Add DiscardOnSave field to Interaction type

The DiscardOnSave field may be set to true by hooks, in order to
discard an interaction before saving it on disk.

See issue #80 for some background info.

cassette/cassette.go

@@ -123,10 +123,23 @@ type Response struct {
 // Interaction type contains a pair of request/response for a
 // single HTTP interaction between a client and a server
 type Interaction struct {
-	ID       int      `yaml:"id"`
-	Request  Request  `yaml:"request"`
+	// ID is the id of the interaction
+	ID int `yaml:"id"`
+
+	// Request is the recorded request
+	Request Request `yaml:"request"`
+
+	// Response is the recorded response
 	Response Response `yaml:"response"`
-	replayed bool     `yaml:"-"`
+
+	// DiscardOnSave if set to true will discard the interaction
+	// as a whole and it will not be part of the final
+	// interactions when saving the cassette on disk.
+	DiscardOnSave bool `yaml:"-"`
+
+	// replayed is true when this interaction has been played
+	// already.
+	replayed bool `yaml:"-"`
 }
 
 // GetHTTPRequest converts the recorded interaction request to
@@ -301,6 +314,20 @@ func (c *Cassette) Save() error {
 		}
 	}
 
+	// Filter out interactions which should be discarded. While
+	// discarding interactions we should also fix the interaction
+	// IDs, so that we don't introduce gaps in the final results.
+	nextId := 0
+	interactions := make([]*Interaction, 0)
+	for _, i := range c.Interactions {
+		if !i.DiscardOnSave {
+			i.ID = nextId
+			interactions = append(interactions, i)
+			nextId += 1
+		}
+	}
+	c.Interactions = interactions
+
 	// Marshal to YAML and save interactions
 	data, err := yaml.Marshal(c)
 	if err != nil {

recorder/recorder_test.go

@@ -1225,3 +1225,82 @@ func TestInvalidRecorderMode(t *testing.T) {
 		t.Fatal("expected recorder to fail with invalid mode")
 	}
 }
+
+func TestDiscardInteractionsOnSave(t *testing.T) {
+	tests := []testCase{
+		{
+			method:            http.MethodPost,
+			body:              "foo",
+			wantBody:          "POST go-vcr\nfoo",
+			wantStatus:        http.StatusOK,
+			wantContentLength: 15,
+			path:              "/api/v1/foo",
+		},
+		{
+			method:            http.MethodPost,
+			body:              "bar",
+			wantBody:          "POST go-vcr\nbar",
+			wantStatus:        http.StatusOK,
+			wantContentLength: 15,
+			path:              "/api/v1/bar",
+		},
+	}
+
+	server := newEchoHttpServer()
+	serverUrl := server.URL
+
+	cassPath, err := newCassettePath("test_discard_interactions_on_save")
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	// Create recorder
+	rec, err := recorder.New(cassPath)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	if rec.Mode() != recorder.ModeRecordOnce {
+		t.Fatal("recorder is not in the correct mode")
+	}
+
+	if rec.IsRecording() != true {
+		t.Fatal("recorder is not recording")
+	}
+
+	// The following hook function will be used to determine
+	// whether an interaction is to be discarded when saving the
+	// cassette on disk.
+	hook := func(i *cassette.Interaction) error {
+		if i.Request.Method == http.MethodPost && i.Request.Body == "foo" {
+			i.DiscardOnSave = true
+		}
+
+		return nil
+	}
+	rec.AddHook(hook, recorder.AfterCaptureHook)
+
+	ctx := context.Background()
+	client := rec.GetDefaultClient()
+	for _, test := range tests {
+		if err := test.run(client, ctx, serverUrl); err != nil {
+			t.Fatal(err)
+		}
+	}
+
+	// Stop recorder and verify cassette
+	rec.Stop()
+
+	cass, err := cassette.Load(cassPath)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	// We should have one interaction less than our test cases
+	// when reading the cassette from disk.
+	wantInteractions := len(tests) - 1
+	gotInteractions := len(cass.Interactions)
+	if wantInteractions != gotInteractions {
+		t.Fatalf("expected %d interactions, got %d", wantInteractions, gotInteractions)
+	}
+}