runtime: signal forwarding for darwin/amd64

Follows the linux signal forwarding semantics from
http://golang.org/cl/8712, sharing the implementation of sigfwdgo.
Forwarding for 386, arm, and arm64 will follow.

Change-Id: I6bf30d563d19da39b6aec6900c7fe12d82ed4f62
Reviewed-on: https://go-review.googlesource.com/9302
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/runtime/signal_unix.go

@@ -6,9 +6,43 @@
 
 package runtime
 
-import _ "unsafe" // for go:linkname
+import "unsafe"
 
 //go:linkname os_sigpipe os.sigpipe
 func os_sigpipe() {
 	systemstack(sigpipe)
 }
+
+// Determines if the signal should be handled by Go and if not, forwards the
+// signal to the handler that was installed before Go's.  Returns whether the
+// signal was forwarded.
+//go:nosplit
+func sigfwdgo(sig uint32, info *siginfo, ctx unsafe.Pointer) bool {
+	g := getg()
+	c := &sigctxt{info, ctx}
+	if sig >= uint32(len(sigtable)) {
+		return false
+	}
+	fwdFn := fwdSig[sig]
+	flags := sigtable[sig].flags
+
+	// If there is no handler to forward to, no need to forward.
+	if fwdFn == _SIG_DFL {
+		return false
+	}
+	// Only forward synchronous signals.
+	if c.sigcode() == _SI_USER || flags&_SigPanic == 0 {
+		return false
+	}
+	// Determine if the signal occurred inside Go code.  We test that:
+	//   (1) we were in a goroutine (i.e., m.curg != nil), and
+	//   (2) we weren't in CGO (i.e., m.curg.syscallsp == 0).
+	if g != nil && g.m != nil && g.m.curg != nil && g.m.curg.syscallsp == 0 {
+		return false
+	}
+	// Signal not handled by Go, forward it.
+	if fwdFn != _SIG_IGN {
+		sigfwd(fwdFn, sig, info, ctx)
+	}
+	return true
+}