"ardi attach-and-watch" fails on platforms with onboard USB-serial support

[egnor]

Some Arduino platforms (like the Uno) have a dedicated USB-serial chip, so the USB-serial device is consistently present throughout the program-run-reset cycle. However, others (like the Leonardo and Micro) use the same chip for USB-serial and for running Arduino programs, which means that on reset the device goes away (at least momentarily).

The one I'm using now is from Adafruit's "Bluefruit" line of nRF52-based Arduino-compatible boards.

Because the USB-serial device doesn't exist for a moment after programming (until the chip boots and the USB-serial device is recognized by the host), ardi attach-and-watch can fail:

% ardi attach-and-watch
ardi    Using compile opts from build definition: blinky 
ardi    Using baud from build definition: blinky      baud=9600
ardi    Compiling...                                  fqbn="adafruit:nrf52:cplaynrf52840" sketch=blinky.ino
Sketch uses 40596 bytes (4%) of program storage space. Maximum is 815104 bytes.
Global variables use 7140 bytes (3%) of dynamic memory, leaving 230428 bytes for local variables. Maximum is 237568 bytes.
ardi    Compilation successful                        fqbn="adafruit:nrf52:cplaynrf52840" sketch=blinky.ino
ardi    Uploading...                                  build=. device=/dev/ttyACM0 fqbn="adafruit:nrf52:cplaynrf52840"
TOUCH: error during reset: setting DTR to OFF: protocol error
Upgrading target on /dev/ttyACM0 with DFU package /tmp/arduino-sketch-3015A9A7226BD79EC290418CB735FCEF/bluefruit.ino.zip. Flow control is disabled, Single bank, Touch disabled
########################################
########################################
Activating new firmware
Device programmed.
ardi    Upload successful                             build=. device=/dev/ttyACM0 fqbn="adafruit:nrf52:cplaynrf52840"
ardi    Watching blinky.ino for changes              
ardi    Attaching to port                             port=/dev/ttyACM0
ardi Failed to read from device                    baud=9600 error="no such file or directory" name=/dev/ttyACM0

Note that once the chip boots (mere milliseconds later), the serial device shows up and can be accessed just fine; a separate ardi attach afterward works:

% ardi attach
ardi    Attaching to port                             port=/dev/ttyACM0
Hello!
Hello!
Hello!
...

Probably this is best dealt with by polling for the serial port to exist? Note that if the chip gets reset (e.g. someone hits a reset button) the serial port will wink out of existence for a moment, so it's probably best if after a serial read error "attach" returned to polling for the device? (That's secondary though.)