Why won't chrony sync to GPS when serial data flows through socat? - gps

Question

I'm using gpsd to sync time to a GPS. When I connect my GPS to /dev/ttyUSB0, and tell gpsd to listen on that port, chrony is happy to use it as a time source.
gpsd -D 5 -N -n /dev/ttyUSB0
However, as soon as I try and pipe that data through socat (which is how it needs to work in our production system), chrony won't use it as a source. This is the case even though gpsd, cgps, and gpsmon all seem perfectly happy with the GPS data they are getting.
Here's my socat:
socat -d -d pty,rawer,echo=0,link=/tmp/ttyVSP0 /dev/ttyUSB0,b4800
(my gpsd command is the same as above but with /tmp/ttyVSP0 as the port to listen to in this case).
I'm using chronyc sources to confirm when GPS is a chrony source.
My refclock line in my /etc/chrony/chrony.conf looks like this:
refclock SHM 0 refid GPS

Answer 1

pty ports are prevented from talking to ntp (and thus, chrony) by an early return meant to prevent code from being executed during testing.
void ntpshm_link_activate(struct gps_device_t *session)
/* set up ntpshm storage for a session */
{
/* don't talk to NTP when we're running inside the test harness */
if (session->sourcetype == source_pty)
return;
if (session->sourcetype != source_pps ) {
/* allocate a shared-memory segment for "NMEA" time data */
session->shm_clock = ntpshm_alloc(session->context);
if (session->shm_clock == NULL) {
gpsd_log(&session->context->errout, LOG_WARN,
"NTP: ntpshm_alloc() failed\n");
return;
}
}
Discovered thanks to this bug report