runtime: allow signal.Ignore of user-generated throwing signals

Today, signal.Ignore(syscall.SIGTRAP) does nothing
while signal.Notify(make(chan os.Signal), syscall.SIGTRAP)
correctly discards user-generated SIGTRAPs.
The same applies to any signal that we throw on.

Make signal.Ignore work for these signals.

Fixes #12906.

Change-Id: Iba244813051e0ce23fa32fbad3e3fa596a941094
Reviewed-on: https://go-review.googlesource.com/18348Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/runtime/crash_unix_test.go

@@ -151,3 +151,11 @@ func TestSignalExitStatus(t *testing.T) {
 		t.Errorf("got %v; expected SIGTERM", ee)
 	}
 }
+
+func TestSignalIgnoreSIGTRAP(t *testing.T) {
+	output := runTestProg(t, "testprognet", "SignalIgnoreSIGTRAP")
+	want := "OK\n"
+	if output != want {
+		t.Fatalf("want %s, got %s\n", want, output)
+	}
+}

src/runtime/signal_386.go

@@ -107,6 +107,10 @@ func sighandler(sig uint32, info *siginfo, ctxt unsafe.Pointer, gp *g) {
 		}
 	}
 
+	if c.sigcode() == _SI_USER && signal_ignored(sig) {
+		return
+	}
+
 	if flags&_SigKill != 0 {
 		dieFromSignal(int32(sig))
 	}

src/runtime/signal_amd64x.go

@@ -139,6 +139,10 @@ func sighandler(sig uint32, info *siginfo, ctxt unsafe.Pointer, gp *g) {
 		}
 	}
 
+	if c.sigcode() == _SI_USER && signal_ignored(sig) {
+		return
+	}
+
 	if flags&_SigKill != 0 {
 		dieFromSignal(int32(sig))
 	}

src/runtime/signal_arm.go

@@ -99,6 +99,10 @@ func sighandler(sig uint32, info *siginfo, ctxt unsafe.Pointer, gp *g) {
 		}
 	}
 
+	if c.sigcode() == _SI_USER && signal_ignored(sig) {
+		return
+	}
+
 	if flags&_SigKill != 0 {
 		dieFromSignal(int32(sig))
 	}

src/runtime/signal_arm64.go

@@ -115,6 +115,10 @@ func sighandler(sig uint32, info *siginfo, ctxt unsafe.Pointer, gp *g) {
 		}
 	}
 
+	if c.sigcode() == _SI_USER && signal_ignored(sig) {
+		return
+	}
+
 	if flags&_SigKill != 0 {
 		dieFromSignal(int32(sig))
 	}

src/runtime/signal_mips64x.go

@@ -117,6 +117,10 @@ func sighandler(sig uint32, info *siginfo, ctxt unsafe.Pointer, gp *g) {
 		}
 	}
 
+	if c.sigcode() == _SI_USER && signal_ignored(sig) {
+		return
+	}
+
 	if flags&_SigKill != 0 {
 		dieFromSignal(int32(sig))
 	}

src/runtime/signal_ppc64x.go

@@ -121,6 +121,10 @@ func sighandler(sig uint32, info *siginfo, ctxt unsafe.Pointer, gp *g) {
 		}
 	}
 
+	if c.sigcode() == _SI_USER && signal_ignored(sig) {
+		return
+	}
+
 	if flags&_SigKill != 0 {
 		dieFromSignal(int32(sig))
 	}

src/runtime/sigqueue.go

@@ -37,6 +37,7 @@ var sig struct {
 	note    note
 	mask    [(_NSIG + 31) / 32]uint32
 	wanted  [(_NSIG + 31) / 32]uint32
+	ignored [(_NSIG + 31) / 32]uint32
 	recv    [(_NSIG + 31) / 32]uint32
 	state   uint32
 	inuse   bool
@@ -146,6 +147,7 @@ func signal_enable(s uint32) {
 		return
 	}
 	sig.wanted[s/32] |= 1 << (s & 31)
+	sig.ignored[s/32] &^= 1 << (s & 31)
 	sigenable(s)
 }
 
@@ -166,9 +168,15 @@ func signal_ignore(s uint32) {
 		return
 	}
 	sig.wanted[s/32] &^= 1 << (s & 31)
+	sig.ignored[s/32] |= 1 << (s & 31)
 	sigignore(s)
 }
 
+// Checked by signal handlers.
+func signal_ignored(s uint32) bool {
+	return sig.ignored[s/32]&(1<<(s&31)) != 0
+}
+
 // This runs on a foreign stack, without an m or a g.  No stack split.
 //go:nosplit
 //go:norace

src/runtime/testdata/testprognet/signal.go

+// Copyright 2016 The Go Authors.  All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// +build !windows,!plan9,!nacl
+
+// This is in testprognet instead of testprog because testprog
+// must not import anything (like net, but also like os/signal)
+// that kicks off background goroutines during init.
+
+package main
+
+import (
+	"os/signal"
+	"syscall"
+)
+
+func init() {
+	register("SignalIgnoreSIGTRAP", SignalIgnoreSIGTRAP)
+}
+
+func SignalIgnoreSIGTRAP() {
+	signal.Ignore(syscall.SIGTRAP)
+	syscall.Kill(syscall.Getpid(), syscall.SIGTRAP)
+	println("OK")
+}