auto merge of #4958 : cpeterso/rust/reseed-rng, r=graydon

For Issue #4709:

**c531506 rt: rand.rs expects `rust_next()` to return `uint32_t`, not `size_t`**

rand.rs expects `rustrt::rand_next()` to return `u32`, but the `rand_next()` C function returns `size_t`: https://github.com/mozilla/rust/blob/ca71c6ec5bb8106121cbe33eec5a6a9ca7786039/src/libcore/rand.rs#L34

**f4320b6 move isaac RNG utility functions to new rust_rng.cpp file**
**665e900 encapsulate isaac RNG in `rust_rng` struct**

Move isaac's `randctx` into a `rust_rng` struct to make names similar to `rand::Rng` function names and prepare for auto-reseeding in the next commit.

**9a78dc9 reseed `rust_rng` after generating 32KB**

Precedents from other languages:
* Haskell's `GenAutoReseed` generator reseeds itself after generating 32KB: http://hackage.haskell.org/packages/archive/DRBG/0.1.2/doc/html/Crypto-Random-DRBG.html#t:GenAutoReseed

* Go's RNG reseeds itself after generating 1MB: https://code.google.com/p/go/source/browse/src/pkg/crypto/rand/rand_unix.go?name=go1.0.3#94

**9a76d71 don't deplete RNG entropy when there is only one runnable task**

`rust_sched_loop::schedule_task()` unnecessarily calls `isaac_rand()` for the common case when there is only 1 runnable task, thus depleting RNG entropy and incurring unnecessary overhead.

1 files changed, 5 insertions, 5 deletions

diff --git a/src/rt/rust_sched_loop.cpp b/src/rt/rust_sched_loop.cpp
index 0d0eaaee962..7143bf88d46 100644
--- a/src/rt/rust_sched_loop.cpp
+++ b/src/rt/rust_sched_loop.cpp
@@ -41,7 +41,7 @@ rust_sched_loop::rust_sched_loop(rust_scheduler *sched, int id, bool killed) :
     name("main")
 {
     LOGPTR(this, "new dom", (uintptr_t)this);
-    isaac_init(kernel, &rctx, NULL);
+    rng_init(kernel, &rng, NULL);
 
     if (!tls_initialized)
         init_tls();
@@ -150,10 +150,10 @@ rust_sched_loop::release_task(rust_task *task) {
 rust_task *
 rust_sched_loop::schedule_task() {
     lock.must_have_lock();
-    if (running_tasks.length() > 0) {
-        size_t k = isaac_rand(&rctx);
-        size_t i = k % running_tasks.length();
-        return (rust_task *)running_tasks[i];
+    size_t tasks = running_tasks.length();
+    if (tasks > 0) {
+        size_t i = (tasks > 1) ? (rng_gen_u32(kernel, &rng) % tasks) : 0;
+        return running_tasks[i];
     }
     return NULL;
 }