https://bugs.torproject.org/25600#comment:14

Snowflake in Tor Browser has been hanging after surfing for a while.
(Tor reports "no running bridges".) It only began happening after commit
12922a23, which caused snowflake-client not to log to a file by
default (leaving it to log to its default stderr). What seems to be
happening is that tor doesn't read from its PT clients' stderr, leaving
a buffer to fill up that eventually causes a hang.