AP_Baro: added semaphore for safe multi-thread use of APIs

libraries/AP_Baro/AP_Baro.cpp

@@ -737,6 +737,8 @@ bool AP_Baro::should_log() const
  */
 void AP_Baro::update(void)
 {
+    WITH_SEMAPHORE(_rsem);
+
     if (fabsf(_alt_offset - _alt_offset_active) > 0.01f) {
         // If there's more than 1cm difference then slowly slew to it via LPF.
         // The EKF does not like step inputs so this keeps it happy.

libraries/AP_Baro/AP_Baro.h

@@ -181,6 +181,11 @@ public:
     void set_log_baro_bit(uint32_t bit) { _log_baro_bit = bit; }
     bool should_log() const;
 
+    // allow threads to lock against baro update
+    HAL_Semaphore &get_semaphore(void) {
+        return _rsem;
+    }
+    
 private:
     // singleton
     static AP_Baro *_singleton;
@@ -246,6 +251,9 @@ private:
     void _probe_i2c_barometers(void);
     AP_Int8                            _filter_range;  // valid value range from mean value
     AP_Int32                           _baro_probe_ext;
+
+    // semaphore for API access from threads
+    HAL_Semaphore_Recursive            _rsem;
 };
 
 namespace AP {