Introduce more context to logging

A new log formatter is introduced that adds program contex to the
log output, e.g. source file, and line of code where a log call
is made. Additional program context can be added consistently.
Datetime is formatted in ISO format and an option is provided to
use UTC or local timezones.

cmd/roy/roy.go

@@ -27,6 +27,7 @@ import (
 
 	"github.com/richardlehane/siegfried"
 	"github.com/richardlehane/siegfried/internal/chart"
+	"github.com/richardlehane/siegfried/internal/logformatter"
 	"github.com/richardlehane/siegfried/pkg/config"
 	"github.com/richardlehane/siegfried/pkg/core"
 	"github.com/richardlehane/siegfried/pkg/loc"
@@ -116,6 +117,18 @@ Additional flags:
       Use a different siegfried home directory.
 `
 
+func setLogger(utc bool) {
+	lw := new(logformatter.LogWriter)
+	lw.Appname = "roy"
+	lw.UTC = utc
+	log.SetOutput(lw)
+}
+
+func init() {
+	// format the application logger before all other init.
+	setLogger(true)
+}
+
 var (
 	// BUILD, ADD flag sets
 	build         = flag.NewFlagSet("build | add", flag.ExitOnError)

cmd/sf/sf.go

@@ -31,6 +31,7 @@ import (
 
 	"github.com/richardlehane/siegfried"
 	"github.com/richardlehane/siegfried/internal/checksum"
+	"github.com/richardlehane/siegfried/internal/logformatter"
 	"github.com/richardlehane/siegfried/internal/logger"
 	"github.com/richardlehane/siegfried/pkg/config"
 	"github.com/richardlehane/siegfried/pkg/core"
@@ -42,6 +43,18 @@ import (
 // defaults
 const maxMulti = 1024
 
+func setLogger(utc bool) {
+	lw := new(logformatter.LogWriter)
+	lw.Appname = "sf"
+	lw.UTC = utc
+	log.SetOutput(lw)
+}
+
+func init() {
+	// format the application logger before all other init.
+	setLogger(true)
+}
+
 // flags
 var (
 	updateShort    = flag.Bool("u", false, "update or install the default signature file")

internal/logformatter/log.go

@@ -0,0 +1,39 @@
+package logformatter
+
+import (
+	"fmt"
+	"log"
+	"os"
+	"time"
+)
+
+type LogWriter struct {
+	Appname string
+	UTC     bool
+}
+
+const logTimeFormat = "2006-01-02 15:04:05"
+
+// Write enables us to format a logging prefix for the application. The
+// text will appear before the log message output by the caller.
+//
+// e.g.
+//
+//	`// 2023-11-27 11:36:57 ERROR :: golang-app:100:main() :: this is an error message, ...some diagnosis`
+func (lw *LogWriter) Write(logString []byte) (int, error) {
+	logTime := time.Now().UTC().Format(logTimeFormat)
+	if !lw.UTC {
+		logTime = time.Now().Format(logTimeFormat)
+	}
+	return fmt.Fprintf(os.Stderr, "%s :: %s :: %s",
+		logTime,
+		lw.Appname,
+		string(logString),
+	)
+}
+
+func init() {
+	// Configure logging to use a custom log writer with sensible defaults.
+	log.SetFlags(0 | log.Lshortfile | log.LUTC)
+	log.SetOutput(new(LogWriter))
+}