uhd: Fix radio_control-related method constness

The const-ness of some radio_control differed between base class and
implementation. This fixes the consistency, but also makes sure these
methods follow the rules for when to make methods 'const'.

The following rules apply:
- Methods that query static capabilities are const. Here, we made
  get_tx_lo_sources() const (the RX version was already const).
- Getters that may have to interact with the device (e.g., peek
  a register) are not const, because the act of peeking is usually also
  non-const. Here, we changed get_rx_lo_export_enabled() to non-const.
- All base classes are fixed such that the derived classes and the base
  classes have the same const-ness. Clang was warning about differences.
  This can cause very tricky bugs, where the radio_control_impl version
  can get called instead of the intended child class.

2 files changed, 8 insertions, 7 deletions

diff --git a/host/lib/usrp/dboard/magnesium/magnesium_radio_control.cpp b/host/lib/usrp/dboard/magnesium/magnesium_radio_control.cpp
index 3881c9285..b84fcee6d 100644
--- a/host/lib/usrp/dboard/magnesium/magnesium_radio_control.cpp
+++ b/host/lib/usrp/dboard/magnesium/magnesium_radio_control.cpp
@@ -767,7 +767,7 @@ void magnesium_radio_control_impl::set_rx_lo_source(
 
 const std::string magnesium_radio_control_impl::get_rx_lo_source(
     const std::string& name, const size_t /*chan*/
-) const
+)
 {
     if (name == MAGNESIUM_LO1) {
         // TODO: should we use this from cache?
@@ -1073,7 +1073,7 @@ eeprom_map_t magnesium_radio_control_impl::get_db_eeprom()
 /**************************************************************************
  * Sensor API
  *************************************************************************/
-std::vector<std::string> magnesium_radio_control_impl::get_rx_sensor_names(size_t)
+std::vector<std::string> magnesium_radio_control_impl::get_rx_sensor_names(size_t) const
 {
     auto sensor_names = _rpcc->request_with_token<std::vector<std::string>>(
         this->_rpc_prefix + "get_sensors", "RX");
@@ -1092,7 +1092,7 @@ sensor_value_t magnesium_radio_control_impl::get_rx_sensor(
         _rpc_prefix + "get_sensor", "RX", name, chan));
 }
 
-std::vector<std::string> magnesium_radio_control_impl::get_tx_sensor_names(size_t)
+std::vector<std::string> magnesium_radio_control_impl::get_tx_sensor_names(size_t) const
 {
     auto sensor_names = _rpcc->request_with_token<std::vector<std::string>>(
         this->_rpc_prefix + "get_sensors", "TX");
diff --git a/host/lib/usrp/dboard/magnesium/magnesium_radio_control.hpp b/host/lib/usrp/dboard/magnesium/magnesium_radio_control.hpp
index 897e00868..f80f75f00 100644
--- a/host/lib/usrp/dboard/magnesium/magnesium_radio_control.hpp
+++ b/host/lib/usrp/dboard/magnesium/magnesium_radio_control.hpp
@@ -116,13 +116,14 @@ public:
         const std::string& name, const size_t chan) const override;
     void set_rx_lo_source(
         const std::string& src, const std::string& name, const size_t chan) override;
-    const std::string get_rx_lo_source(const std::string& name, const size_t chan) const;
+    const std::string get_rx_lo_source(
+        const std::string& name, const size_t chan) override;
     double set_rx_lo_freq(
         double freq, const std::string& name, const size_t chan) override;
     double get_rx_lo_freq(const std::string& name, const size_t chan) override;
     std::vector<std::string> get_tx_lo_names(const size_t chan) const override;
     std::vector<std::string> get_tx_lo_sources(
-        const std::string& name, const size_t chan) const;
+        const std::string& name, const size_t chan) const override;
     freq_range_t get_tx_lo_freq_range(
         const std::string& name, const size_t chan) override;
     void set_tx_lo_source(
@@ -150,9 +151,9 @@ public:
     /**************************************************************************
      * Sensor API
      *************************************************************************/
-    std::vector<std::string> get_rx_sensor_names(size_t chan);
+    std::vector<std::string> get_rx_sensor_names(size_t chan) const override;
     uhd::sensor_value_t get_rx_sensor(const std::string& name, size_t chan) override;
-    std::vector<std::string> get_tx_sensor_names(size_t chan);
+    std::vector<std::string> get_tx_sensor_names(size_t chan) const override;
     uhd::sensor_value_t get_tx_sensor(const std::string& name, size_t chan) override;
 
     /**************************************************************************