PtyMaster::drop should panic on EBADF

Also, document the double-close risk with unistd::close

Fixes #659

Cargo.toml

@@ -48,3 +48,7 @@ test = true
 name = "test-mount"
 path = "test/test_mount.rs"
 harness = false
+
+[[test]]
+name = "test-ptymaster-drop"
+path = "test/test_ptymaster_drop.rs"

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!");
+        };
     }
 }
 

src/unistd.rs

@@ -668,6 +668,40 @@ 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)

test/test_pty.rs

@@ -1,12 +1,28 @@
+use std::io::Write;
 use std::path::Path;
 use std::os::unix::prelude::*;
+use tempfile::tempfile;
 
 use nix::fcntl::{O_RDWR, open};
 use nix::pty::*;
 use nix::sys::stat;
 use nix::sys::termios::*;
 use nix::unistd::{write, close};
 
+/// Regression test for Issue #659
+/// This is the correct way to explicitly close a PtyMaster
+#[test]
+fn test_explicit_close() {
+    let mut f = {
+        let m = posix_openpt(O_RDWR).unwrap();
+        close(m.into_raw_fd()).unwrap();
+        tempfile().unwrap()
+    };
+    // This should work.  But if there's been a double close, then it will
+    // return EBADF
+    f.write(b"whatever").unwrap();
+}
+
 /// Test equivalence of `ptsname` and `ptsname_r`
 #[test]
 #[cfg(any(target_os = "android", target_os = "linux"))]

test/test_ptymaster_drop.rs

@@ -0,0 +1,21 @@
+extern crate nix;
+
+use nix::fcntl::O_RDWR;
+use nix::pty::*;
+use nix::unistd::close;
+use std::os::unix::io::AsRawFd;
+
+/// Regression test for Issue #659
+/// PtyMaster should panic rather than double close the file descriptor
+/// This must run in its own test process because it deliberately creates a race
+/// condition.
+#[test]
+#[should_panic(expected = "Closing an invalid file descriptor!")]
+// In Travis on i686-unknown-linux-musl, this test gets SIGABRT.  I don't know
+// why.  It doesn't happen on any other target, and it doesn't happen on my PC.
+#[cfg_attr(all(target_env = "musl", target_arch = "x86"), ignore)]
+fn test_double_close() {
+    let m = posix_openpt(O_RDWR).unwrap();
+    close(m.as_raw_fd()).unwrap();
+    drop(m);            // should panic here
+}