types/logger: add WithPrefix, use it in two places

Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

ipn/ipnserver/server.go

@@ -169,11 +169,8 @@ func Run(rctx context.Context, logf logger.Logf, logid string, opts Options, e w
 		oldS = s
 
 		go func(ctx context.Context, bs *ipn.BackendServer, s net.Conn, i int) {
-			// TODO: move this prefixing-Logf code into a new helper in types/logger?
-			si := fmt.Sprintf("%d: ", i)
-			pump(func(fmt string, args ...interface{}) {
-				logf(si+fmt, args...)
-			}, ctx, bs, s)
+			logf := logger.WithPrefix(logf, fmt.Sprintf("%d: ", i))
+			pump(logf, ctx, bs, s)
 			if !opts.SurviveDisconnects || bs.GotQuit {
 				bs.Reset()
 				s.Close()

ipn/local.go

@@ -174,9 +174,7 @@ func (b *LocalBackend) Start(opts Options) error {
 		persist = &controlclient.Persist{}
 	}
 	cli, err := controlclient.New(controlclient.Options{
-		Logf: func(fmt string, args ...interface{}) {
-			b.logf("control: "+fmt, args...)
-		},
+		Logf:            logger.WithPrefix(b.logf, "control: "),
 		Persist:         *persist,
 		ServerURL:       b.serverURL,
 		Hostinfo:        hi,

types/logger/logger.go

@@ -7,4 +7,12 @@
 // types around.
 package logger
 
-type Logf func(fmt string, args ...interface{})
+// Logf is the basic Tailscale logger type: a printf-like func.
+type Logf func(format string, args ...interface{})
+
+// WithPrefix wraps f, prefixing each format with the provided prefix.
+func WithPrefix(f Logf, prefix string) Logf {
+	return func(format string, args ...interface{}) {
+		f(prefix+format, args...)
+	}
+}