Move rust's uv implementation to its own crate

There are a few reasons that this is a desirable move to take:

1. Proof of concept that a third party event loop is possible
2. Clear separation of responsibility between rt::io and the uv-backend
3. Enforce in the future that the event loop is "pluggable" and replacable

Here's a quick summary of the points of this pull request which make this
possible:

* Two new lang items were introduced: event_loop, and event_loop_factory.
  The idea of a "factory" is to define a function which can be called with no
  arguments and will return the new event loop as a trait object. This factory
  is emitted to the crate map when building an executable. The factory doesn't
  have to exist, and when it doesn't then an empty slot is in the crate map and
  a basic event loop with no I/O support is provided to the runtime.

* When building an executable, then the rustuv crate will be linked by default
  (providing a default implementation of the event loop) via a similar method to
  injecting a dependency on libstd. This is currently the only location where
  the rustuv crate is ever linked.

* There is a new #[no_uv] attribute (implied by #[no_std]) which denies
  implicitly linking to rustuv by default

Closes #5019

1 files changed, 29 insertions, 3 deletions

diff --git a/src/libstd/rt/mod.rs b/src/libstd/rt/mod.rs
index 21fdf0e50a1..d8d07f14021 100644
--- a/src/libstd/rt/mod.rs
+++ b/src/libstd/rt/mod.rs
@@ -68,7 +68,6 @@ use rt::sched::{Scheduler, Shutdown};
 use rt::sleeper_list::SleeperList;
 use rt::task::UnwindResult;
 use rt::task::{Task, SchedTask, GreenTask, Sched};
-use rt::uv::uvio::UvEventLoop;
 use unstable::atomics::{AtomicInt, AtomicBool, SeqCst};
 use unstable::sync::UnsafeArc;
 use vec::{OwnedVector, MutableVector, ImmutableVector};
@@ -123,6 +122,7 @@ pub mod io;
 pub mod rtio;
 
 /// libuv and default rtio implementation.
+#[cfg(stage0)]
 pub mod uv;
 
 /// The Local trait for types that are accessible via thread-local
@@ -287,7 +287,7 @@ fn run_(main: ~fn(), use_main_sched: bool) -> int {
         rtdebug!("inserting a regular scheduler");
 
         // Every scheduler is driven by an I/O event loop.
-        let loop_ = ~UvEventLoop::new() as ~rtio::EventLoop;
+        let loop_ = new_event_loop();
         let mut sched = ~Scheduler::new(loop_,
                                         work_queue.clone(),
                                         work_queues.clone(),
@@ -311,7 +311,7 @@ fn run_(main: ~fn(), use_main_sched: bool) -> int {
         // set.
         let work_queue = WorkQueue::new();
 
-        let main_loop = ~UvEventLoop::new() as ~rtio::EventLoop;
+        let main_loop = new_event_loop();
         let mut main_sched = ~Scheduler::new_special(main_loop,
                                                      work_queue,
                                                      work_queues.clone(),
@@ -462,3 +462,29 @@ pub fn in_green_task_context() -> bool {
         }
     }
 }
+
+#[cfg(stage0)]
+pub fn new_event_loop() -> ~rtio::EventLoop {
+    use rt::uv::uvio::UvEventLoop;
+    ~UvEventLoop::new() as ~rtio::EventLoop
+}
+
+#[cfg(not(stage0))]
+pub fn new_event_loop() -> ~rtio::EventLoop {
+    #[fixed_stack_segment]; #[allow(cstack)];
+
+    match crate_map::get_crate_map() {
+        None => {}
+        Some(map) => {
+            match map.event_loop_factory {
+                None => {}
+                Some(factory) => return factory()
+            }
+        }
+    }
+
+    // If the crate map didn't specify a factory to create an event loop, then
+    // instead just use a basic event loop missing all I/O services to at least
+    // get the scheduler running.
+    return basic::event_loop();
+}