log: Add TimestampFormat and Timeformat.

log/value.go

@@ -32,13 +32,52 @@ func containsValuer(keyvals []interface{}) bool {
 	return false
 }
 
-// Timestamp returns a Valuer that invokes the underlying function when bound,
-// returning a time.Time. Users will probably want to use DefaultTimestamp or
-// DefaultTimestampUTC.
+// Timestamp returns a timestamp Valuer. It invokes the t function to get the
+// time; unless you are doing something tricky, pass time.Now.
+//
+// Most users will want to use DefaultTimestamp or DefaultTimestampUTC, which
+// are TimestampFormats that use the RFC3339Nano format.
 func Timestamp(t func() time.Time) Valuer {
 	return func() interface{} { return t() }
 }
 
+// TimestampFormat returns a timestamp Valuer with a custom time format. It
+// invokes the t function to get the time to format; unless you are doing
+// something tricky, pass time.Now. The layout string is passed to
+// Time.Format.
+//
+// Most users will want to use DefaultTimestamp or DefaultTimestampUTC, which
+// are TimestampFormats that use the RFC3339Nano format.
+func TimestampFormat(t func() time.Time, layout string) Valuer {
+	return func() interface{} {
+		return timeFormat{
+			time:   t(),
+			layout: layout,
+		}
+	}
+}
+
+// A timeFormat represents an instant in time and a layout used when
+// marshaling to a text format.
+type timeFormat struct {
+	time   time.Time
+	layout string
+}
+
+func (tf timeFormat) String() string {
+	return tf.time.Format(tf.layout)
+}
+
+// MarshalText implements encoding.TextMarshaller.
+func (tf timeFormat) MarshalText() (text []byte, err error) {
+	// The following code adapted from the standard library time.Time.Format
+	// method. Using the same undocumented magic constant to extend the size
+	// of the buffer as seen there.
+	b := make([]byte, 0, len(tf.layout)+10)
+	b = tf.time.AppendFormat(b, tf.layout)
+	return b, nil
+}
+
 // Caller returns a Valuer that returns a file and line from a specified depth
 // in the callstack. Users will probably want to use DefaultCaller.
 func Caller(depth int) Valuer {
@@ -48,15 +87,14 @@ func Caller(depth int) Valuer {
 var (
 	// DefaultTimestamp is a Valuer that returns the current wallclock time,
 	// respecting time zones, when bound.
-	DefaultTimestamp = Valuer(func() interface{} {
-		return time.Now().Format(time.RFC3339Nano)
-	})
+	DefaultTimestamp = TimestampFormat(time.Now, time.RFC3339Nano)
 
 	// DefaultTimestampUTC is a Valuer that returns the current time in UTC
 	// when bound.
-	DefaultTimestampUTC = Valuer(func() interface{} {
-		return time.Now().UTC().Format(time.RFC3339Nano)
-	})
+	DefaultTimestampUTC = TimestampFormat(
+		func() time.Time { return time.Now().UTC() },
+		time.RFC3339Nano,
+	)
 
 	// DefaultCaller is a Valuer that returns the file and line where the Log
 	// method was invoked. It can only be used with log.With.

log/value_test.go

@@ -1,7 +1,9 @@
 package log_test
 
 import (
+	"encoding"
 	"fmt"
+	"reflect"
 	"testing"
 	"time"
 
@@ -34,7 +36,7 @@ func TestValueBinding(t *testing.T) {
 	if want, have := start.Add(time.Second), timestamp; want != have {
 		t.Errorf("output[1]: want %v, have %v", want, have)
 	}
-	if want, have := "value_test.go:29", fmt.Sprint(output[3]); want != have {
+	if want, have := "value_test.go:31", fmt.Sprint(output[3]); want != have {
 		t.Errorf("output[3]: want %s, have %s", want, have)
 	}
 
@@ -47,7 +49,7 @@ func TestValueBinding(t *testing.T) {
 	if want, have := start.Add(2*time.Second), timestamp; want != have {
 		t.Errorf("output[1]: want %v, have %v", want, have)
 	}
-	if want, have := "value_test.go:42", fmt.Sprint(output[3]); want != have {
+	if want, have := "value_test.go:44", fmt.Sprint(output[3]); want != have {
 		t.Errorf("output[3]: want %s, have %s", want, have)
 	}
 }
@@ -90,6 +92,43 @@ func TestValueBinding_loggingZeroKeyvals(t *testing.T) {
 	}
 }
 
+func TestTimestampFormat(t *testing.T) {
+	t.Parallel()
+
+	start := time.Date(2015, time.April, 25, 0, 0, 0, 0, time.UTC)
+	now := start
+	mocktime := func() time.Time {
+		now = now.Add(time.Second)
+		return now
+	}
+
+	tv := log.TimestampFormat(mocktime, time.RFC822)
+
+	if want, have := now.Add(time.Second).Format(time.RFC822), fmt.Sprint(tv()); want != have {
+		t.Errorf("wrong time format: want %v, have %v", want, have)
+	}
+
+	if want, have := now.Add(2*time.Second).Format(time.RFC822), fmt.Sprint(tv()); want != have {
+		t.Errorf("wrong time format: want %v, have %v", want, have)
+	}
+
+	mustMarshal := func(v interface{}) []byte {
+		b, err := v.(encoding.TextMarshaler).MarshalText()
+		if err != nil {
+			t.Fatal("error marshaling text:", err)
+		}
+		return b
+	}
+
+	if want, have := now.Add(3*time.Second).AppendFormat(nil, time.RFC822), mustMarshal(tv()); !reflect.DeepEqual(want, have) {
+		t.Errorf("wrong time format: want %s, have %s", want, have)
+	}
+
+	if want, have := now.Add(4*time.Second).AppendFormat(nil, time.RFC822), mustMarshal(tv()); !reflect.DeepEqual(want, have) {
+		t.Errorf("wrong time format: want %s, have %s", want, have)
+	}
+}
+
 func BenchmarkValueBindingTimestamp(b *testing.B) {
 	logger := log.NewNopLogger()
 	lc := log.With(logger, "ts", log.DefaultTimestamp)