Rust: don't crash kernel CPU when no flash kernel is present.

artiq/runtime.rs/src/sched.rs

@@ -214,13 +214,6 @@ unsafe impl Send for WaitEvent {}
 pub struct Waiter<'a>(&'a Yielder<WaitResult, WaitRequest, OwnedStack>);
 
 impl<'a> Waiter<'a> {
-    pub fn relinquish(&self) {
-        self.0.suspend(WaitRequest {
-            timeout: None,
-            event:   None
-        });
-    }
-
     pub fn sleep(&self, duration: Duration) -> Result<()> {
         let request = WaitRequest {
             timeout: Some(Instant::now() + duration),
@@ -242,6 +235,13 @@ impl<'a> Waiter<'a> {
         }
     }
 
+    pub fn relinquish(&self) -> Result<()> {
+        self.suspend(WaitRequest {
+            timeout: None,
+            event:   None
+        })
+    }
+
     pub fn join(&self, thread: ThreadHandle) -> Result<()> {
         self.suspend(WaitRequest {
             timeout: None,

artiq/runtime.rs/src/session.rs

@@ -320,7 +320,7 @@ fn host_kernel_worker(waiter: Waiter,
             }
         }
 
-        waiter.relinquish()
+        try!(waiter.relinquish())
     }
 }
 
@@ -330,6 +330,13 @@ fn flash_kernel_worker(waiter: Waiter,
     let mut session = Session::new(congress);
 
     let kernel = config::read_to_end(config_key);
+    if kernel.len() == 0 {
+        info!("no kernel present in config key {}", config_key);
+        loop {
+            try!(waiter.relinquish())
+        }
+    }
+
     try!(unsafe { kern_load(waiter, &mut session, &kernel) });
     try!(kern_run(&mut session));
 
@@ -410,6 +417,6 @@ pub fn handler(waiter: Waiter, spawner: Spawner) {
             })
         }
 
-        waiter.relinquish()
+        let _ = waiter.relinquish();
     }
 }