Copter: factor out an rc_throttle_failsafe_checks method

NFC, preparing to call this from elsewhere

ArduCopter/AP_Arming.cpp

@@ -63,6 +63,34 @@ bool AP_Arming_Copter::run_pre_arm_checks(bool display_failure)
         & AP_Arming::pre_arm_checks(display_failure);
 }
 
+bool AP_Arming_Copter::rc_throttle_failsafe_checks(bool display_failure) const
+{
+    // throttle failsafe.  In this case the parameter also gates the
+    // no-pulses RC failure case - the radio-in value can be zero due
+    // to not having received any RC pulses at all.  Note that if we
+    // have ever seen RC and then we *lose* RC then these checks are
+    // likely to pass if the user is relying on no-pulses to detect RC
+    // failure.  However, arming is precluded in that case by being in
+    // RC failsafe.
+    if (copter.g.failsafe_throttle == FS_THR_DISABLED) {
+        return true;
+    }
+
+#if FRAME_CONFIG == HELI_FRAME
+    const char *rc_item = "Collective";
+#else
+    const char *rc_item = "Throttle";
+#endif
+
+    // check throttle is not too low - must be above failsafe throttle
+    if (copter.channel_throttle->get_radio_in() < copter.g.failsafe_throttle_value) {
+        check_failed(ARMING_CHECK_RC, true, "%s below failsafe", rc_item);
+        return false;
+    }
+
+    return true;
+}
+
 bool AP_Arming_Copter::barometer_checks(bool display_failure)
 {
     if (!AP_Arming::barometer_checks(display_failure)) {
@@ -597,17 +625,15 @@ bool AP_Arming_Copter::arm_checks(AP_Arming::Method method)
 
     // check throttle
     if ((checks_to_perform == ARMING_CHECK_ALL) || (checks_to_perform & ARMING_CHECK_RC)) {
+        if (!rc_throttle_failsafe_checks(true)) {
+            return false;
+        }
+
          #if FRAME_CONFIG == HELI_FRAME
         const char *rc_item = "Collective";
         #else
         const char *rc_item = "Throttle";
         #endif
-        // check throttle is not too low - must be above failsafe throttle
-        if (copter.g.failsafe_throttle != FS_THR_DISABLED && copter.channel_throttle->get_radio_in() < copter.g.failsafe_throttle_value) {
-            check_failed(ARMING_CHECK_RC, true, "%s below failsafe", rc_item);
-            return false;
-        }
-
         // check throttle is not too high - skips checks if arming from GCS in Guided
         if (!(method == AP_Arming::Method::MAVLINK && (copter.flightmode->mode_number() == Mode::Number::GUIDED || copter.flightmode->mode_number() == Mode::Number::GUIDED_NOGPS))) {
             // above top of deadband is too always high

ArduCopter/AP_Arming.h

@@ -48,6 +48,7 @@ protected:
     bool gcs_failsafe_check(bool display_failure);
     bool winch_checks(bool display_failure) const;
     bool alt_checks(bool display_failure);
+    bool rc_throttle_failsafe_checks(bool display_failure) const;
 
     void set_pre_arm_check(bool b);