Best solution that's type-safe but also extensible

Supply an `io.Writer` but check it against the `WriterResetter`
interface to see if we can call `Reset()`.  Which is a neat solution!

Add a new test to check that a supplied `bytes.Buffer` gets reset
between API calls.

mastodon.go

@@ -24,6 +24,11 @@ type Config struct {
 	AccessToken  string
 }
 
+type WriterResetter interface {
+	io.Writer
+	Reset()
+}
+
 // Client is a API client for mastodon.
 type Client struct {
 	http.Client
@@ -128,6 +133,9 @@ func (c *Client) doAPI(ctx context.Context, method string, uri string, params in
 	}
 
 	if c.JSONWriter != nil {
+		if resetter, ok := c.JSONWriter.(WriterResetter); ok {
+			resetter.Reset()
+		}
 		return json.NewDecoder(io.TeeReader(resp.Body, c.JSONWriter)).Decode(&res)
 	} else {
 		return json.NewDecoder(resp.Body).Decode(&res)

status_test.go

@@ -39,6 +39,70 @@ func TestGetFavourites(t *testing.T) {
 	}
 }
 
+func TestGetFavouritesSavedJSONTwice(t *testing.T) {
+	ourJSON := `[{"content": "foo"}, {"content": "bar"}]`
+	ts := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		fmt.Fprintln(w, ourJSON)
+	}))
+	defer ts.Close()
+
+	client := NewClient(&Config{
+		Server:       ts.URL,
+		ClientID:     "foo",
+		ClientSecret: "bar",
+		AccessToken:  "zoo",
+	})
+
+	var buf bytes.Buffer
+	client.JSONWriter = &buf
+
+	favs, err := client.GetFavourites(context.Background(), nil)
+	if err != nil {
+		t.Fatalf("should not be fail: %v", err)
+	}
+	if len(favs) != 2 {
+		t.Fatalf("result should be two: %d", len(favs))
+	}
+	if favs[0].Content != "foo" {
+		t.Fatalf("want %q but %q", "foo", favs[0].Content)
+	}
+	if favs[1].Content != "bar" {
+		t.Fatalf("want %q but %q", "bar", favs[1].Content)
+	}
+
+	// We get a trailing `\n` from the API which we need to trim
+	// off before we compare it with our literal above.
+	theirJSON := strings.TrimSpace(string(buf.Bytes()))
+
+	if theirJSON != ourJSON {
+		t.Fatalf("want %q but %q", ourJSON, theirJSON)
+	}
+
+	// Now we call the API again to see if we get the same or doubled JSON.
+
+	favs, err = client.GetFavourites(context.Background(), nil)
+	if err != nil {
+		t.Fatalf("should not be fail: %v", err)
+	}
+	if len(favs) != 2 {
+		t.Fatalf("result should be two: %d", len(favs))
+	}
+	if favs[0].Content != "foo" {
+		t.Fatalf("want %q but %q", "foo", favs[0].Content)
+	}
+	if favs[1].Content != "bar" {
+		t.Fatalf("want %q but %q", "bar", favs[1].Content)
+	}
+
+	// We get a trailing `\n` from the API which we need to trim
+	// off before we compare it with our literal above.
+	theirJSON = strings.TrimSpace(string(buf.Bytes()))
+
+	if theirJSON != ourJSON {
+		t.Fatalf("want %q but %q", ourJSON, theirJSON)
+	}
+}
+
 func TestGetFavouritesSavedJSON(t *testing.T) {
 	ourJSON := `[{"content": "foo"}, {"content": "bar"}]`
 	ts := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {