runtime: sigaction problem when using cgo on FreeBSD

[billziss-gh]

What version of Go are you using (go version)?

go version go1.10.1 freebsd/amd64

Does this issue reproduce with the latest release?

Likely.

What operating system and processor architecture are you using (go env)?

GOARCH="amd64"
GOBIN=""
GOCACHE="/home/billziss/.cache/go-build"
GOEXE=""
GOHOSTARCH="amd64"
GOHOSTOS="freebsd"
GOOS="freebsd"
GOPATH="/home/billziss/go"
GORACE=""
GOROOT="/usr/local/go"
GOTMPDIR=""
GOTOOLDIR="/usr/local/go/pkg/tool/freebsd_amd64"
GCCGO="gccgo"
CC="clang"
CXX="clang++"
CGO_ENABLED="1"
CGO_CFLAGS="-g -O2"
CGO_CPPFLAGS=""
CGO_CXXFLAGS="-g -O2"
CGO_FFLAGS="-g -O2"
CGO_LDFLAGS="-g -O2"
PKG_CONFIG="pkg-config"
GOGCCFLAGS="-fPIC -m64 -pthread -fno-caret-diagnostics -Qunused-arguments -fmessage-length=0 -fdebug-prefix-map=/tmp/go-build089165738=/tmp/go-build -gno-record-gcc-switches"

What did you do?

The following simple cgo program:

package main

/*
#include <stdio.h>
#include <signal.h>

static void sigprint(int sig) {
	struct sigaction sa;
	int ret = sigaction(sig, 0, &sa);
	fprintf(stderr, "ret=%d handler=%p\n", ret, sa.sa_handler);
}
*/
import "C"
import "syscall"

func main() {
	C.sigprint(C.int(syscall.SIGTERM))
}

Succeeds (prints a non-0 signal handler) on Darwin and Linux:

$ go run sigbug.go
ret=0 handler=0x4034150

But fails (prints a 0 signal handler) on FreeBSD:

$ go run sigbug.go
ret=0 handler=0x0

What did you expect to see?

I expected to see the signal handler that the go runtime installs.

What did you see instead?

The C sigaction function thinks that there is no signal handler.

Investigation

I believe the reason for the "mysterious" sigaction behavior is that the Go runtime invokes sigaction using SYSCALL. However FreeBSD expects that C (and cgo) programs should use the libc sigaction, which is "interposed" by libthr to __thr_sigaction. Libthr maintains its own copy of sigaction's and overrides what the kernel returns.

A likely fix for this is to do what Linux does for sigaction. Invoke it using SYSCALL if "!cgo" or use the C library sigaction if "cgo". See rt_sigaction.

Additional information at billziss-gh/cgofuse#18.

[ianlancetaylor]

I believe that this is already fixed on tip and therefore in the upcoming 1.11 release as part of the work on #14327. I'm going to close this issue but please do comment if I turn out to be mistaken.

[billziss-gh]

Fantastic! I can confirm this is now fixed against the current tip:

$ go version
go version devel +2ce295e954 Wed Jun 6 01:47:31 2018 +0000 freebsd/amd64
$ go run sigbug.go
ret=0 handler=0x44a9c0