compose: Disable "Message X" button for DMs when DMs are not allowed.

When in a private narrow, the "Message X" button is disabled if direct
messages are not allowed in the organisation and the current recipient
is not a bot.

Note that when the recipient is a user group with 1, more or all bots,
the button is disabled then too as such PMs are not allowed. Only when
the recipient is a single bot, then it's not disabled, as DMs with one
bot are allowed even in organisations where DMs are disabled.

web/src/compose_closed_ui.js

@@ -53,6 +53,15 @@ export function get_recipient_label(message) {
     return "";
 }
 
+function update_reply_button_state(disable = false, title) {
+    $(".compose_reply_button").attr("disabled", disable);
+    if (!title) {
+        const title_text = $t({defaultMessage: "Reply to selected message"});
+        title = title_text + " (r)";
+    }
+    $(".compose_reply_button").prop("title", title);
+}
+
 function update_stream_button(btn_text, title) {
     $("#left_bar_compose_stream_button_big").text(btn_text);
     $("#left_bar_compose_stream_button_big").prop("title", title);
@@ -63,17 +72,31 @@ function update_conversation_button(btn_text, title) {
     $("#left_bar_compose_private_button_big").prop("title", title);
 }
 
-function update_buttons(text_stream) {
+function update_buttons(text_stream, disable_reply, title_reply) {
     const title_stream = text_stream + " (c)";
     const text_conversation = $t({defaultMessage: "New direct message"});
     const title_conversation = text_conversation + " (x)";
     update_stream_button(text_stream, title_stream);
     update_conversation_button(text_conversation, title_conversation);
+    update_reply_button_state(disable_reply, title_reply);
 }
 
 export function update_buttons_for_private() {
     const text_stream = $t({defaultMessage: "New stream message"});
-    update_buttons(text_stream);
+    if (
+        !narrow_state.pm_ids_string() ||
+        people.user_can_direct_message(narrow_state.pm_ids_string())
+    ) {
+        update_buttons(text_stream);
+        return;
+    }
+    // disable the [Message X] button when in a private narrow
+    // if the user cannot dm the current recipient
+    const disable_reply = true;
+    const title_reply = $t({
+        defaultMessage: "You are not allowed to send private messages in this organization.",
+    });
+    update_buttons(text_stream, disable_reply, title_reply);
 }
 
 export function update_buttons_for_stream() {

web/src/people.js

@@ -702,6 +702,26 @@ export function user_is_bot(user_id) {
     return user.is_bot;
 }
 
+export function user_can_direct_message(recipient_ids_string) {
+    // Common function for checking if a user can send a direct
+    // message to the target user (or group of users) represented by a
+    // user ids string.
+
+    // Regardless of policy, we allow sending private messages to bots.
+    const recipient_ids = user_ids_string_to_ids_array(recipient_ids_string);
+    if (recipient_ids.length === 1 && user_is_bot(recipient_ids[0])) {
+        return true;
+    }
+
+    if (
+        page_params.realm_private_message_policy ===
+        settings_config.private_message_policy_values.disabled.code
+    ) {
+        return false;
+    }
+    return true;
+}
+
 function gravatar_url_for_email(email) {
     const hash = md5(email.toLowerCase());
     const avatar_url = "https://secure.gravatar.com/avatar/" + hash + "?d=identicon";

web/tests/compose.test.js

@@ -12,6 +12,8 @@ const blueslip = require("./lib/zblueslip");
 const $ = require("./lib/zjquery");
 const {page_params, user_settings} = require("./lib/zpage_params");
 
+const settings_config = zrequire("settings_config");
+
 const noop = () => {};
 
 set_global("document", {
@@ -37,6 +39,7 @@ const compose_fade = mock_esm("../src/compose_fade");
 const compose_pm_pill = mock_esm("../src/compose_pm_pill");
 const loading = mock_esm("../src/loading");
 const markdown = mock_esm("../src/markdown");
+const narrow_state = mock_esm("../src/narrow_state");
 const reminder = mock_esm("../src/reminder");
 const rendered_markdown = mock_esm("../src/rendered_markdown");
 const resize = mock_esm("../src/resize");
@@ -77,6 +80,7 @@ const alice = {
     email: "alice@example.com",
     user_id: 31,
     full_name: "Alice",
+    is_bot: false,
 };
 
 const bob = {
@@ -85,12 +89,20 @@ const bob = {
     full_name: "Bob",
 };
 
+const bot = {
+    email: "bot@example.com",
+    user_id: 33,
+    full_name: "Bot",
+    is_bot: true,
+};
+
 people.add_active_user(new_user);
 people.add_active_user(me);
 people.initialize_current_user(me.user_id);
 
 people.add_active_user(alice);
 people.add_active_user(bob);
+people.add_active_user(bot);
 
 const social = {
     stream_id: 101,
@@ -767,7 +779,29 @@ test_ui("create_message_object", ({override, override_rewire}) => {
     people.email_list_to_user_ids_string = email_list_to_user_ids_string;
 });
 
-test_ui("narrow_button_titles", () => {
+test_ui("DM policy disabled", ({override, override_rewire}) => {
+    // Disable dms in the organisation
+    override(
+        page_params,
+        "realm_private_message_policy",
+        settings_config.private_message_policy_values.disabled.code,
+    );
+    let reply_disabled = false;
+    override_rewire(compose_closed_ui, "update_reply_button_state", (disabled = false) => {
+        reply_disabled = disabled;
+    });
+    // For single bot recipient, Bot, the "Message X" button is not disabled
+    override(narrow_state, "pm_ids_string", () => "33");
+    compose_closed_ui.update_buttons_for_private();
+    assert.ok(!reply_disabled);
+    // For human user, Alice, the "Message X" button is disabled
+    override(narrow_state, "pm_ids_string", () => "31");
+    compose_closed_ui.update_buttons_for_private();
+    assert.ok(reply_disabled);
+});
+
+test_ui("narrow_button_titles", ({override}) => {
+    override(narrow_state, "pm_ids_string", () => "31");
     compose_closed_ui.update_buttons_for_private();
     assert.equal(
         $("#left_bar_compose_stream_button_big").text(),