io: Implement process wait timeouts

This implements set_timeout() for std::io::Process which will affect wait()
operations on the process. This follows the same pattern as the rest of the
timeouts emerging in std::io::net.

The implementation was super easy for everything except libnative on unix
(backwards from usual!), which required a good bit of signal handling. There's a
doc comment explaining the strategy in libnative. Internally, this also required
refactoring the "helper thread" implementation used by libnative to allow for an
extra helper thread (not just the timer).

This is a breaking change in terms of the io::Process API. It is now possible
for wait() to fail, and subsequently wait_with_output(). These two functions now
return IoResult<T> due to the fact that they can time out.

Additionally, the wait_with_output() function has moved from taking `&mut self`
to taking `self`. If a timeout occurs while waiting with output, the semantics
are undesirable in almost all cases if attempting to re-wait on the process.
Equivalent functionality can still be achieved by dealing with the output
handles manually.

[breaking-change]

cc #13523

src/libnative/io/timer_unix.rs

@@ -52,11 +52,14 @@ use std::os;
 use std::ptr;
 use std::rt::rtio;
 use std::sync::atomics;
+use std::comm;
 
 use io::IoResult;
 use io::c;
 use io::file::FileDesc;
-use io::timer_helper;
+use io::helper_thread::Helper;
+
+helper_init!(static mut HELPER: Helper<Req>)
 
 pub struct Timer {
     id: uint,
@@ -79,9 +82,6 @@ pub enum Req {
     // Remove a timer based on its id and then send it back on the channel
     // provided
     RemoveTimer(uint, Sender<Box<Inner>>),
-
-    // Shut down the loop and then ACK this channel once it's shut down
-    Shutdown,
 }
 
 // returns the current time (in milliseconds)
@@ -93,7 +93,7 @@ pub fn now() -> u64 {
     }
 }
 
-fn helper(input: libc::c_int, messages: Receiver<Req>) {
+fn helper(input: libc::c_int, messages: Receiver<Req>, _: ()) {
     let mut set: c::fd_set = unsafe { mem::init() };
 
     let mut fd = FileDesc::new(input, true);
@@ -163,7 +163,7 @@ fn helper(input: libc::c_int, messages: Receiver<Req>) {
             1 => {
                 loop {
                     match messages.try_recv() {
-                        Ok(Shutdown) => {
+                        Err(comm::Disconnected) => {
                             assert!(active.len() == 0);
                             break 'outer;
                         }
@@ -202,7 +202,7 @@ fn helper(input: libc::c_int, messages: Receiver<Req>) {
 
 impl Timer {
     pub fn new() -> IoResult<Timer> {
-        timer_helper::boot(helper);
+        unsafe { HELPER.boot(|| {}, helper); }
 
         static mut ID: atomics::AtomicUint = atomics::INIT_ATOMIC_UINT;
         let id = unsafe { ID.fetch_add(1, atomics::Relaxed) };
@@ -235,7 +235,7 @@ impl Timer {
             Some(i) => i,
             None => {
                 let (tx, rx) = channel();
-                timer_helper::send(RemoveTimer(self.id, tx));
+                unsafe { HELPER.send(RemoveTimer(self.id, tx)); }
                 rx.recv()
             }
         }
@@ -261,7 +261,7 @@ impl rtio::RtioTimer for Timer {
         inner.interval = msecs;
         inner.target = now + msecs;
 
-        timer_helper::send(NewTimer(inner));
+        unsafe { HELPER.send(NewTimer(inner)); }
         return rx;
     }
 
@@ -275,7 +275,7 @@ impl rtio::RtioTimer for Timer {
         inner.interval = msecs;
         inner.target = now + msecs;
 
-        timer_helper::send(NewTimer(inner));
+        unsafe { HELPER.send(NewTimer(inner)); }
         return rx;
     }
 }