types/logger: add AsJSON

Printing out JSON representation things in log output is pretty common.

Updates #cleanup

Change-Id: Ife2d2e321a18e6e1185efa8b699a23061ac5e5a4
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

types/logger/logger.go

@@ -353,3 +353,23 @@ func LogfCloser(logf Logf) (newLogf Logf, close func()) {
 	}
 	return newLogf, close
 }
+
+// AsJSON returns a formatter that formats v as JSON. The value is suitable to
+// passing to a regular %v printf argument. (%s is not required)
+//
+// If json.Marshal returns an error, the output is "%%!JSON-ERROR:" followed by
+// the error string.
+func AsJSON(v any) fmt.Formatter {
+	return asJSONResult{v}
+}
+
+type asJSONResult struct{ v any }
+
+func (a asJSONResult) Format(s fmt.State, verb rune) {
+	v, err := json.Marshal(a.v)
+	if err != nil {
+		fmt.Fprintf(s, "%%!JSON-ERROR:%v", err)
+		return
+	}
+	s.Write(v)
+}

types/logger/logger_test.go

@@ -221,3 +221,33 @@ func TestJSON(t *testing.T) {
 		t.Errorf("mismatch\n got: %q\nwant: %q\n", got, want)
 	}
 }
+
+func TestAsJSON(t *testing.T) {
+	got := fmt.Sprintf("got %v", AsJSON(struct {
+		Foo string
+		Bar int
+	}{"hi", 123}))
+	const want = `got {"Foo":"hi","Bar":123}`
+	if got != want {
+		t.Errorf("got %#q; want %#q", got, want)
+	}
+
+	got = fmt.Sprintf("got %v", AsJSON(func() {}))
+	const wantErr = `got %!JSON-ERROR:json: unsupported type: func()`
+	if got != wantErr {
+		t.Errorf("for marshal error, got %#q; want %#q", got, wantErr)
+	}
+
+	var buf bytes.Buffer
+	n := int(testing.AllocsPerRun(1000, func() {
+		buf.Reset()
+		fmt.Fprintf(&buf, "got %v", AsJSON("hi"))
+	}))
+	if n > 2 {
+		// the JSON AsMarshal itself + boxing
+		// the asJSONResult into an interface (which needs
+		// to happen at some point to get to fmt, so might
+		// as well return an interface from AsJSON))
+		t.Errorf("allocs = %v; want max 2", n)
+	}
+}