multi_usrp: Provide valid return value for multi_usrp::get_device()

For RFNoC devices, multi_usrp::get_device() no longer returns a device
pointer, rather, it returns a nullptr.
This is intentional because access to the underlying device is no longer
allowed. However, legacy code can segfault (e.g. portions ofr gr-uhd).

This patch returns a faux uhd::device class, which almost mimicks the
original behaviour perfectly, by redirecting its class methods back to
multi_usrp_rfnoc. The only exception is recv_async_msg(), which requires
a TX streamer. This function will always return false now.

2 files changed, 21 insertions, 5 deletions

diff --git a/host/include/uhd/device.hpp b/host/include/uhd/device.hpp
index f4594504d..a70a396a7 100644
--- a/host/include/uhd/device.hpp
+++ b/host/include/uhd/device.hpp
@@ -88,11 +88,18 @@ public:
      */
     virtual tx_streamer::sptr get_tx_stream(const stream_args_t& args) = 0;
 
-    /*!
-     * Receive and asynchronous message from the device.
+    /*! Receive asynchronous message from the device
+     *
+     * Note that when calling multi_usrp::get_device() on an RFNoC-capable device,
+     * the returned uhd::device (while still being a valid object) will not be
+     * able to execute this method. Instead, query the asynchronous messages
+     * from the appropriate Tx streamer.
+     *
      * \param async_metadata the metadata to be filled in
      * \param timeout the timeout in seconds to wait for a message
      * \return true when the async_metadata is valid, false for timeout
+     * \throws uhd::runtime_error if called on a device returned by
+     *         multi_usrp::get_device on an RFNoC device.
      */
     virtual bool recv_async_msg(
         async_metadata_t& async_metadata, double timeout = 0.1) = 0;
diff --git a/host/include/uhd/usrp/multi_usrp.hpp b/host/include/uhd/usrp/multi_usrp.hpp
index ac80cc15a..93ef7d090 100644
--- a/host/include/uhd/usrp/multi_usrp.hpp
+++ b/host/include/uhd/usrp/multi_usrp.hpp
@@ -120,9 +120,18 @@ public:
      */
     static sptr make(const device_addr_t& dev_addr);
 
-    /*!
-     * Get the underlying device object.
-     * This is needed to get access to the streaming API and properties.
+    /*! Get the underlying device object
+     *
+     * Note that it is not recommended to use this method. The property tree can
+     * be accessed by calling get_tree() on this object, and the streamers own
+     * all the streaming-related functionality. get_tx_stream() and
+     * get_rx_stream() can also be called on this object.
+     *
+     * For RFNoC devices, this won't return a true uhd::device anyway, because
+     * direct device access is locked for those. The returned pointer will
+     * still point to a valid device object, however, it has reduced
+     * functionality (in particular, recv_async_msg() won't work).
+     *
      * \return the device object within this USRP
      */
     virtual device::sptr get_device(void) = 0;