From a57d3e0c152a293885f65cdf44d4b98a5e2f71cc Mon Sep 17 00:00:00 2001
From: Ben Blum <bblum@andrew.cmu.edu>
Date: Wed, 15 Aug 2012 14:11:39 -0400
Subject: [PATCH] Fix :broadcast_heavy in condvars.

---
 src/libstd/sync.rs | 48 ++++++++++++++++++++++++++--------------------
 1 file changed, 27 insertions(+), 21 deletions(-)

diff --git a/src/libstd/sync.rs b/src/libstd/sync.rs
index a1ac6bb8945..bf44e3f25ea 100644
--- a/src/libstd/sync.rs
+++ b/src/libstd/sync.rs
@@ -28,6 +28,11 @@ type signal_end = pipes::chan_one<()>;
 struct waitqueue { head: pipes::port<signal_end>;
                    tail: pipes::chan<signal_end>; }
 
+fn new_waitqueue() -> waitqueue {
+    let (block_tail, block_head) = pipes::stream();
+    waitqueue { head: block_head, tail: block_tail }
+}
+
 // Signals one live task from the queue.
 #[doc(hidden)]
 fn signal_waitqueue(q: &waitqueue) -> bool {
@@ -70,18 +75,15 @@ enum sem<Q: send> = Exclusive<sem_inner<Q>>;
 
 #[doc(hidden)]
 fn new_sem<Q: send>(count: int, +q: Q) -> sem<Q> {
-    let (wait_tail, wait_head)  = pipes::stream();
     sem(exclusive(sem_inner {
-        mut count: count,
-        waiters: waitqueue { head: wait_head, tail: wait_tail },
-        blocked: q }))
+        mut count: count, waiters: new_waitqueue(), blocked: q }))
 }
 #[doc(hidden)]
-fn new_sem_and_signal(count: int, num_condvars: uint) -> sem<~[waitqueue]> {
-    let mut queues = ~[];
+fn new_sem_and_signal(count: int, num_condvars: uint)
+        -> sem<~[mut waitqueue]> {
+    let mut queues = ~[mut];
     for num_condvars.times {
-        let (block_tail, block_head) = pipes::stream();
-        vec::push(queues, waitqueue { head: block_head, tail: block_tail });
+        vec::push(queues, new_waitqueue());
     }
     new_sem(count, queues)
 }
@@ -136,7 +138,7 @@ impl &sem<()> {
     }
 }
 #[doc(hidden)]
-impl &sem<~[waitqueue]> {
+impl &sem<~[mut waitqueue]> {
     fn access<U>(blk: fn() -> U) -> U {
         let mut release = none;
         unsafe {
@@ -158,13 +160,13 @@ struct sem_release {
 }
 #[doc(hidden)]
 struct sem_and_signal_release {
-    sem: &sem<~[waitqueue]>;
-    new(sem: &sem<~[waitqueue]>) { self.sem = sem; }
+    sem: &sem<~[mut waitqueue]>;
+    new(sem: &sem<~[mut waitqueue]>) { self.sem = sem; }
     drop { self.sem.release(); }
 }
 
 /// A mechanism for atomic-unlock-and-deschedule blocking and signalling.
-struct condvar { priv sem: &sem<~[waitqueue]>; drop { } }
+struct condvar { priv sem: &sem<~[mut waitqueue]>; drop { } }
 
 impl &condvar {
     /**
@@ -232,8 +234,8 @@ impl &condvar {
         // mutex during unwinding. As long as the wrapper (mutex, etc) is
         // bounded in when it gets released, this shouldn't hang forever.
         struct sem_and_signal_reacquire {
-            sem: &sem<~[waitqueue]>;
-            new(sem: &sem<~[waitqueue]>) { self.sem = sem; }
+            sem: &sem<~[mut waitqueue]>;
+            new(sem: &sem<~[mut waitqueue]>) { self.sem = sem; }
             drop unsafe {
                 // Needs to succeed, instead of itself dying.
                 do task::unkillable {
@@ -268,19 +270,23 @@ impl &condvar {
     /// As broadcast, but with a specified condvar_id. See wait_on.
     fn broadcast_on(condvar_id: uint) -> uint {
         let mut out_of_bounds = none;
-        let mut result = 0;
+        let mut queue = none;
         unsafe {
             do (**self.sem).with |state| {
                 if condvar_id < vec::len(state.blocked) {
-                    // FIXME(#3145) fix :broadcast_heavy
-                    result = broadcast_waitqueue(&state.blocked[condvar_id])
+                    // To avoid :broadcast_heavy, we make a new waitqueue,
+                    // swap it out with the old one, and broadcast on the
+                    // old one outside of the little-lock.
+                    queue = some(util::replace(&mut state.blocked[condvar_id],
+                                               new_waitqueue()));
                 } else {
                     out_of_bounds = some(vec::len(state.blocked));
                 }
             }
         }
         do check_cvar_bounds(out_of_bounds, condvar_id, "cond.signal_on()") {
-            result
+            let queue = option::swap_unwrap(&mut queue);
+            broadcast_waitqueue(&queue)
         }
     }
 }
@@ -303,7 +309,7 @@ fn check_cvar_bounds<U>(out_of_bounds: option<uint>, id: uint, act: &str,
 }
 
 #[doc(hidden)]
-impl &sem<~[waitqueue]> {
+impl &sem<~[mut waitqueue]> {
     // The only other place that condvars get built is rwlock_write_mode.
     fn access_cond<U>(blk: fn(c: &condvar) -> U) -> U {
         do self.access { blk(&condvar { sem: self }) }
@@ -354,7 +360,7 @@ impl &semaphore {
  * A task which fails while holding a mutex will unlock the mutex as it
  * unwinds.
  */
-struct mutex { priv sem: sem<~[waitqueue]>; }
+struct mutex { priv sem: sem<~[mut waitqueue]>; }
 
 /// Create a new mutex, with one associated condvar.
 fn mutex() -> mutex { mutex_with_condvars(1) }
@@ -402,7 +408,7 @@ struct rwlock_inner {
  */
 struct rwlock {
     /* priv */ order_lock:  semaphore;
-    /* priv */ access_lock: sem<~[waitqueue]>;
+    /* priv */ access_lock: sem<~[mut waitqueue]>;
     /* priv */ state:       Exclusive<rwlock_inner>;
 }