libmpv: Segfault when setting demuxer property immediately after loadfile command

[jaseg]

Important Information

Provide following Information:
mpv version: built from source: v0.32.0-585-gfba1c681b8
Platform: up-to-date Debian 10

Reproduction steps

Using my python libmpv wrapper I get a segfault that seems to be caused by a dead heap canary when I set the demuxer player property immediately after issuing a loadfile command.

$ pip3 install python-mpv

segfault:

import mpv
import time

p = mpv.MPV(log_handler=print, loglevel='debug')

p.play('test.webm') # issues "loadfile test.webm append" via mpv_command

p.demuxer = 'foo' # sets demuxer property via mpv_set_property_string
time.sleep(1000)

no segfault:

import mpv
import time

p = mpv.MPV(log_handler=print, loglevel='debug')

p.play('test.webm') # issues "loadfile test.webm append" via mpv_command

time.sleep(1) # Wait before setting property
p.demuxer = 'foo' # sets demuxer property via mpv_set_property_string
time.sleep(1000)

Expected behavior

No memory corruption. If possible an error message ("hey im busy k thx").

Actual behavior

Program terminates with segmentation fault inside libmpv.

Log file

https://0x0.st/ivZi.txt

Sample files

I used this here but anything else will work too.
https://raw.githubusercontent.com/jaseg/python-mpv/master/test.webm

[jaseg]

Merged with #7920