pkg/process: Alive(): fix PID 0, -1, negative values

unix.Kill() does not produce an error for PID 0, -1. As a result, checking
process.Alive() would return "true" for both 0 and -1 on macOS (and previously
on Linux as well).

Let's shortcut these values to consider them "not alive", to prevent someone
trying to kill them.

A basic test was added to check the behavior.

Given that the intent of these functions is to handle single processes, this patch
also prevents 0 and negative values to be used.

From KILL(2): https://man7.org/linux/man-pages/man2/kill.2.html

    If pid is positive, then signal sig is sent to the process with
    the ID specified by pid.

    If pid equals 0, then sig is sent to every process in the process
    group of the calling process.

    If pid equals -1, then sig is sent to every process for which the
    calling process has permission to send signals, except for
    process 1 (init), but see below.

    If pid is less than -1, then sig is sent to every process in the
    process group whose ID is -pid.

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

pkg/process/process_test.go

@@ -0,0 +1,40 @@
+package process
+
+import (
+	"fmt"
+	"os"
+	"os/exec"
+	"runtime"
+	"testing"
+)
+
+func TestAlive(t *testing.T) {
+	for _, pid := range []int{0, -1, -123} {
+		t.Run(fmt.Sprintf("invalid process (%d)", pid), func(t *testing.T) {
+			if Alive(pid) {
+				t.Errorf("PID %d should not be alive", pid)
+			}
+		})
+	}
+	t.Run("current process", func(t *testing.T) {
+		if pid := os.Getpid(); !Alive(pid) {
+			t.Errorf("current PID (%d) should be alive", pid)
+		}
+	})
+	t.Run("exited process", func(t *testing.T) {
+		if runtime.GOOS == "windows" {
+			t.Skip("TODO: make this work on Windows")
+		}
+
+		// Get a PID of an exited process.
+		cmd := exec.Command("echo", "hello world")
+		err := cmd.Run()
+		if err != nil {
+			t.Fatal(err)
+		}
+		exitedPID := cmd.ProcessState.Pid()
+		if Alive(exitedPID) {
+			t.Errorf("PID %d should not be alive", exitedPID)
+		}
+	})
+}

pkg/process/process_unix.go

@@ -14,8 +14,14 @@ import (
 	"golang.org/x/sys/unix"
 )
 
-// Alive returns true if process with a given pid is running.
+// Alive returns true if process with a given pid is running. It only considers
+// positive PIDs; 0 (all processes in the current process group), -1 (all processes
+// with a PID larger than 1), and negative (-n, all processes in process group
+// "n") values for pid are never considered to be alive.
 func Alive(pid int) bool {
+	if pid < 1 {
+		return false
+	}
 	switch runtime.GOOS {
 	case "darwin":
 		// OS X does not have a proc filesystem. Use kill -0 pid to judge if the
@@ -35,8 +41,16 @@ func Alive(pid int) bool {
 	}
 }
 
-// Kill force-stops a process.
+// Kill force-stops a process. It only considers positive PIDs; 0 (all processes
+// in the current process group), -1 (all processes with a PID larger than 1),
+// and negative (-n, all processes in process group "n") values for pid are
+// ignored. Refer to [KILL(2)] for details.
+//
+// [KILL(2)]: https://man7.org/linux/man-pages/man2/kill.2.html
 func Kill(pid int) error {
+	if pid < 1 {
+		return fmt.Errorf("invalid PID (%d): only positive PIDs are allowed", pid)
+	}
 	err := unix.Kill(pid, unix.SIGKILL)
 	if err != nil && err != unix.ESRCH {
 		return err
@@ -44,9 +58,16 @@ func Kill(pid int) error {
 	return nil
 }
 
-// Zombie return true if process has a state with "Z"
-// http://man7.org/linux/man-pages/man5/proc.5.html
+// Zombie return true if process has a state with "Z". It only considers positive
+// PIDs; 0 (all processes in the current process group), -1 (all processes with
+// a PID larger than 1), and negative (-n, all processes in process group "n")
+// values for pid are ignored. Refer to [PROC(5)] for details.
+//
+// [PROC(5)]: https://man7.org/linux/man-pages/man5/proc.5.html
 func Zombie(pid int) (bool, error) {
+	if pid < 1 {
+		return false, nil
+	}
 	data, err := os.ReadFile(fmt.Sprintf("/proc/%d/stat", pid))
 	if err != nil {
 		if os.IsNotExist(err) {