Reading from dump file leads to assert in vsm.c (varnishlog -r)

[lassekarstensenmt]

Issue: When attempting to use python-varnishapi to read varnishlog dumped to binary file on disk, opening it with Varnishapi.VarnishLog() triggers assert in vsm.c.

Assert error in VSM_Get(), vsm.c line 1019:
  Condition((vd->attached) != 0) not true.

python-varnishapi from pypi, version 60.24.
debian testing with Varnish 6.0.7 from packagecloud varnish60lts.

Using this snippet based on the examples:

#!/usr/bin/env python3
import varnishapi
from time import sleep

def cbline(vap, cbd, priv):
    print(cbd)

def cbvxid(vap, priv):
    print("VXID")

def cbgroup(vap, priv):
    print("GROUP")

def main():
#    vsl = varnishapi.VarnishLog(["-g", "request"])
    vsl = varnishapi.VarnishLog(["-r", "foo.dump", "-g", "request"])
    arg = {
        "cb": cbline,
        "vxidcb": cbvxid,
        "groupcb": cbgroup,
        "maxread": 0,
    }

    while 1:
        ret = vsl.Dispatch(**arg)
        if 0 == ret:
            sleep(0.5)
        print(".", end="")
    vsl.Fini()

if __name__ == "__main__":
    main()