types/logger: add adapters for Logf to std Logger/Writer

types/logger/logger.go

@@ -7,6 +7,11 @@
 // types around.
 package logger
 
+import (
+	"io"
+	"log"
+)
+
 // Logf is the basic Tailscale logger type: a printf-like func.
 type Logf func(format string, args ...interface{})
 
@@ -16,3 +21,20 @@ func WithPrefix(f Logf, prefix string) Logf {
 		f(prefix+format, args...)
 	}
 }
+
+// FuncWriter returns an io.Writer that writes to f.
+func FuncWriter(f Logf) io.Writer {
+	return funcWriter{f}
+}
+
+// StdLogger returns a standard library logger from a Logf.
+func StdLogger(f Logf) *log.Logger {
+	return log.New(FuncWriter(f), "", 0)
+}
+
+type funcWriter struct{ f Logf }
+
+func (w funcWriter) Write(p []byte) (int, error) {
+	w.f("%s", p)
+	return len(p), nil
+}

types/logger/logger_test.go

@@ -0,0 +1,21 @@
+// Copyright (c) 2020 Tailscale Inc & AUTHORS All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package logger
+
+import (
+	"log"
+	"testing"
+)
+
+func TestFuncWriter(t *testing.T) {
+	w := FuncWriter(t.Logf)
+	lg := log.New(w, "prefix: ", 0)
+	lg.Printf("plumbed through")
+}
+
+func TestStdLogger(t *testing.T) {
+	lg := StdLogger(t.Logf)
+	lg.Printf("plumbed through")
+}