proposal: os: add Number method to Signal interface

[berkant]

Signals have portable numbers identifying them on various operating systems (Plan9 excluded). But it looks like there is no direct way to retrieve these numbers from the wrapped os.Signal other than a String method which lazily looks up the name of that signal and returns it or using reflection package or type assertion. That's why I'm proposing to seal a Number(): int method to os.Signal interface and adapt current implementations to it. It will be supposed to return the original receiver provided it's an integer (or a numeric type that can be converted to integer) otherwise returning -1.

cf.
repo:^github.com/golang/go$ file:^src/syscall/ type Signal|Note
repo:^github.com/golang/go$ file:^src/syscall/ SIGINT count:1000

[DeedleFake]

I'm not sure what the point is. In the definitions for the signals on Windows in golang.org/x/sys/windows:

const (
    // More invented values for signals
    SIGHUP  = Signal(0x1)
    SIGINT  = Signal(0x2)
    SIGQUIT = Signal(0x3)
    SIGILL  = Signal(0x4)
    SIGTRAP = Signal(0x5)
    SIGABRT = Signal(0x6)
    SIGBUS  = Signal(0x7)
    SIGFPE  = Signal(0x8)
    SIGKILL = Signal(0x9)
    SIGSEGV = Signal(0xb)
    SIGPIPE = Signal(0xd)
    SIGALRM = Signal(0xe)
    SIGTERM = Signal(0xf)
)

In other words, these values are completely arbitrary. I don't think relying on them for something is a good idea. If you're going to have to treat signals differently on different systems anyways, then you may as well just check them against different values. For example:

// signal.go
c := make(chan os.Signal)
signal.Notify(c)
handleSignal(<-c)

// signal_unix.go
func handleSignal(sig os.Signal) {
  switch sig {
  case os.Interrupt:
    println("UNIX interrupt.")
  case syscall.SIGHUP:
    println("Hup.")
  }
}

// signal_windows.go
func handleSignal(sig os.Signal) {
  switch os.Interrupt:
    println("Windows interrupt.")
  }
}

[ianlancetaylor]

If you need to deal with signal numbers, you must be running on a Unix systems. And in that case you can write sig.(syscall.Signal) to get a number.

[bradfitz]

Sorry, we can't expand os.Signal, as that would break the Go 1 compatibility promise. It doesn't have any unexported methods, so it's frozen: https://golang.org/pkg/os/#Signal

(contrast to, say, testing.TB: https://golang.org/pkg/testing/#TB)