How to get UHD 'rx_time' / 'rx_freq' after 'tune'? (Python)

[walter]

Hi,

(see subject line)

SETUP:

I'm using GR / Python / USRP (one or more B210s) for Rx frequency-hopping in a 
no-GUI application.

I created a block that sends 'tune' messages to the uhd, based on the docs here:
        REF: 
https://www.gnuradio.org/doc/doxygen/page_uhd.html#uhd_command_syntax_cmds
(the messages from that block are also sent to a ZMQ PUB Message Sink for 
logging). 

I've done extensive testing, and I know the tune commands are being processed 
by the B210, i.e. it's changing center_freq and LO Offset.

PROBLEM:

Although I know when the command was sent, I don't know when it was processed. 
I want to either: 

1. get 'rx_time' and 'rx_freq' messages after the 'tune' command is executed, 
or 
2. access <my_uhd_block>.get_time_now() / <my_uhd_block>.get_center_freq() from 
within a python block.  I'm unclear on how to get the <my_uhd_block> reference.

Note the rx_time, rx_freq, and rx_samp tags are attached to sample 0, but no 
other.  It's not clear from my reading of the gr and usrp docs how I might 
issue the tune request as a timed command to make use of the info at sample 0 
(or recover from underflow).

I'm hoping that I've 'missed something obvious' ... any suggestions welcome.