Extract several packages to form a public API

framework/log/writer.go

@@ -0,0 +1,77 @@
+package log
+
+import (
+	"fmt"
+	"io"
+	"os"
+	"strings"
+	"time"
+)
+
+type wcOutput struct {
+	timestamps bool
+	wc         io.WriteCloser
+}
+
+func (w wcOutput) Write(stamp time.Time, debug bool, msg string) {
+	builder := strings.Builder{}
+	if w.timestamps {
+		builder.WriteString(stamp.UTC().Format("2006-01-02T15:04:05.000Z "))
+	}
+	if debug {
+		builder.WriteString("[debug] ")
+	}
+	builder.WriteString(msg)
+	builder.WriteRune('\n')
+	if _, err := io.WriteString(w.wc, builder.String()); err != nil {
+		fmt.Fprintf(os.Stderr, "!!! Failed to write message to log: %v\n", err)
+	}
+}
+
+func (w wcOutput) Close() error {
+	return w.wc.Close()
+}
+
+// WriteCloserOutput returns a log.Output implementation that
+// will write formatted messages to the provided io.Writer.
+//
+// Closing returned log.Output object will close the underlying
+// io.WriteCloser.
+//
+// Written messages will include timestamp formatted with millisecond
+// precision and [debug] prefix for debug messages.
+// If timestamps argument is false, timestamps will not be added.
+//
+// Returned log.Output does not provide its own serialization
+// so goroutine-safety depends on the io.Writer. Most operating
+// systems have atomic (read: thread-safe) implementations for
+// stream I/O, so it should be safe to use WriterOutput with os.File.
+func WriteCloserOutput(wc io.WriteCloser, timestamps bool) Output {
+	return wcOutput{timestamps, wc}
+}
+
+type nopCloser struct {
+	io.Writer
+}
+
+func (nc nopCloser) Close() error {
+	return nil
+}
+
+// WriterOutput returns a log.Output implementation that
+// will write formatted messages to the provided io.Writer.
+//
+// Closing returned log.Output object will have no effect on the
+// underlying io.Writer.
+//
+// Written messages will include timestamp formatted with millisecond
+// precision and [debug] prefix for debug messages.
+// If timestamps argument is false, timestamps will not be added.
+//
+// Returned log.Output does not provide its own serialization
+// so goroutine-safety depends on the io.Writer. Most operating
+// systems have atomic (read: thread-safe) implementations for
+// stream I/O, so it should be safe to use WriterOutput with os.File.
+func WriterOutput(w io.Writer, timestamps bool) Output {
+	return wcOutput{timestamps, nopCloser{os.Stderr}}
+}