AC_PrecLand: rename BUFFER to LAG and add alloc failure check

avoids issue where LAG parameter is set too large which would lead to trying to access unallocated memory
inertial buffer is only allocated at startup to avoid memory leak

libraries/AC_PrecLand/AC_PrecLand.cpp

@@ -90,14 +90,15 @@ const AP_Param::GroupInfo AC_PrecLand::var_info[] = {
     // @User: Advanced
     AP_GROUPINFO("BUS",    8, AC_PrecLand, _bus, -1),
 
-    // @Param: BUFFER
-    // @DisplayName: Inertial History Buffer
-    // @Description: Length of inertial history buffer in ms, to cope with variable landing_target latency
+    // @Param: LAG
+    // @DisplayName: Precision Landing sensor lag
+    // @Description: Precision Landing sensor lag in ms, to cope with variable landing_target latency
     // @Range: 1 250
     // @Increment: 1
-    // @User: Advanced
     // @Units: ms
-    AP_GROUPINFO("BUFFER", 9, AC_PrecLand, _inertial_buffer_length, 20), // 20ms is the old default buffer size (8 frames @ 400hz/2.5ms)
+    // @User: Advanced
+    // @RebootRequired: True
+    AP_GROUPINFO("LAG", 9, AC_PrecLand, _lag_ms, 20), // 20ms is the old default buffer size (8 frames @ 400hz/2.5ms)
 
     AP_GROUPEND
 };
@@ -125,7 +126,7 @@ void AC_PrecLand::init()
     _backend_state.healthy = false;
 
     // Create inertial history buffer
-    inertial_buffer();
+    check_inertial_buffer();
     /* NOTE: If this check below returns without creating buffer and backend then SITL crashes */
     if (_inertial_history == nullptr) {
         return; // allocation failed
@@ -161,29 +162,31 @@ void AC_PrecLand::init()
     }
 }
 
-// inertial_buffer - create/update inertial buffer
-void AC_PrecLand::inertial_buffer()
+// create inertial buffer
+void AC_PrecLand::check_inertial_buffer()
 {
-    // If PLND_LATENCY parameter is not within bounds, return without doing anything
-    if ( _inertial_buffer_length <= 0 || _inertial_buffer_length > 250 ) {
+    // if lag parameter is not within bounds, return without doing anything
+    if (_lag_ms <= 0 || _lag_ms > 250) {
         return;
     }
 
     // calculate inertial buffer size from delay length (loop runs at 400hz)
-    _inertial_buffer_size = (uint16_t)roundf(_inertial_buffer_length / (1000/400.0f));
+    uint16_t inertial_buffer_size = (uint16_t)roundf(_lag_ms / (1000/400.0f));
 
-    // Create buffer if it doesn't exist or if latency parameter has changed
-    if (_inertial_history == nullptr || _inertial_buffer_size != _inertial_history->size()) {
+    // create buffer if it doesn't exist or if latency parameter has changed
+    if (_inertial_history == nullptr || inertial_buffer_size != _inertial_history->size()) {
         // instantiate ring buffer to hold inertial history
-        _inertial_history = new ObjectArray<inertial_data_frame_s>(_inertial_buffer_size);
+        _inertial_history = new ObjectArray<inertial_data_frame_s>(inertial_buffer_size);
     }
 }
 
 // update - give chance to driver to get updates from sensor
 void AC_PrecLand::update(float rangefinder_alt_cm, bool rangefinder_alt_valid)
 {
-    // Recreate inertial buffer if latency parameter has changed
-    inertial_buffer();
+    // exit immediately if not enabled
+    if (_backend == nullptr || _inertial_history == nullptr) {
+        return;
+    }
 
     // append current velocity and attitude correction into history buffer
     struct inertial_data_frame_s inertial_data_newest;

libraries/AC_PrecLand/AC_PrecLand.h

@@ -60,7 +60,7 @@ public:
     uint32_t last_update_ms() const { return _last_update_ms; }
 
     // create or update the inertial buffer
-    void inertial_buffer();
+    void check_inertial_buffer();
 
     // returns time of last time target was seen
     uint32_t last_backend_los_meas_ms() const { return _last_backend_los_meas_ms; }
@@ -122,14 +122,13 @@ private:
     AP_Int8                     _type;              // precision landing sensor type
     AP_Int8                     _bus;               // which sensor bus
     AP_Int8                     _estimator_type;    // precision landing estimator type
-    AP_Int16                    _inertial_buffer_length; // inertial buffer length in ms
+    AP_Int16                    _lag_ms;            // sensor lag in ms
     AP_Float                    _yaw_align;         // Yaw angle from body x-axis to sensor x-axis.
     AP_Float                    _land_ofs_cm_x;     // Desired landing position of the camera forward of the target in vehicle body frame
     AP_Float                    _land_ofs_cm_y;     // Desired landing position of the camera right of the target in vehicle body frame
     AP_Float                    _accel_noise;       // accelometer process noise
     AP_Vector3f                 _cam_offset;        // Position of the camera relative to the CG
 
-    uint16_t                    _inertial_buffer_size;   // inertial buffer queue length, calculated from _inertial_buffer_length
     uint32_t                    _last_update_ms;    // system time in millisecond when update was last called
     bool                        _target_acquired;   // true if target has been seen recently
     uint32_t                    _last_backend_los_meas_ms;  // system time target was last seen