When the publisher is created, it calls msgq_init_publisher which resets the reader count. If you send data (in msgq_msg_send) before the subcriber connects, you will have no readers to signal to here:

https://github.com/commaai/cereal/blob/bd31b25aacc5b39f36cedcb0dabd05db471da59f/messaging/msgq.cc#L295-L299

This PR finds all current readers and sends a SIGUSR2 signal. Verified that the subscriber polling doesn't finish early when a publisher is created.

from cereal.messaging import SubMaster, PubMaster, new_message, sub_sock
from cereal import log
import time

sock = sub_sock('sendcan')
sock.setTimeout(100)

t = time.time()
while True:
  print('recv', sock.receive())
  print(time.time() - t)
  print()

from cereal.messaging import PubMaster, new_message
from cereal import log
import time

times = []

for _ in range(1000):
  pm = PubMaster(['sendcan'])
  t = time.monotonic()
  while 1:
    if pm.sock['sendcan'].all_readers_updated():
      times.append(time.monotonic() - t)
      break

print(times)
print(sum(times) / len(times))

Time for all readers to connect (should be half of subscriber dt on average):

0.05434682522984804 mean, 0.049833057644082523 std

After change:

2.469095983542502e-06 mean, 7.680831147408531e-07 std