Please don't ignore SIGPIPE in the mosquitto client library

[nmeum]

Hello,

Presently, the mosquitto MQTT library ignores SIGPIPE:

mosquitto/lib/mosquitto.c

Lines 112 to 114 in d5970ca

	#ifndef WIN32
	signal(SIGPIPE, SIG_IGN);
	#endif

This is a problem since this will affect all threads of the process running the mosquitto client library. Furthermore, as per signal(7) “A child created via fork(2) inherits a copy of its parent's signal dispositions.”. That is, any process forked from the process which uses the mosquitto client library will also ignore SIGPIPE by default. I believe it to be a very bad idea to have a C library introduce this sort of side-effect since code using the C library may require SIGPIPE to not be ignored.

We noticed this at Alpine Linux where we use mosquitto as part of our build infrastructure. See:

• Restore default SIGPIPE handler after fork() ncopa/mqtt-exec#7
• https://gitlab.alpinelinux.org/alpine/infra/infra/-/issues/10758

Opening this issue here in the hopes that you might consider not modifying signal dispositions in the mosquitto library.

[ericonr]

I can think of two alternatives here:

• use send(2) with MSG_NOSIGNAL on platforms where it's available
• ignore the signal but with a dummy function instead, which can't be inherited (is still uncooperative behavior from a C library, but at least less disruptive for some use cases)

[ralight]

Agreed, this is bad behaviour. Thanks for pointing it out.