slog: JSONHandler

Add a Handler that produces JSON output.

Change-Id: Ifbadfb3bea1afbd987a1c48319c0494ebf629d46
Reviewed-on: https://go-review.googlesource.com/c/exp/+/429835
Reviewed-by: Alan Donovan <[email protected]>
Run-TryBot: Jonathan Amsterdam <[email protected]>

Author: jba

slog/handler.go

@@ -143,7 +143,9 @@ func (h *commonHandler) handle(r Record) error {
 		val := r.Time().Round(0) // strip monotonic to match Attr behavior
 		if rep == nil {
 			app.appendKey(key)
-			app.appendTime(val)
+			if err := app.appendTime(val); err != nil {
+				return err
+			}
 		} else {
 			replace(Time(key, val))
 		}
@@ -226,13 +228,15 @@ func appendAttr(app appender, a Attr) {
 
 // An appender appends keys and values to a buffer.
 // TextHandler and JSONHandler both implement it.
+// It factors out the format-specific parts of the job.
+// Other than growing the buffer, none of the methods should allocate.
 type appender interface {
 	appendStart()                       // start a sequence of Attrs
 	appendEnd()                         // end a sequence of Attrs
 	appendSep()                         // separate one Attr from the next
 	appendKey(key string)               // append a key
 	appendString(string)                // append a string that may need to be escaped
-	appendTime(time.Time)               // append a time
+	appendTime(time.Time) error         // append a time
 	appendSource(file string, line int) // append file:line
 	appendAttrValue(a Attr) error       // append the Attr's value (but not its key)
 }

slog/handler_test.go

@@ -105,12 +105,16 @@ type memAppender struct {
 
 func (a *memAppender) set(v any) { a.m[a.key] = v }
 
-func (a *memAppender) appendStart()           {}
-func (a *memAppender) appendSep()             {}
-func (a *memAppender) appendEnd()             {}
-func (a *memAppender) appendKey(key string)   { a.key = key }
-func (a *memAppender) appendString(s string)  { a.set(s) }
-func (a *memAppender) appendTime(t time.Time) { a.set(t) }
+func (a *memAppender) appendStart()          {}
+func (a *memAppender) appendSep()            {}
+func (a *memAppender) appendEnd()            {}
+func (a *memAppender) appendKey(key string)  { a.key = key }
+func (a *memAppender) appendString(s string) { a.set(s) }
+
+func (a *memAppender) appendTime(t time.Time) error {
+	a.set(t)
+	return nil
+}
 
 func (a *memAppender) appendSource(file string, line int) {
 	a.set(fmt.Sprintf("%s:%d", file, line))
@@ -121,6 +125,24 @@ func (a *memAppender) appendAttrValue(at Attr) error {
 	return nil
 }
 
+const rfc3339Millis = "2006-01-02T15:04:05.000Z07:00"
+
+func TestAppendTimeRFC3339(t *testing.T) {
+	for _, tm := range []time.Time{
+		time.Date(2000, 1, 2, 3, 4, 5, 0, time.UTC),
+		time.Date(2000, 1, 2, 3, 4, 5, 400, time.Local),
+		time.Date(2000, 11, 12, 3, 4, 500, 5e7, time.UTC),
+	} {
+		want := tm.Format(rfc3339Millis)
+		var buf []byte
+		buf = appendTimeRFC3339Millis(buf, tm)
+		got := string(buf)
+		if got != want {
+			t.Errorf("got %s, want %s", got, want)
+		}
+	}
+}
+
 func BenchmarkAppendTime(b *testing.B) {
 	buf := make([]byte, 0, 100)
 	tm := time.Date(2022, 3, 4, 5, 6, 7, 823456789, time.Local)

slog/internal/buffer/buffer.go

@@ -32,8 +32,9 @@ func (b *Buffer) Free() {
 		bufPool.Put(b)
 	}
 }
-func (b *Buffer) Write(p []byte) {
+func (b *Buffer) Write(p []byte) (int, error) {
 	*b = append(*b, p...)
+	return len(p), nil
 }
 
 func (b *Buffer) WriteString(s string) {