os/signal: add ability to ignore signals and restore initial signal handlers

There is currently no way to ignore signals using the os/signal package.
It is possible to catch a signal and do nothing but this is not the same
as ignoring it. The new function Ignore allows a set of signals to be
ignored. The new function Reset allows the initial handlers for a set of
signals to be restored.

Fixes #5572

Change-Id: I5c0f07956971e3a9ff9b9d9631e6e3a08c20df15
Reviewed-on: https://go-review.googlesource.com/3580
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/os/signal/sig.s

@@ -24,6 +24,9 @@ TEXT ·signal_disable(SB),NOSPLIT,$0
 TEXT ·signal_enable(SB),NOSPLIT,$0
 	JMP runtime·signal_enable(SB)
 
+TEXT ·signal_ignore(SB),NOSPLIT,$0
+	JMP runtime·signal_ignore(SB)
+
 TEXT ·signal_recv(SB),NOSPLIT,$0
 	JMP runtime·signal_recv(SB)
 

src/os/signal/signal.go

@@ -28,9 +28,55 @@ func (h *handler) set(sig int) {
 	h.mask[sig/32] |= 1 << uint(sig&31)
 }
 
+func (h *handler) clear(sig int) {
+	h.mask[sig/32] &^= 1 << uint(sig&31)
+}
+
+// Stop relaying the signals, sigs, to any channels previously registered to
+// receive them and either reset the signal handlers to their original values
+// (action=disableSignal) or ignore the signals (action=ignoreSignal).
+func cancel(sigs []os.Signal, action func(int)) {
+	handlers.Lock()
+	defer handlers.Unlock()
+
+	remove := func(n int) {
+		var zerohandler handler
+
+		for c, h := range handlers.m {
+			if h.want(n) {
+				handlers.ref[n]--
+				h.clear(n)
+				if h.mask == zerohandler.mask {
+					delete(handlers.m, c)
+				}
+			}
+		}
+
+		action(n)
+	}
+
+	if len(sigs) == 0 {
+		for n := 0; n < numSig; n++ {
+			remove(n)
+		}
+	} else {
+		for _, s := range sigs {
+			remove(signum(s))
+		}
+	}
+}
+
+// Ignore causes the provided signals to be ignored. If they are received by
+// the program, nothing will happen. Ignore undoes the effect of any prior
+// calls to Notify for the provided signals.
+// If no signals are provided, all incoming signals will be ignored.
+func Ignore(sig ...os.Signal) {
+	cancel(sig, ignoreSignal)
+}
+
 // Notify causes package signal to relay incoming signals to c.
-// If no signals are listed, all incoming signals will be relayed to c.
-// Otherwise, just the listed signals will.
+// If no signals are provided, all incoming signals will be relayed to c.
+// Otherwise, just the provided signals will.
 //
 // Package signal will not block sending to c: the caller must ensure
 // that c has sufficient buffer space to keep up with the expected
@@ -85,6 +131,13 @@ func Notify(c chan<- os.Signal, sig ...os.Signal) {
 	}
 }
 
+// Reset undoes the effect of any prior calls to Notify for the provided
+// signals.
+// If no signals are provided, all signal handlers will be reset.
+func Reset(sig ...os.Signal) {
+	cancel(sig, disableSignal)
+}
+
 // Stop causes package signal to stop relaying incoming signals to c.
 // It undoes the effect of all prior calls to Notify using c.
 // When Stop returns, it is guaranteed that c will receive no more signals.

src/os/signal/signal_plan9.go

@@ -14,6 +14,7 @@ var sigtab = make(map[os.Signal]int)
 // In sig.s; jumps to runtime.
 func signal_disable(uint32)
 func signal_enable(uint32)
+func signal_ignore(uint32)
 func signal_recv() string
 
 func init() {
@@ -53,3 +54,7 @@ func enableSignal(sig int) {
 func disableSignal(sig int) {
 	signal_disable(uint32(sig))
 }
+
+func ignoreSignal(sig int) {
+	signal_ignore(uint32(sig))
+}

src/os/signal/signal_test.go

@@ -109,6 +109,72 @@ func TestStress(t *testing.T) {
 	time.Sleep(10 * time.Millisecond)
 }
 
