hid: port logging facilities

Though the regular mode now has timestamps and a new mode for systemd
has been added.

hid/main.go

@@ -28,6 +28,7 @@ import (
 	"io"
 	"io/ioutil"
 	"log"
+	"log/syslog"
 	"net"
 	"os"
 	"os/signal"
@@ -48,6 +49,100 @@ const (
 	projectVersion = "0"
 )
 
+// --- Logging -----------------------------------------------------------------
+
+type logPrio int
+
+const (
+	prioFatal logPrio = iota
+	prioError
+	prioWarning
+	prioStatus
+	prioDebug
+)
+
+func (lp logPrio) prefix() string {
+	switch lp {
+	case prioFatal:
+		return "fatal: "
+	case prioError:
+		return "error: "
+	case prioWarning:
+		return "warning: "
+	case prioStatus:
+		return ""
+	case prioDebug:
+		return "debug: "
+	default:
+		panic("unhandled log priority")
+	}
+}
+
+func (lp logPrio) syslogPrio() syslog.Priority {
+	switch lp {
+	case prioFatal:
+		return syslog.LOG_ERR
+	case prioError:
+		return syslog.LOG_ERR
+	case prioWarning:
+		return syslog.LOG_WARNING
+	case prioStatus:
+		return syslog.LOG_INFO
+	case prioDebug:
+		return syslog.LOG_DEBUG
+	default:
+		panic("unhandled log priority")
+	}
+}
+
+// - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
+
+func logMessageStdio(prio logPrio, format string, args ...interface{}) {
+	// TODO: isatty-enabled colors based on prio.
+	os.Stderr.WriteString(time.Now().Format("2006-01-02 15:04:05 ") +
+		prio.prefix() + fmt.Sprintf(format, args...) + "\n")
+}
+
+func logMessageSystemd(prio logPrio, format string, args ...interface{}) {
+	if prio == prioFatal {
+		// There is no corresponding syslog severity.
+		format = "fatal: " + format
+	}
+	fmt.Fprintf(os.Stderr, "<%d>%s\n",
+		prio.syslogPrio(), fmt.Sprintf(format, args...))
+}
+
+// - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
+
+var logMessage = logMessageStdio
+
+func printDebug(format string, args ...interface{}) {
+	if debugMode {
+		logMessage(prioDebug, format, args...)
+	}
+}
+
+func printStatus(format string, args ...interface{}) {
+	logMessage(prioStatus, format, args...)
+}
+func printWarning(format string, args ...interface{}) {
+	logMessage(prioWarning, format, args...)
+}
+func printError(format string, args ...interface{}) {
+	logMessage(prioError, format, args...)
+}
+
+// "fatal" is reserved for failures that would harm further operation.
+
+func printFatal(format string, args ...interface{}) {
+	logMessage(prioFatal, format, args...)
+}
+
+func exitFatal(format string, args ...interface{}) {
+	printFatal(format, args...)
+	os.Exit(1)
+}
+
 // --- Utilities ---------------------------------------------------------------
 
 // Split a string by a set of UTF-8 delimiters, optionally ignoring empty items.
@@ -3316,6 +3411,7 @@ func main() {
 	version := flag.Bool("version", false, "show version and exit")
 	writeDefaultCfg := flag.Bool("writedefaultcfg", false,
 		"write a default configuration file and exit")
+	systemd := flag.Bool("systemd", false, "log in systemd format")
 
 	flag.Parse()
 
@@ -3327,6 +3423,9 @@ func main() {
 		callSimpleConfigWriteDefault("", configTable)
 		return
 	}
+	if *systemd {
+		logMessage = logMessageSystemd
+	}
 	if flag.NArg() > 0 {
 		flag.Usage()
 		os.Exit(2)