runtime: don't always block all signals on OpenBSD

Implement the changes from CL 10173 on OpenBSD.

Change-Id: I2db1cd8141fd392a34753a1b8113e2e0401173b9
Reviewed-on: https://go-review.googlesource.com/10342
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

Author: Elias Naur

src/runtime/os1_openbsd.go

@@ -149,6 +149,11 @@ func mpreinit(mp *m) {
 }
 
 func msigsave(mp *m) {
+	smask := (*uint32)(unsafe.Pointer(&mp.sigmask))
+	if unsafe.Sizeof(*smask) > unsafe.Sizeof(mp.sigmask) {
+		throw("insufficient storage for signal mask")
+	}
+	*smask = sigprocmask(_SIG_BLOCK, 0)
 }
 
 // Called to initialize a new m (including the bootstrap m).
@@ -161,11 +166,22 @@ func minit() {
 
 	// Initialize signal handling
 	signalstack((*byte)(unsafe.Pointer(_g_.m.gsignal.stack.lo)), 32*1024)
-	sigprocmask(_SIG_SETMASK, sigset_none)
+
+	// restore signal mask from m.sigmask and unblock essential signals
+	nmask := *(*uint32)(unsafe.Pointer(&_g_.m.sigmask))
+	for i := range sigtable {
+		if sigtable[i].flags&_SigUnblock != 0 {
+			nmask &^= 1 << (uint32(i) - 1)
+		}
+	}
+	sigprocmask(_SIG_SETMASK, nmask)
 }
 
 // Called from dropm to undo the effect of an minit.
 func unminit() {
+	_g_ := getg()
+	smask := *(*uint32)(unsafe.Pointer(&_g_.m.sigmask))
+	sigprocmask(_SIG_SETMASK, smask)
 	signalstack(nil, 0)
 }