pkg/signal.CatchAll: ignore SIGURG on Linux

Do not handle SIGURG on Linux, as in go1.14+, the go runtime issues
SIGURG as an interrupt to support preemptable system calls on Linux.

This issue was caught in TestCatchAll, which could fail when updating to Go 1.14 or above;

    === Failed
    === FAIL: pkg/signal TestCatchAll (0.01s)
        signal_linux_test.go:32: assertion failed: urgent I/O condition (string) != continued (string)
        signal_linux_test.go:32: assertion failed: continued (string) != hangup (string)
        signal_linux_test.go:32: assertion failed: hangup (string) != child exited (string)
        signal_linux_test.go:32: assertion failed: child exited (string) != illegal instruction (string)
        signal_linux_test.go:32: assertion failed: illegal instruction (string) != floating point exception (string)

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

pkg/signal/signal.go

@@ -12,9 +12,16 @@ import (
 )
 
 // CatchAll catches all signals and relays them to the specified channel.
+// On Linux, SIGURG is not handled, as it's used by the Go runtime to support
+// preemptable system calls.
 func CatchAll(sigc chan os.Signal) {
 	var handledSigs []os.Signal
 	for _, s := range SignalMap {
+		if isRuntimeSig(s) {
+			// Do not handle SIGURG on Linux, as in go1.14+, the go runtime issues
+			// SIGURG as an interrupt to support preemptable system calls on Linux.
+			continue
+		}
 		handledSigs = append(handledSigs, s)
 	}
 	signal.Notify(sigc, handledSigs...)

pkg/signal/signal_darwin.go

@@ -1,6 +1,7 @@
 package signal // import "github.com/docker/docker/pkg/signal"
 
 import (
+	"os"
 	"syscall"
 )
 
@@ -39,3 +40,7 @@ var SignalMap = map[string]syscall.Signal{
 	"XCPU":   syscall.SIGXCPU,
 	"XFSZ":   syscall.SIGXFSZ,
 }
+
+func isRuntimeSig(_ os.Signal) bool {
+	return false
+}

pkg/signal/signal_freebsd.go

@@ -1,6 +1,7 @@
 package signal // import "github.com/docker/docker/pkg/signal"
 
 import (
+	"os"
 	"syscall"
 )
 
@@ -41,3 +42,7 @@ var SignalMap = map[string]syscall.Signal{
 	"XCPU":   syscall.SIGXCPU,
 	"XFSZ":   syscall.SIGXFSZ,
 }
+
+func isRuntimeSig(_ os.Signal) bool {
+	return false
+}

pkg/signal/signal_linux.go

@@ -3,6 +3,7 @@
 package signal // import "github.com/docker/docker/pkg/signal"
 
 import (
+	"os"
 	"syscall"
 
 	"golang.org/x/sys/unix"
@@ -81,3 +82,7 @@ var SignalMap = map[string]syscall.Signal{
 	"RTMAX-1":  sigrtmax - 1,
 	"RTMAX":    sigrtmax,
 }
+
+func isRuntimeSig(s os.Signal) bool {
+	return s == unix.SIGURG
+}

pkg/signal/signal_linux_mipsx.go

@@ -4,6 +4,7 @@
 package signal // import "github.com/docker/docker/pkg/signal"
 
 import (
+	"os"
 	"syscall"
 
 	"golang.org/x/sys/unix"
@@ -82,3 +83,7 @@ var SignalMap = map[string]syscall.Signal{
 	"RTMAX-1":  sigrtmax - 1,
 	"RTMAX":    sigrtmax,
 }
+
+func isRuntimeSig(s os.Signal) bool {
+	return s == unix.SIGURG
+}

pkg/signal/signal_linux_test.go

@@ -6,6 +6,7 @@ import (
 	"os"
 	"syscall"
 	"testing"
+	"time"
 
 	"gotest.tools/v3/assert"
 	is "gotest.tools/v3/assert/cmp"
@@ -34,6 +35,22 @@ func TestCatchAll(t *testing.T) {
 	}
 }
 
+func TestCatchAllIgnoreSigUrg(t *testing.T) {
+	sigs := make(chan os.Signal, 1)
+	CatchAll(sigs)
+	defer StopCatch(sigs)
+
+	err := syscall.Kill(syscall.Getpid(), syscall.SIGURG)
+	assert.NilError(t, err)
+	timer := time.NewTimer(1 * time.Second)
+	defer timer.Stop()
+	select {
+	case <-timer.C:
+	case s := <-sigs:
+		t.Fatalf("expected no signals to be handled, but received %q", s.String())
+	}
+}
+
 func TestStopCatch(t *testing.T) {
 	signal := SignalMap["HUP"]
 	channel := make(chan os.Signal, 1)

pkg/signal/signal_windows.go

@@ -1,6 +1,7 @@
 package signal // import "github.com/docker/docker/pkg/signal"
 
 import (
+	"os"
 	"syscall"
 )
 
@@ -24,3 +25,7 @@ var SignalMap = map[string]syscall.Signal{
 	"KILL": syscall.SIGKILL,
 	"TERM": syscall.SIGTERM,
 }
+
+func isRuntimeSig(_ os.Signal) bool {
+	return false
+}