From b30734a105def55f82520f1ecea6ccd479398ddc Mon Sep 17 00:00:00 2001
From: whitequark <whitequark@whitequark.org>
Date: Tue, 1 Nov 2016 13:22:22 +0000
Subject: [PATCH] runtime: fix a race condition with async RPCs.

session.rs has code like:

    while !rpc_queue::empty() {
        try!(process_kern_queued_rpc(stream, &mut session))
    }

    // A

    if mailbox::receive() != 0 {
        try!(process_kern_message(waiter, Some(stream), &mut session));
    }

If both an async and a mailbox RPC (async or large sync) are posted
at point A then they will be processed out of order.
This commit fixes the issue by flushing the async RPC queue before
posting any RPC to the mailbox.
---
 artiq/runtime.rs/libksupport/lib.rs | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/artiq/runtime.rs/libksupport/lib.rs b/artiq/runtime.rs/libksupport/lib.rs
index dc1c853e4..7e4d66b59 100644
--- a/artiq/runtime.rs/libksupport/lib.rs
+++ b/artiq/runtime.rs/libksupport/lib.rs
@@ -107,6 +107,7 @@ extern fn send_rpc(service: u32, tag: *const u8, data: *const *const ()) {
     extern { fn strlen(s: *const c_char) -> size_t; }
     let tag = unsafe { slice::from_raw_parts(tag, strlen(tag as *const c_char)) };
 
+    while !rpc_queue::empty() {}
     send(&RpcSend {
         async:   false,
         service: service,
@@ -130,6 +131,7 @@ extern fn send_async_rpc(service: u32, tag: *const u8, data: *const *const ()) {
     }).unwrap_or_else(|err| {
         assert!(err.kind() == std::io::ErrorKind::WriteZero);
 
+        while !rpc_queue::empty() {}
         send(&RpcSend {
             async:   true,
             service: service,