logtail: always send a json array

The code goes to some effort to send a single JSON object
when there's only a single line and a JSON array when there
are multiple lines.

It makes the code more complex and more expensive;
when we add a second line, we have to use a second buffer
to duplicate the first one after adding a leading square brackets.

The savings come to two bytes. Instead, always send an array.

Signed-off-by: Josh Bleecher Snyder <josh@tailscale.com>

logtail/logtail.go

@@ -203,6 +203,7 @@ func (l *Logger) drainBlock() (shuttingDown bool) {
 // If no logs are available, drainPending blocks until logs are available.
 func (l *Logger) drainPending() (res []byte) {
 	buf := new(bytes.Buffer)
+	buf.WriteByte('[')
 	entries := 0
 
 	var batchDone bool
@@ -242,28 +243,15 @@ func (l *Logger) drainPending() (res []byte) {
 			b = l.encodeText(b, true)
 		}
 
-		switch {
-		case entries == 0:
-			buf.Write(b)
-		case entries == 1:
-			buf2 := new(bytes.Buffer)
-			buf2.WriteByte('[')
-			buf2.Write(buf.Bytes())
-			buf2.WriteByte(',')
-			buf2.Write(b)
-			buf.Reset()
-			buf.Write(buf2.Bytes())
-		default:
+		if entries > 0 {
 			buf.WriteByte(',')
-			buf.Write(b)
 		}
+		buf.Write(b)
 		entries++
 	}
 
-	if entries > 1 {
-		buf.WriteByte(']')
-	}
-	if buf.Len() == 0 {
+	buf.WriteByte(']')
+	if buf.Len() <= len("[]") {
 		return nil
 	}
 	return buf.Bytes()

logtail/logtail_test.go

@@ -117,12 +117,7 @@ func TestEncodeAndUploadMessages(t *testing.T) {
 		io.WriteString(l, tt.log)
 		body := <-ts.uploaded
 
-		data := make(map[string]interface{})
-		err := json.Unmarshal(body, &data)
-		if err != nil {
-			t.Error(err)
-		}
-
+		data := unmarshalOne(t, body)
 		got := data["text"]
 		if got != tt.want {
 			t.Errorf("%s: got %q; want %q", tt.name, got.(string), tt.want)
@@ -154,11 +149,7 @@ func TestEncodeSpecialCases(t *testing.T) {
 	// JSON log message already contains a logtail field.
 	io.WriteString(l, `{"logtail": "LOGTAIL", "text": "text"}`)
 	body := <-ts.uploaded
-	data := make(map[string]interface{})
-	err := json.Unmarshal(body, &data)
-	if err != nil {
-		t.Error(err)
-	}
+	data := unmarshalOne(t, body)
 	errorHasLogtail, ok := data["error_has_logtail"]
 	if ok {
 		if errorHasLogtail != "LOGTAIL" {
@@ -186,11 +177,7 @@ func TestEncodeSpecialCases(t *testing.T) {
 	l.skipClientTime = true
 	io.WriteString(l, "text")
 	body = <-ts.uploaded
-	data = make(map[string]interface{})
-	err = json.Unmarshal(body, &data)
-	if err != nil {
-		t.Error(err)
-	}
+	data = unmarshalOne(t, body)
 	_, ok = data["logtail"]
 	if ok {
 		t.Errorf("skipClientTime: unexpected logtail map present: %v", data)
@@ -204,11 +191,7 @@ func TestEncodeSpecialCases(t *testing.T) {
 	longStr := strings.Repeat("0", 512)
 	io.WriteString(l, longStr)
 	body = <-ts.uploaded
-	data = make(map[string]interface{})
-	err = json.Unmarshal(body, &data)
-	if err != nil {
-		t.Error(err)
-	}
+	data = unmarshalOne(t, body)
 	text, ok := data["text"]
 	if !ok {
 		t.Errorf("lowMem: no text %v", data)
@@ -219,7 +202,7 @@ func TestEncodeSpecialCases(t *testing.T) {
 
 	// -------------------------------------------------------------------------
 
-	err = l.Shutdown(context.Background())
+	err := l.Shutdown(context.Background())
 	if err != nil {
 		t.Error(err)
 	}
@@ -326,3 +309,16 @@ func TestPublicIDUnmarshalText(t *testing.T) {
 		t.Errorf("allocs = %v; want 0", n)
 	}
 }
+
+func unmarshalOne(t *testing.T, body []byte) map[string]interface{} {
+	t.Helper()
+	var entries []map[string]interface{}
+	err := json.Unmarshal(body, &entries)
+	if err != nil {
+		t.Error(err)
+	}
+	if len(entries) != 1 {
+		t.Fatalf("expected one entry, got %d", len(entries))
+	}
+	return entries[0]
+}