Merge #677

677: PtyMaster::drop should panic on EBADF r=asomers

Fixes #659

2 files changed, 45 insertions, 4 deletions

diff --git a/src/pty.rs b/src/pty.rs
index 88c9cc14..fd8ad98e 100644
--- a/src/pty.rs
+++ b/src/pty.rs
@@ -45,10 +45,17 @@ impl IntoRawFd for PtyMaster {
 
 impl Drop for PtyMaster {
     fn drop(&mut self) {
-        // Errors when closing are ignored because we don't actually know if the file descriptor
-        // was closed. If we retried, it's possible that descriptor was reallocated in the mean
-        // time and the wrong file descriptor could be closed.
-        let _ = ::unistd::close(self.0);
+        // On drop, we ignore errors like EINTR and EIO because there's no clear
+        // way to handle them, we can't return anything, and (on FreeBSD at
+        // least) the file descriptor is deallocated in these cases.  However,
+        // we must panic on EBADF, because it is always an error to close an
+        // invalid file descriptor.  That frequently indicates a double-close
+        // condition, which can cause confusing errors for future I/O
+        // operations.
+        let e = ::unistd::close(self.0);
+        if e == Err(Error::Sys(Errno::EBADF)) {
+            panic!("Closing an invalid file descriptor!");
+        };
     }
 }
 
diff --git a/src/unistd.rs b/src/unistd.rs
index 3209bb98..196232b6 100644
--- a/src/unistd.rs
+++ b/src/unistd.rs
@@ -670,6 +670,40 @@ pub fn gethostname<'a>(buffer: &'a mut [u8]) -> Result<&'a CStr> {
     })
 }
 
+/// Close a raw file descriptor
+///
+/// Be aware that many Rust types implicitly close-on-drop, including
+/// `std::fs::File`.  Explicitly closing them with this method too can result in
+/// a double-close condition, which can cause confusing `EBADF` errors in
+/// seemingly unrelated code.  Caveat programmer.
+///
+/// # Examples
+///
+/// ```no_run
+/// extern crate tempfile;
+/// extern crate nix;
+///
+/// use std::os::unix::io::AsRawFd;
+/// use nix::unistd::close;
+///
+/// fn main() {
+///     let mut f = tempfile::tempfile().unwrap();
+///     close(f.as_raw_fd()).unwrap();   // Bad!  f will also close on drop!
+/// }
+/// ```
+///
+/// ```rust
+/// extern crate tempfile;
+/// extern crate nix;
+///
+/// use std::os::unix::io::IntoRawFd;
+/// use nix::unistd::close;
+///
+/// fn main() {
+///     let mut f = tempfile::tempfile().unwrap();
+///     close(f.into_raw_fd()).unwrap(); // Good.  into_raw_fd consumes f
+/// }
+/// ```
 pub fn close(fd: RawFd) -> Result<()> {
     let res = unsafe { libc::close(fd) };
     Errno::result(res).map(drop)