From 9d6a5c6f0428387a41ad25708973bb9eb0777310 Mon Sep 17 00:00:00 2001
From: Andrew Tridgell <andrew@tridgell.net>
Date: Mon, 24 Oct 2016 13:35:39 +1100
Subject: [PATCH] HAL_PX4: make ready_to_arm state latching

this fixes a race condition in the handling of the px4io safety switch
---
 libraries/AP_HAL_PX4/RCOutput.cpp | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/libraries/AP_HAL_PX4/RCOutput.cpp b/libraries/AP_HAL_PX4/RCOutput.cpp
index 4ada16dcb4..583932572e 100644
--- a/libraries/AP_HAL_PX4/RCOutput.cpp
+++ b/libraries/AP_HAL_PX4/RCOutput.cpp
@@ -385,7 +385,11 @@ void PX4RCOutput::_arm_actuators(bool arm)
 
 	_armed.timestamp = hrt_absolute_time();
     _armed.armed = arm;
-    _armed.ready_to_arm = arm;
+    if (arm) {
+        // this latches ready_to_arm to true once set once. Otherwise
+        // we have a race condition with px4io safety switch update
+        _armed.ready_to_arm = true;
+    }
     _armed.lockdown = false;
     _armed.force_failsafe = false;