Recover idle process. Make shell wait for the user it spawned.

kernel/src/fs.rs

@@ -29,9 +29,10 @@ pub fn shell() {
             continue;
         }
         if let Ok(file) = root.borrow().lookup(name.as_str()) {
-            let len = file.borrow().read_at(0, &mut *buf).unwrap();
             use process::*;
-            processor().add(Context::new_user(&buf[..len]));
+            let len = file.borrow().read_at(0, &mut *buf).unwrap();
+            let pid = processor().add(Context::new_user(&buf[..len]));
+            processor().current_wait_for(pid);
         } else {
             println!("Program not exist");
         }

kernel/src/process/mod.rs

@@ -10,11 +10,18 @@ type Processor = Processor_<Context, StrideScheduler>;
 
 pub fn init() {
     PROCESSOR.call_once(||
-        SpinNoIrqLock::new(Processor::new(
+        SpinNoIrqLock::new({
+            let mut processor = Processor::new(
                 unsafe { Context::new_init() },
                 // NOTE: max_time_slice <= 5 to ensure 'priority' test pass
                 StrideScheduler::new(5),
-        ))
+            );
+            extern fn idle(arg: usize) -> ! {
+                loop {}
+            }
+            processor.add(Context::new_kernel(idle, 0));
+            processor
+        })
     );
 }