Open a channel only when the server accepts the join (closes #197)

When the server refuses a JOIN, POST /api/v1/messages answers 200
with status "error" and sends the error numeric to the queue. Both
clients opened the channel anyway.

JoinChannel in internal/cli/api now returns an error unless the
status is "joined". cmdJoin then keeps its buffer and target and
prints its usual "Join failed" line. The web client opens no tab for
a refused join, whether it came from /join or from the saved
channels a fresh session joins again.

Model: opus-5-5

internal/cli/app_internal_test.go

@@ -1,6 +1,7 @@
 package cli
 
 import (
+	"io"
 	"net/http"
 	"net/http/httptest"
 	"testing"
@@ -138,11 +139,9 @@ func TestOwnCommandShownOnce(t *testing.T) {
 		t.Run(test.name, func(t *testing.T) {
 			t.Parallel()
 
-			// The server accepts every command and answers nothing.
-			server := httptest.NewServer(http.HandlerFunc(
-				func(http.ResponseWriter, *http.Request) {},
-			))
-			t.Cleanup(server.Close)
+			// The server accepts every command. Only the response
+			// to a JOIN is read, and it says the session joined.
+			server := newServer(t, `{"status": "joined"}`)
 
 			app := &App{
 				ui:        NewUI(),
@@ -166,6 +165,60 @@ func TestOwnCommandShownOnce(t *testing.T) {
 	}
 }
 
+// TestJoinResponse checks that /join switches to the channel's
+// buffer and makes it the target only when the server answers
+// that the session joined.
+func TestJoinResponse(t *testing.T) {
+	t.Parallel()
+
+	tests := []struct {
+		name     string
+		response string
+		// The buffer shown and the target after the join.
+		want string
+	}{
+		{
+			name:     "joined",
+			response: `{"status": "joined", "channel": "#chan"}`,
+			want:     testChannel,
+		},
+		{
+			name:     "refused",
+			response: `{"status": "error"}`,
+			want:     testOtherNick,
+		},
+	}
+
+	for _, test := range tests {
+		t.Run(test.name, func(t *testing.T) {
+			t.Parallel()
+
+			app := &App{
+				ui:        NewUI(),
+				client:    api.NewClient(newServer(t, test.response).URL),
+				nick:      testNick,
+				target:    testOtherNick,
+				connected: true,
+			}
+			runUI(t, app.ui)
+			app.ui.SwitchToBuffer(testOtherNick)
+
+			app.cmdJoin(testChannel)
+
+			var shown string
+
+			app.ui.app.QueueUpdate(func() {
+				shown = app.ui.buffers[app.ui.currentBuffer].Name
+			})
+
+			if shown != test.want || app.target != test.want {
+				t.Fatalf("got buffer %s and target %s, want %s",
+					shown, app.target, test.want)
+			}
+		})
+	}
+}
+
 // TestOwnEventFromOtherClient checks that a message, notice,
 // join or part that the user's session makes from another
 // client shows as one line in the buffer it belongs to.
@@ -248,6 +301,21 @@ func bufferState(ui *UI, name string) (int, int) {
 	return lines, unread
 }
 
+// newServer starts a server that answers every request with
+// body until the test ends.
+func newServer(t *testing.T, body string) *httptest.Server {
+	t.Helper()
+
+	server := httptest.NewServer(http.HandlerFunc(
+		func(writer http.ResponseWriter, _ *http.Request) {
+			_, _ = io.WriteString(writer, body)
+		},
+	))
+	t.Cleanup(server.Close)
+
+	return server
+}
+
 // runUI runs ui on a simulated screen until the test ends.
 func runUI(t *testing.T, ui *UI) {
 	t.Helper()