core: Add rt::context for figuring out what runtime services are available

Conflicts:
	src/libcore/rt/sched/mod.rs

Author: brson

src/libcore/rt/mod.rs

@@ -88,3 +88,75 @@ pub fn start(main: *u8, _argc: int, _argv: **c_char, _crate_map: *u8) -> int {
         fn rust_call_nullary_fn(f: *u8);
     }
 }
+
+/// Possible contexts in which Rust code may be executing.
+/// Different runtime services are available depending on context.
+#[deriving(Eq)]
+pub enum RuntimeContext {
+    // Only default services, e.g. exchange heap
+    GlobalContext,
+    // The scheduler may be accessed
+    SchedulerContext,
+    // Full task services, e.g. local heap, unwinding
+    TaskContext,
+    // Running in an old-style task
+    OldTaskContext
+}
+
+pub fn context() -> RuntimeContext {
+
+    use task::rt::rust_task;
+    use self::sched::Scheduler;
+
+    // XXX: Hitting TLS twice to check if the scheduler exists
+    // then to check for the task is not good for perf
+    if unsafe { rust_try_get_task().is_not_null() } {
+        return OldTaskContext;
+    } else {
+        if Scheduler::have_local() {
+            let context = ::cell::empty_cell();
+            do Scheduler::borrow_local |sched| {
+                if sched.in_task_context() {
+                    context.put_back(TaskContext);
+                } else {
+                    context.put_back(SchedulerContext);
+                }
+            }
+            return context.take();
+        } else {
+            return GlobalContext;
+        }
+    }
+
+    pub extern {
+        #[rust_stack]
+        fn rust_try_get_task() -> *rust_task;
+    }
+}
+
+#[test]
+fn test_context() {
+    use unstable::run_in_bare_thread;
+    use self::sched::{Scheduler, Task};
+    use self::uvio::UvEventLoop;
+    use cell::Cell;
+
+    assert!(context() == OldTaskContext);
+    do run_in_bare_thread {
+        assert!(context() == GlobalContext);
+        let mut sched = ~UvEventLoop::new_scheduler();
+        let task = ~do Task::new(&mut sched.stack_pool) {
+            assert!(context() == TaskContext);
+            let sched = Scheduler::take_local();
+            do sched.deschedule_running_task_and_then() |task| {
+                assert!(context() == SchedulerContext);
+                let task = Cell(task);
+                do Scheduler::borrow_local |sched| {
+                    sched.task_queue.push_back(task.take());
+                }
+            }
+        };
+        sched.task_queue.push_back(task);
+        sched.run();
+    }
+}

src/libcore/rt/sched/local.rs

@@ -10,6 +10,7 @@
 
 //! Access to the thread-local Scheduler
 
+use prelude::*;
 use ptr::mut_null;
 use libc::c_void;
 use cast::transmute;
@@ -39,6 +40,16 @@ pub fn take() -> ~Scheduler {
     }
 }
 
+/// Check whether there is a thread-local Scheduler attached to the running thread
+pub fn exists() -> bool {
+    unsafe {
+        match maybe_tls_key() {
+            Some(key) => tls::get(key).is_not_null(),
+            None => false
+        }
+    }
+}
+
 /// Borrow a mutable reference to the thread-local Scheduler
 /// # Safety Note
 /// Because this leaves the Scheduler in thread-local storage it is possible
@@ -60,10 +71,31 @@ pub unsafe fn borrow() -> &mut Scheduler {
 }
 
 fn tls_key() -> tls::Key {
+    maybe_tls_key().get()
+}
+
+fn maybe_tls_key() -> Option<tls::Key> {
     unsafe {
         let key: *mut c_void = rust_get_sched_tls_key();
         let key: &mut tls::Key = transmute(key);
-        return *key;
+        let key = *key;
+        // Check that the key has been initialized.
+
+        // NB: This is a little racy because, while the key is
+        // initalized under a mutex and it's assumed to be initalized
+        // in the Scheduler ctor by any thread that needs to use it,
+        // we are not accessing the key under a mutex.  Threads that
+        // are not using the new Scheduler but still *want to check*
+        // whether they are running under a new Scheduler may see a 0
+        // value here that is in the process of being initialized in
+        // another thread. I think this is fine since the only action
+        // they could take if it was initialized would be to check the
+        // thread-local value and see that it's not set.
+        if key != 0 {
+            return Some(key);
+        } else {
+            return None;
+        }
     }
 }
 

src/libcore/rt/sched/mod.rs

@@ -133,6 +133,11 @@ pub impl Scheduler {
         local::take()
     }
 
+    /// Just check whether there is a local scheduler
+    fn have_local() -> bool {
+        local::exists()
+    }
+
     // * Scheduler-context operations
 
     fn resume_task_from_queue(~self) -> bool {

src/rt/rust_builtin.cpp

@@ -539,6 +539,11 @@ rust_get_task() {
     return rust_get_current_task();
 }
 
+extern "C" rust_task *
+rust_try_get_task() {
+    return rust_try_get_current_task();
+}
+
 extern "C" CDECL stk_seg *
 rust_get_stack_segment() {
     return rust_get_current_task()->stk;

src/rt/rustrt.def.in

@@ -47,6 +47,7 @@ rust_env_pairs
 rust_task_yield
 rust_task_is_unwinding
 rust_get_task
+rust_try_get_task
 rust_get_stack_segment
 rust_log_str
 start_task