More documentation on pipes, and moving assert in runtime.

src/libcore/pipes.rs

@@ -41,6 +41,21 @@ buffer that is carried along in the endpoint data structures.
 
 FIXME (#3072) - This is still incomplete
 
+
+
+## Invariants
+
+This section attempts to document the invariants that must hold to
+avoid races. These primarily deal with the state and blocked_task
+fields on packet_headers.
+
+1. If the sender reads a some(task) out of blocked_task, then the task
+that is pointed there will remain live for any events that the sender
+might signal.
+
+2. The sender may only read the blocked_task field if it first ensures
+that the packet's state field is blocked.
+
 */
 
 import unsafe::{forget, reinterpret_cast, transmute};

src/rt/rust_task.cpp

@@ -678,11 +678,11 @@ MUST_CHECK bool rust_task::wait_event(void **result) {
 
 void
 rust_task::signal_event(void *event) {
-    scoped_lock with(lifecycle_lock);
-
     assert(task_state_blocked == state ||
            task_state_running == state);
 
+    scoped_lock with(lifecycle_lock);
+
     this->event = event;
     event_reject = true;
     if(task_state_blocked == state) {