Implement Debug trait for PollFd

This is useful when using printf-style debugging to observe the variables of
the program.

Also includes a test.

Fixes #885.

Author: kccqzy

src/poll.rs

@@ -4,6 +4,7 @@ use sys::time::TimeSpec;
 #[cfg(any(target_os = "android", target_os = "dragonfly", target_os = "freebsd", target_os = "linux"))]
 use sys::signal::SigSet;
 use std::os::unix::io::RawFd;
+use std::fmt;
 
 use libc;
 use Result;
@@ -19,7 +20,6 @@ use errno::Errno;
 /// retrieved by calling [`revents()`](#method.revents) on the `PollFd`.
 #[repr(C)]
 #[derive(Clone, Copy)]
-#[allow(missing_debug_implementations)]
 pub struct PollFd {
     pollfd: libc::pollfd,
 }
@@ -43,6 +43,23 @@ impl PollFd {
     }
 }
 
+impl fmt::Debug for PollFd {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        let pfd = self.pollfd;
+        let mut ds = f.debug_struct("PollFd");
+        ds.field("fd", &pfd.fd);
+        match EventFlags::from_bits(pfd.events) {
+            None => ds.field("events", &pfd.events),
+            Some(ef) => ds.field("events", &ef),
+        };
+        match EventFlags::from_bits(pfd.revents) {
+            None => ds.field("revents", &pfd.revents),
+            Some(ef) => ds.field("revents", &ef),
+        };
+        ds.finish()
+    }
+}
+
 libc_bitflags! {
     /// These flags define the different events that can be monitored by `poll` and `ppoll`
     pub struct EventFlags: libc::c_short {

test/test_poll.rs

@@ -1,7 +1,7 @@
 use nix::poll::{EventFlags, poll, PollFd};
 use nix::sys::signal::SigSet;
 use nix::sys::time::{TimeSpec, TimeValLike};
-use nix::unistd::{write, pipe};
+use nix::unistd::{write, pipe, close};
 
 #[test]
 fn test_poll() {
@@ -21,6 +21,24 @@ fn test_poll() {
     assert!(fds[0].revents().unwrap().contains(EventFlags::POLLIN));
 }
 
+#[test]
+fn test_poll_debug() {
+    assert_eq!(format!("{:?}", PollFd::new(0, EventFlags::empty())),
+               "PollFd { fd: 0, events: (empty), revents: (empty) }");
+    assert_eq!(format!("{:?}", PollFd::new(1, EventFlags::POLLIN)),
+               "PollFd { fd: 1, events: POLLIN, revents: (empty) }");
+
+    // Testing revents requires doing some I/O
+    let (r, w) = pipe().unwrap();
+    let mut fds = [PollFd::new(r, EventFlags::POLLIN)];
+    write(w, b" ").unwrap();
+    close(w).unwrap();
+    poll(&mut fds, -1).unwrap();
+    assert_eq!(format!("{:?}", fds[0]),
+               format!("PollFd {{ fd: {}, events: POLLIN, revents: POLLIN | POLLHUP }}", r));
+    close(r).unwrap();
+}
+
 // ppoll(2) is the same as poll except for how it handles timeouts and signals.
 // Repeating the test for poll(2) should be sufficient to check that our
 // bindings are correct.