+func testCancel(t *testing.T, ignore bool) {
+	// Send SIGWINCH. By default this signal should be ignored.
+	syscall.Kill(syscall.Getpid(), syscall.SIGWINCH)
+	time.Sleep(100 * time.Millisecond)
+
+	// Ask to be notified on c1 when a SIGWINCH is received.
+	c1 := make(chan os.Signal, 1)
+	Notify(c1, syscall.SIGWINCH)
+	defer Stop(c1)
+
+	// Ask to be notified on c2 when a SIGHUP is received.
+	c2 := make(chan os.Signal, 1)
+	Notify(c2, syscall.SIGHUP)
+	defer Stop(c2)
+
+	// Send this process a SIGWINCH and wait for notification on c1.
+	syscall.Kill(syscall.Getpid(), syscall.SIGWINCH)
+	waitSig(t, c1, syscall.SIGWINCH)
+
+	// Send this process a SIGHUP and wait for notification on c2.
+	syscall.Kill(syscall.Getpid(), syscall.SIGHUP)
+	waitSig(t, c2, syscall.SIGHUP)
+
+	// Ignore, or reset the signal handlers for, SIGWINCH and SIGHUP.
+	if ignore {
+		Ignore(syscall.SIGWINCH, syscall.SIGHUP)
+	} else {
+		Reset(syscall.SIGWINCH, syscall.SIGHUP)
+	}
+
+	// Send this process a SIGWINCH. It should be ignored.
+	syscall.Kill(syscall.Getpid(), syscall.SIGWINCH)
+
+	// If ignoring, Send this process a SIGHUP. It should be ignored.
+	if ignore {
+		syscall.Kill(syscall.Getpid(), syscall.SIGHUP)
+	}
+
+	select {
+	case s := <-c1:
+		t.Fatalf("unexpected signal %v", s)
+	case <-time.After(100 * time.Millisecond):
+		// nothing to read - good
+	}
+
+	select {
+	case s := <-c2:
+		t.Fatalf("unexpected signal %v", s)
+	case <-time.After(100 * time.Millisecond):
+		// nothing to read - good
+	}
+
+	// Reset the signal handlers for all signals.
+	Reset()
+}
+
+// Test that Reset cancels registration for listed signals on all channels.
+func TestReset(t *testing.T) {
+	testCancel(t, false)
+}
+
+// Test that Ignore cancels registration for listed signals on all channels.
+func TestIgnore(t *testing.T) {
+	testCancel(t, true)
+}
+
 var sendUncaughtSighup = flag.Int("send_uncaught_sighup", 0, "send uncaught SIGHUP during TestStop")
 
 // Test that Stop cancels the channel's registrations.

src/os/signal/signal_unix.go

@@ -14,6 +14,7 @@ import (
 // In assembly.
 func signal_disable(uint32)
 func signal_enable(uint32)
+func signal_ignore(uint32)
 func signal_recv() uint32
 
 func loop() {
@@ -51,3 +52,7 @@ func enableSignal(sig int) {
 func disableSignal(sig int) {
 	signal_disable(uint32(sig))
 }
+
+func ignoreSignal(sig int) {
+	signal_ignore(uint32(sig))
+}

src/runtime/os1_nacl.go

@@ -174,6 +174,7 @@ func munmap(addr unsafe.Pointer, n uintptr)               {}
 func resetcpuprofiler(hz int32)                           {}
 func sigdisable(uint32)                                   {}
 func sigenable(uint32)                                    {}
+func sigignore(uint32)                                    {}
 func closeonexec(int32)                                   {}
 
 var writelock uint32 // test-and-set spin lock for write

src/runtime/os1_windows_386.go

@@ -118,3 +118,6 @@ func sigenable(sig uint32) {
 
 func sigdisable(sig uint32) {
 }
+
+func sigignore(sig uint32) {
+}

src/runtime/os1_windows_amd64.go

@@ -137,3 +137,6 @@ func sigenable(sig uint32) {
 
 func sigdisable(sig uint32) {
 }
+
+func sigignore(sig uint32) {
+}

src/runtime/os3_plan9.go

@@ -106,6 +106,9 @@ func sigenable(sig uint32) {
 func sigdisable(sig uint32) {
 }
 
+func sigignore(sig uint32) {
+}
+
 func resetcpuprofiler(hz int32) {
 	// TODO: Enable profiling interrupts.
 	getg().m.profilehz = hz

src/runtime/signal1_unix.go

@@ -78,6 +78,18 @@ func sigdisable(sig uint32) {
 	}
 }
 
+func sigignore(sig uint32) {
+	if sig >= uint32(len(sigtable)) {
+		return
+	}
+
+	t := &sigtable[sig]
+	if t.flags&_SigNotify != 0 {
+		t.flags &^= _SigHandling
+		setsig(int32(sig), _SIG_IGN, true)
+	}
+}
+
 func resetcpuprofiler(hz int32) {
 	var it itimerval
 	if hz == 0 {

src/runtime/sigqueue.go

@@ -153,6 +153,15 @@ func signal_disable(s uint32) {
 	sigdisable(s)
 }
 
+// Must only be called from a single goroutine at a time.
+func signal_ignore(s uint32) {
+	if int(s) >= len(sig.wanted)*32 {
+		return
+	}
+	sig.wanted[s/32] &^= 1 << (s & 31)
+	sigignore(s)
+}
+
 // This runs on a foreign stack, without an m or a g.  No stack split.
 //go:nosplit
 func badsignal(sig uintptr) {

src/runtime/sigqueue_plan9.go

@@ -113,3 +113,7 @@ func signal_enable(s uint32) {
 // Must only be called from a single goroutine at a time.
 func signal_disable(s uint32) {
 }
+
+// Must only be called from a single goroutine at a time.
+func signal_ignore(s uint32) {
+}