Unix: Close the fd on ChannelClosed

For unix OSes make sure to close the file descriptor on ChannelClosed.
The file descriptors should be closed in select to avoid leaking fds
after removal from pollfds (servo#96). After e06edbc this is safe and
avoids the previously seen race condition due to file descriptor
reuse.

src/platform/unix/mod.rs

@@ -14,7 +14,7 @@ use libc::{SO_LINGER, S_IFMT, S_IFSOCK, c_char, c_int, c_void, getsockopt};
 use libc::{iovec, mkstemp, mode_t, msghdr, nfds_t, off_t, poll, pollfd, recvmsg, sendmsg};
 use libc::{setsockopt, size_t, sockaddr, sockaddr_un, socketpair, socklen_t, sa_family_t};
 use std::cmp;
-use std::collections::{HashMap, HashSet};
+use std::collections::HashMap;
 use std::ffi::{CStr, CString};
 use std::fmt::{self, Debug, Formatter};
 use std::hash::BuildHasherDefault;
@@ -453,7 +453,6 @@ impl OsIpcReceiverSet {
             return Err(UnixError::last())
         }
 
-        let mut hangups = HashSet::new();
         for pollfd in self.pollfds.iter_mut() {
             if (pollfd.revents & POLLIN) != 0 {
                 match recv(pollfd.fd, BlockingMode::Blocking) {
@@ -465,19 +464,23 @@ impl OsIpcReceiverSet {
                                 shared_memory_regions));
                     }
                     Err(err) if err.channel_is_closed() => {
-                        hangups.insert(pollfd.fd);
-                        selection_results.push(OsIpcSelectionResult::ChannelClosed(
-                                    *self.fdids.get(&pollfd.fd).unwrap()))
+                        let id = self.fdids.remove(&pollfd.fd).unwrap();
+                        unsafe {
+                            libc::close(pollfd.fd);
+                        }
+                        selection_results.push(OsIpcSelectionResult::ChannelClosed(id))
                     }
                     Err(err) => return Err(err),
                 }
                 pollfd.revents = pollfd.revents & !POLLIN
             }
         }
 
-        if !hangups.is_empty() {
-            self.pollfds.retain(|pollfd| !hangups.contains(&pollfd.fd));
-        }
+        // File descriptors not in fdids are closed channels, and the descriptor
+        // has been closed. This must be done after we have finished iterating over
+        // the pollfds vec.
+        let fdids = &self.fdids;
+        self.pollfds.retain(|pollfd| fdids.contains_key(&pollfd.fd));
 
         Ok(selection_results)
     }