x/sys/unix: add SignalNum() to convert signal name to a number

[kolyshkin]

A SignalName() method was recently (#25134) added to x/sys/unix to convert a numeric signal id to a name. It uses signalList array which contains names and numeric values for all known signals on every platform.

This is a proposal to add a complementary method to convert a string representation to a number.

// SignalNum returns the syscall.Signal for signal named s,
// or an error if a signal with such name is not found.
func SignalNum(s string) (syscall.Signal, error)

For example, SignalNum("SIGTERM") is to return 15, nil, and it should probably do the same for SignalNum("TERM") and SignalNum("term").

Update: s/SignalId/SignalNum/

[ianlancetaylor]

Now that we have SignalName, you can write this yourself. Is this really a common enough desire to put it in the package?

[kolyshkin]

Currently quite a few packages have to implement it, here are a few examples:

• x/crypto/ssh
• Docker
• containerd
• runc

[kolyshkin]

An alternative would be to expose signalList, which I think is much less maintainable long term.

[tklauser]

@ianlancetaylor I don't think we can use SignalName to implement that as it maps a signal number to a string. But what's proposed here is the reverse operation (string to signal number). Or am I missing something?

@kolyshkin I'd propose to name the proposed function SignalNum instead of SignalId, as the signal(2) and signal(7) use the term signal number to refer to the numeric signal id.

[ianlancetaylor]

func SignalNum(s string) syscall.Signal {
	for i := syscall.Signal(1); ; i++ {
		sn := unix.SignalName(i)
		if sn == s {
			return i
		}
		if sn == "" {
			return 0
		}
	}
}

[tklauser]

Are signal number guaranteed to be contiguous? At least on linux they seem to be. But looking at the recently added zerrors_aix_*.go I see:

var signalList = [...]struct {
        num  syscall.Signal
        name string
        desc string
}{
// ...
        {39, "SIGWAITING", "signal 39"},
        {48, "SIGSYSERROR", "signal 48"},
        {49, "SIGCAPI", "signal 49"},
        {58, "SIGRECONFIG", "signal 58"},
        {59, "SIGCPUFAIL", "CPU Failure Predicted"},
        {60, "SIGGRANT", "monitor mode granted"},
        {61, "SIGRETRACT", "monitor mode retracted"},
        {62, "SIGSOUND", "sound completed"},
        {63, "SIGMAX32", "secure attention"},
        {255, "SIGMAX", "signal 255"},
}

so on aix something like

num := SignalNum("SIGSYSERROR")

wouldn't work.

[tklauser]

We could add unix.NSIG though and then do:

func SignalNum(s string) syscall.Signal {
	for i := syscall.Signal(1); i < syscall.Signal(unix.NSIG); i++ {
		sn := unix.SignalName(i)
		if sn == s {
			return i
		}
	}
	return 0
}

[ondrej-fabry]

@tklauser It also needs some check if s == "" otherwise it will return first undefined signal.

EDIT: by the way, why not loop over signalList slice?

func SignalNum(name string) syscall.Signal {
	for _, s := range signalList {
		if s.name == name {
			return s.num
		}
	}
	return 0
}

oh I guess you were actually talking about implementation outside proposed package..

[tklauser]

@ondrej-fabry oops, right. Thank you. The proposed SignalNum is supposed to be implemented outside of golang.org/x/sys/unix (see #28027 (comment)) thus we cannot use signalList directly.

[ondrej-fabry]

@tklauser right, got that

btw, loop could start from 0 to avoid need for extra check of s == "" and loop until 255 for maximum possible signal number to eliminate need to define unix.NSIG.

func SignalNum(s string) syscall.Signal {
	for i := syscall.Signal(0); i < syscall.Signal(255); i++ {
		sn := unix.SignalName(i)
		if sn == s {
			return i
		}
	}
	return 0
}

SignalNum("") // will return 0

[kolyshkin]

As previous comments demonstrate, it is slightly less than trivial to write SignalNum() implementation based on unix.SignalName(). Perhaps it is easier to add SignalNum() right into the package?

[kolyshkin]

I have updated title and description to use SignalNum() rather than SignalId() as per @tklauser suggestion at #28027 (comment)