
+### Changed
+- `pipe2` now calls `libc::pipe2` where available. Previously it was emulated
+  using `pipe`, which meant that setting `O_CLOEXEC` was not atomic.
+  ([#427](https://github.com/nix-rust/nix/pull/427))
+
 ## [0.7.0] 2016-09-09
 
 ### Added

src/unistd.rs

@@ -1,8 +1,8 @@
 //! Standard symbolic constants and types
 //!
 use {Errno, Error, Result, NixPath};
-use fcntl::{fcntl, OFlag, O_NONBLOCK, O_CLOEXEC, FD_CLOEXEC};
-use fcntl::FcntlArg::{F_SETFD, F_SETFL};
+use fcntl::{fcntl, OFlag, O_CLOEXEC, FD_CLOEXEC};
+use fcntl::FcntlArg::F_SETFD;
 use libc::{self, c_char, c_void, c_int, c_uint, size_t, pid_t, off_t, uid_t, gid_t, mode_t};
 use std::mem;
 use std::ffi::{CString, CStr, OsString};
@@ -360,21 +360,42 @@ pub fn pipe() -> Result<(RawFd, RawFd)> {
     }
 }
 
+// libc only defines `pipe2` in `libc::notbsd`.
+#[cfg(any(target_os = "linux",
+          target_os = "android",
+          target_os = "emscripten"))]
 pub fn pipe2(flags: OFlag) -> Result<(RawFd, RawFd)> {
-    unsafe {
-        let mut fds: [c_int; 2] = mem::uninitialized();
+    let mut fds: [c_int; 2] = unsafe { mem::uninitialized() };
 
-        let res = libc::pipe(fds.as_mut_ptr());
+    let res = unsafe { libc::pipe2(fds.as_mut_ptr(), flags.bits()) };
 
-        try!(Errno::result(res));
+    try!(Errno::result(res));
+
+    Ok((fds[0], fds[1]))
+}
 
-        try!(pipe2_setflags(fds[0], fds[1], flags));
+#[cfg(not(any(target_os = "linux",
+              target_os = "android",
+              target_os = "emscripten")))]
+pub fn pipe2(flags: OFlag) -> Result<(RawFd, RawFd)> {
+    let mut fds: [c_int; 2] = unsafe { mem::uninitialized() };
 
-        Ok((fds[0], fds[1]))
-    }
+    let res = unsafe { libc::pipe(fds.as_mut_ptr()) };
+
+    try!(Errno::result(res));
+
+    try!(pipe2_setflags(fds[0], fds[1], flags));
+
+    Ok((fds[0], fds[1]))
 }
 
+#[cfg(not(any(target_os = "linux",
+              target_os = "android",
+              target_os = "emscripten")))]
 fn pipe2_setflags(fd1: RawFd, fd2: RawFd, flags: OFlag) -> Result<()> {
+    use fcntl::O_NONBLOCK;
+    use fcntl::FcntlArg::F_SETFL;
+
     let mut res = Ok(0);
 
     if flags.contains(O_CLOEXEC) {