Handle failure during select nicely. Fixes #3176

Author: eholk

src/libcore/pipes.rs

@@ -530,16 +530,20 @@ fn sender_terminate<T: send>(p: *packet<T>) {
 #[doc(hidden)]
 fn receiver_terminate<T: send>(p: *packet<T>) {
     let p = unsafe { &*p };
-    assert p.header.blocked_task.is_null();
     match swap_state_rel(p.header.state, terminated) {
       empty => {
+        assert p.header.blocked_task.is_null();
         // the sender will clean up
       }
       blocked => {
-        // this shouldn't happen.
-        fail ~"terminating a blocked packet"
+        let old_task = swap_task(p.header.blocked_task, ptr::null());
+        if !old_task.is_null() {
+            rustrt::rust_task_deref(old_task);
+            assert old_task == rustrt::rust_get_task();
+        }
       }
       terminated | full => {
+        assert p.header.blocked_task.is_null();
         // I have to clean up, use drop_glue
       }
     }

src/test/run-pass/issue-3176.rs

@@ -0,0 +1,23 @@
+import pipes::{select2, selectable};
+
+fn main() {
+    let (c,p) = pipes::stream();
+    do task::try {
+        let (c2,p2) = pipes::stream();
+        do task::spawn {
+            p2.recv();
+            #error["brother fails"];
+            fail;
+        }   
+        let (c3,p3) = pipes::stream();
+        c.send(c3);
+        c2.send(());
+        #error["child blocks"];
+        let (c, p) = pipes::stream();
+        (p, p3).select();
+        c.send(());
+    };  
+    #error["parent tries"];
+    assert !p.recv().try_send(());
+    #error("all done!");
+}