cherry-pick: 3157 excessive ptrs

Merge in DNS/adguard-home from 3157-excessive-ptrs to master

Updates #3157.

Squashed commit of the following:

commit 6803988240dca2f147bb80a5b3f78d7749d2fa14
Author: Eugene Burkov <E.Burkov@AdGuard.COM>
Date:   Tue Apr 19 14:50:01 2022 +0300

    aghnet: and again

commit 1a7f4d1dbc8fd4d3ae620349917526a75fa71b47
Author: Eugene Burkov <E.Burkov@AdGuard.COM>
Date:   Tue Apr 19 14:49:20 2022 +0300

    aghnet: docs again

commit d88da1fc7135f3cd03aff10b02d9957c8ffdfd30
Author: Eugene Burkov <E.Burkov@AdGuard.COM>
Date:   Tue Apr 19 14:47:36 2022 +0300

    aghnet: imp docs

commit c45dbc7800e882c6c4110aab640c32b03046f89a
Author: Eugene Burkov <E.Burkov@AdGuard.COM>
Date:   Tue Apr 19 14:41:19 2022 +0300

    aghnet: keep alphabetical order

commit b61781785d096ef43f60fb4f1905a4ed3cdf7c68
Author: Eugene Burkov <E.Burkov@AdGuard.COM>
Date:   Tue Apr 19 13:50:56 2022 +0300

    aghnet: imp code quality

commit 578dbd71ed2f2089c69343d7d4bf8bbc29150ace
Author: Eugene Burkov <E.Burkov@AdGuard.COM>
Date:   Tue Apr 12 17:02:38 2022 +0300

    aghnet: imp arp container

internal/aghos/os.go

@@ -52,24 +52,27 @@ func HaveAdminRights() (bool, error) {
 	return haveAdminRights()
 }
 
-// MaxCmdOutputSize is the maximum length of performed shell command output.
-const MaxCmdOutputSize = 2 * 1024
+// MaxCmdOutputSize is the maximum length of performed shell command output in
+// bytes.
+const MaxCmdOutputSize = 64 * 1024
 
 // RunCommand runs shell command.
-func RunCommand(command string, arguments ...string) (int, string, error) {
+func RunCommand(command string, arguments ...string) (code int, output []byte, err error) {
 	cmd := exec.Command(command, arguments...)
 	out, err := cmd.Output()
 	if len(out) > MaxCmdOutputSize {
 		out = out[:MaxCmdOutputSize]
 	}
 
-	if errors.As(err, new(*exec.ExitError)) {
-		return cmd.ProcessState.ExitCode(), string(out), nil
-	} else if err != nil {
-		return 1, "", fmt.Errorf("exec.Command(%s) failed: %w: %s", command, err, string(out))
+	if err != nil {
+		if eerr := new(exec.ExitError); errors.As(err, &eerr) {
+			return eerr.ExitCode(), eerr.Stderr, nil
+		}
+
+		return 1, nil, fmt.Errorf("command %q failed: %w: %s", command, err, out)
 	}
 
-	return cmd.ProcessState.ExitCode(), string(out), nil
+	return cmd.ProcessState.ExitCode(), out, nil
 }
 
 // PIDByCommand searches for process named command and returns its PID ignoring
@@ -172,3 +175,13 @@ func RootDirFS() (fsys fs.FS) {
 	// behavior is undocumented but it currently works.
 	return os.DirFS("")
 }
+
+// NotifyShutdownSignal notifies c on receiving shutdown signals.
+func NotifyShutdownSignal(c chan<- os.Signal) {
+	notifyShutdownSignal(c)
+}
+
+// IsShutdownSignal returns true if sig is a shutdown signal.
+func IsShutdownSignal(sig os.Signal) (ok bool) {
+	return isShutdownSignal(sig)
+}

internal/aghos/os_unix.go

@@ -0,0 +1,27 @@
+//go:build darwin || freebsd || linux || openbsd
+// +build darwin freebsd linux openbsd
+
+package aghos
+
+import (
+	"os"
+	"os/signal"
+
+	"golang.org/x/sys/unix"
+)
+
+func notifyShutdownSignal(c chan<- os.Signal) {
+	signal.Notify(c, unix.SIGINT, unix.SIGQUIT, unix.SIGTERM)
+}
+
+func isShutdownSignal(sig os.Signal) (ok bool) {
+	switch sig {
+	case
+		unix.SIGINT,
+		unix.SIGQUIT,
+		unix.SIGTERM:
+		return true
+	default:
+		return false
+	}
+}

internal/aghos/os_windows.go

@@ -4,6 +4,10 @@
 package aghos
 
 import (
+	"os"
+	"os/signal"
+	"syscall"
+
 	"golang.org/x/sys/windows"
 )
 
@@ -35,3 +39,20 @@ func haveAdminRights() (bool, error) {
 func isOpenWrt() (ok bool) {
 	return false
 }
+
+func notifyShutdownSignal(c chan<- os.Signal) {
+	// syscall.SIGTERM is processed automatically.  See go doc os/signal,
+	// section Windows.
+	signal.Notify(c, os.Interrupt)
+}
+
+func isShutdownSignal(sig os.Signal) (ok bool) {
+	switch sig {
+	case
+		os.Interrupt,
+		syscall.SIGTERM:
+		return true
+	default:
+		return false
+	}
+}