Partial implementation of close for stdin/out/err (rust-lang#40032)

This is only the low-level code required for Unix.

src/libstd/sys/unix/fd.rs

@@ -243,6 +243,30 @@ impl FileDesc {
         }
         cvt(unsafe { libc::fcntl(fd, libc::F_DUPFD, 0) }).and_then(make_filedesc)
     }
+
+    /// Replace the open file referred to by `self` with the open file
+    /// referred to by `other`.  This does not change the file
+    /// descriptor _number_ within self; rather, it changes what open
+    /// file that number refers to.
+    ///
+    /// On success, the file formerly referred to by `self` is
+    /// transferred to a newly-allocated file descriptor and returned.
+    /// On failure, the file referred to by `self` is unchanged.
+    /// Regardless of success or failure, `other` is consumed, which
+    /// means the file descriptor formerly held by `other` will be
+    /// closed.
+    ///
+    /// The file underlying `self` is replaced atomically, but the
+    /// _overall_ operation is not atomic; concurrent threads that snoop on
+    /// the set of valid file descriptors (which they shouldn't) can observe
+    /// intermediate states.
+    pub fn replace(&mut self, other: FileDesc) -> io::Result<FileDesc> {
+        let previous = self.duplicate()?;
+        let fd = cvt(unsafe { libc::dup2(other.fd, self.fd) })?;
+
+        assert!(fd == self.fd);
+        Ok(previous)
+    }
 }
 
 impl<'a> Read for &'a FileDesc {

src/libstd/sys/unix/stdio.rs

@@ -11,11 +11,25 @@
 use io;
 use libc;
 use sys::fd::FileDesc;
+use sys::fs::{File, OpenOptions};
+use ffi::CStr;
 
 pub struct Stdin(());
 pub struct Stdout(());
 pub struct Stderr(());
 
+// FIXME: This duplicates code from process_common.rs.
+fn open_null_device (readable: bool) -> io::Result<FileDesc> {
+    let mut opts = OpenOptions::new();
+    opts.read(readable);
+    opts.write(!readable);
+    let path = unsafe {
+        CStr::from_ptr("/dev/null\0".as_ptr() as *const _)
+    };
+    let fd = File::open_c(&path, &opts)?;
+    Ok(fd.into_fd())
+}
+
 impl Stdin {
     pub fn new() -> io::Result<Stdin> { Ok(Stdin(())) }
 
@@ -25,6 +39,21 @@ impl Stdin {
         fd.into_raw();
         ret
     }
+
+    pub fn close() -> io::Result<()> {
+        // To close stdin, what we actually do is change its well-known
+        // file descriptor number to refer to a file open on the null
+        // device.  This protects against code (perhaps in third-party
+        // libraries) that assumes STDIN_FILENO is always open and always
+        // refers to the same thing as stdin.
+        let mut fd = FileDesc::new(libc::STDIN_FILENO);
+        // If this step succeeds, the "previous" file descriptor returned
+        // by `fd.replace` is dropped and thus closed.
+        fd.replace(open_null_device(true)?)?;
+        // Don't close STDIN_FILENO itself, though!
+        fd.into_raw();
+        Ok(())
+    }
 }
 
 impl Stdout {
@@ -40,6 +69,15 @@ impl Stdout {
     pub fn flush(&self) -> io::Result<()> {
         Ok(())
     }
+
+    pub fn close() -> io::Result<()> {
+        // See commentary for Stdin::close.
+
+        let mut fd = FileDesc::new(libc::STDOUT_FILENO);
+        fd.replace(open_null_device(false)?)?;
+        fd.into_raw();
+        Ok(())
+    }
 }
 
 impl Stderr {
@@ -55,6 +93,15 @@ impl Stderr {
     pub fn flush(&self) -> io::Result<()> {
         Ok(())
     }
+
+    pub fn close() -> io::Result<()> {
+        // See commentary for Stdin::close.
+
+        let mut fd = FileDesc::new(libc::STDERR_FILENO);
+        fd.replace(open_null_device(false)?)?;
+        fd.into_raw();
+        Ok(())
+    }
 }
 
 // FIXME: right now this raw stderr handle is used in a few places because