Add a smarter formatter to simulate the normal log behaviour

This adds a more clever formatter that uses the logger call depth to
return accurate data about the logged message in question.

It can be called with:

capnslog.SetFormatter(
	capnslog.NewLogFormatter(os.Stderr, "<prefix> ", flags)
)

Author: purpleidea

capnslog/formatters.go

@@ -18,6 +18,7 @@ import (
 	"bufio"
 	"fmt"
 	"io"
+	"log"
 	"runtime"
 	"strings"
 	"time"
@@ -104,3 +105,33 @@ func (c *PrettyFormatter) Format(pkg string, l LogLevel, depth int, entries ...i
 func (c *PrettyFormatter) Flush() {
 	c.w.Flush()
 }
+
+// LogFormatter emulates the form of the traditional built-in logger.
+type LogFormatter struct {
+	logger *log.Logger
+	prefix string
+}
+
+// NewLogFormatter is a helper to produce a new LogFormatter struct. It uses the
+// golang log package to actually do the logging work so that logs look similar.
+func NewLogFormatter(w io.Writer, prefix string, flag int) *LogFormatter {
+	return &LogFormatter{
+		logger: log.New(w, "", flag), // don't use prefix here
+		prefix: prefix,               // save it instead
+	}
+}
+
+// Format builds a log message for the LogFormatter. The LogLevel is ignored.
+func (s *LogFormatter) Format(pkg string, l LogLevel, i int, entries ...interface{}) {
+	str := fmt.Sprint(entries...)
+	prefix := s.prefix
+	if pkg != "" {
+		prefix = fmt.Sprintf("%s%s: ", prefix, pkg)
+	}
+	s.logger.Output(5, fmt.Sprintf("%s%v", prefix, str)) // call depth is 5
+}
+
+// Flush is included so that the interface is complete, but is not used.
+func (s *LogFormatter) Flush() {
+	// noop
+}