runtime: write sigsetstack for Darwin, fix sigaction arg

It turns out that the second argument for sigaction on Darwin has a
different type than the first argument.  The second argument is the user
visible sigaction struct, and does not have the sa_tramp field.

I base this on
  http://www.opensource.apple.com/source/Libc/Libc-1081.1.3/sys/sigaction.c
not to mention actual testing.

While I was at it I removed a useless memclr in setsig, a relic of the C
code.

This CL is Darwin-specific changes.  The tests for this CL are in
https://golang.org/cl/17903 .

Change-Id: I61fe305c72311df6a589b49ad7b6e49b6960ca24
Reviewed-on: https://go-review.googlesource.com/18015
Reviewed-by: David Crawshaw <crawshaw@golang.org>

src/runtime/defs_darwin.go

@@ -152,7 +152,7 @@ type StackT C.struct_sigaltstack
 type Sighandler C.union___sigaction_u
 
 type Sigaction C.struct___sigaction // used in syscalls
-// type Sigaction C.struct_sigaction	// used by the C library
+type Usigaction C.struct_sigaction  // used by sigaction second argument
 type Sigval C.union_sigval
 type Siginfo C.siginfo_t
 type Timeval C.struct_timeval

src/runtime/defs_darwin_386.go

@@ -167,6 +167,12 @@ type sigactiont struct {
 	sa_flags      int32
 }
 
+type usigactiont struct {
+	__sigaction_u [4]byte
+	sa_mask       uint32
+	sa_flags      int32
+}
+
 type siginfo struct {
 	si_signo  int32
 	si_errno  int32

src/runtime/defs_darwin_amd64.go

@@ -168,6 +168,12 @@ type sigactiont struct {
 	sa_flags      int32
 }
 
+type usigactiont struct {
+	__sigaction_u [8]byte
+	sa_mask       uint32
+	sa_flags      int32
+}
+
 type siginfo struct {
 	si_signo  int32
 	si_errno  int32

src/runtime/defs_darwin_arm.go

@@ -169,6 +169,12 @@ type sigactiont struct {
 	sa_flags      int32
 }
 
+type usigactiont struct {
+	__sigaction_u [4]byte
+	sa_mask       uint32
+	sa_flags      int32
+}
+
 type siginfo struct {
 	si_signo  int32
 	si_errno  int32

src/runtime/defs_darwin_arm64.go

@@ -168,6 +168,12 @@ type sigactiont struct {
 	sa_flags      int32
 }
 
+type usigactiont struct {
+	__sigaction_u [8]byte
+	sa_mask       uint32
+	sa_flags      int32
+}
+
 type siginfo struct {
 	si_signo  int32
 	si_errno  int32

src/runtime/os1_darwin.go

@@ -444,7 +444,6 @@ func memlimit() uintptr {
 
 func setsig(i int32, fn uintptr, restart bool) {
 	var sa sigactiont
-	memclr(unsafe.Pointer(&sa), unsafe.Sizeof(sa))
 	sa.sa_flags = _SA_SIGINFO | _SA_ONSTACK
 	if restart {
 		sa.sa_flags |= _SA_RESTART
@@ -456,12 +455,22 @@ func setsig(i int32, fn uintptr, restart bool) {
 }
 
 func setsigstack(i int32) {
-	throw("setsigstack")
+	var osa usigactiont
+	sigaction(uint32(i), nil, &osa)
+	handler := *(*uintptr)(unsafe.Pointer(&osa.__sigaction_u))
+	if handler == 0 || handler == _SIG_DFL || handler == _SIG_IGN || osa.sa_flags&_SA_ONSTACK != 0 {
+		return
+	}
+	var sa sigactiont
+	*(*uintptr)(unsafe.Pointer(&sa.__sigaction_u)) = handler
+	sa.sa_tramp = unsafe.Pointer(funcPC(sigtramp))
+	sa.sa_mask = osa.sa_mask
+	sa.sa_flags = osa.sa_flags | _SA_ONSTACK
+	sigaction(uint32(i), &sa, nil)
 }
 
 func getsig(i int32) uintptr {
-	var sa sigactiont
-	memclr(unsafe.Pointer(&sa), unsafe.Sizeof(sa))
+	var sa usigactiont
 	sigaction(uint32(i), nil, &sa)
 	return *(*uintptr)(unsafe.Pointer(&sa.__sigaction_u))
 }

src/runtime/os_darwin.go

@@ -27,7 +27,7 @@ func sysctl(mib *uint32, miblen uint32, out *byte, size *uintptr, dst *byte, nds
 func sigprocmask(how uint32, new, old *sigset)
 
 //go:noescape
-func sigaction(mode uint32, new, old *sigactiont)
+func sigaction(mode uint32, new *sigactiont, old *usigactiont)
 
 //go:noescape
 func sigaltstack(new, old *stackt)