PlanManager: use shorter timeout to download items

When downloading mission items from the autopilot we need to retry more
agressively, so use a shorter timeout. If we don't do this, we might not
re-request a mission item in time and the autopilot has already given up
the overall mission transfer.

This fixes the case where a mission request from QGC to autopilot or a
mission item from autopilot to QGC gets lost on the link and needs to be
requested again.

src/MissionManager/PlanManager.cc

@@ -36,7 +36,6 @@ PlanManager::PlanManager(Vehicle* vehicle, MAV_MISSION_TYPE planType)
 {
     _ackTimeoutTimer = new QTimer(this);
     _ackTimeoutTimer->setSingleShot(true);
-    _ackTimeoutTimer->setInterval(_ackTimeoutMilliseconds);
 
     connect(_ackTimeoutTimer, &QTimer::timeout, this, &PlanManager::_ackTimeout);
 }
@@ -245,6 +244,24 @@ void PlanManager::_ackTimeout(void)
 
 void PlanManager::_startAckTimeout(AckType_t ack)
 {
+    switch (ack) {
+    case AckMissionItem:
+        // We are actively trying to get the mission item, so we don't want to wait as long.
+        _ackTimeoutTimer->setInterval(_retryTimeoutMilliseconds);
+        break;
+    case AckNone:
+        // FALLTHROUGH
+    case AckMissionCount:
+        // FALLTHROUGH
+    case AckMissionRequest:
+        // FALLTHROUGH
+    case AckMissionClearAll:
+        // FALLTHROUGH
+    case AckGuidedItem:
+        _ackTimeoutTimer->setInterval(_ackTimeoutMilliseconds);
+        break;
+    }
+
     _expectedAck = ack;
     _ackTimeoutTimer->start();
 }

src/MissionManager/PlanManager.h

@@ -70,7 +70,10 @@ public:
     } ErrorCode_t;
 
     // These values are public so the unit test can set appropriate signal wait times
+    // When passively waiting for a mission process, use a longer timeout.
     static const int _ackTimeoutMilliseconds = 1000;
+    // When actively retrying to request mission items, use a shorter timeout instead.
+    static const int _retryTimeoutMilliseconds = 250;
     static const int _maxRetryCount = 5;
 
 signals: