broadcasts (#2707)

* add Chattype::Broadcast and create_broadcast_list()

* do not disclose recipients for broadcasts

* allow sending/add-/remove-member for broadcast

* set broadcast subject same as for one-to-one chats

* broadcast-recipient-list does not include SELF

* use special icon for broadcast groups

* generate initial broadcast names

* make clippy happy

* send BCC message unencrypted to avoid unexpected disclosing; encryption is opportunistic anyway. if we have 'protected chats' at some point, we can think that over.

* reword 'To:'-group

* simplify can-send-check

* add broadcast tests

* tweak comments

* Update deltachat-ffi/deltachat.h

Co-authored-by: Hocuri <hocuri@gmx.de>

* change name of can_edit() to is_self_in_chat()

Co-authored-by: Hocuri <hocuri@gmx.de>

deltachat-ffi/deltachat.h

@@ -1299,6 +1299,8 @@ void            dc_accept_chat               (dc_context_t* context, uint32_t ch
  *   explicitly as it may happen that oneself gets removed from a still existing
  *   group
  *
+ * - for broadcasts, all recipients are returned, DC_CONTACT_ID_SELF is not included
+ *
  * - for mailing lists, the behavior is not documented currently, we will decide on that later.
  *   for now, the UI should not show the list for mailing lists.
  *   (we do not know all members and there is not always a global mailing list address,
@@ -1408,6 +1410,36 @@ dc_chat_t*      dc_get_chat                  (dc_context_t* context, uint32_t ch
 uint32_t        dc_create_group_chat         (dc_context_t* context, int protect, const char* name);
 
 
+/**
+ * Create a new broadcast list.
+ *
+ * Broadcast lists are similar to groups on the sending device,
+ * however, recipients get the messages in normal one-to-one chats
+ * and will not be aware of other members.
+ *
+ * Replies to broadcasts go only to the sender
+ * and not to all broadcast recipients.
+ * Moreover, replies will not appear in the broadcast list
+ * but in the one-to-one chat with the person answering.
+ *
+ * The name and the image of the broadcast list is set automatically
+ * and is visible to the sender only.
+ * Not asking for these data allows more focused creation
+ * and we bypass the question who will get which data.
+ * Also, many users will have at most one broadcast list
+ * so, a generic name and image is sufficient at the first place.
+ *
+ * Later on, however, the name can be changed using dc_set_chat_name().
+ * The image cannot be changed to have a unique, recognizable icon in the chat lists.
+ * All in all, this is also what other messengers are doing here.
+ *
+ * @memberof dc_context_t
+ * @param context The context object.
+ * @return The chat ID of the new broadcast list, 0 on errors.
+ */
+uint32_t        dc_create_broadcast_list     (dc_context_t* context);
+
+
 /**
  * Check if a given contact ID is a member of a group chat.
  *
@@ -3044,6 +3076,11 @@ uint32_t        dc_chat_get_id               (const dc_chat_t* chat);
  *   and cannot be changed using this api.
  *   moreover, for now, mailist lists are read-only.
  *
+ * - @ref DC_CHAT_TYPE_BROADCAST - a broadcast list,
+ *   the recipients will get messages in a one-to-one chats and
+ *   the sender will get answers in a one-to-one as well.
+ *   chats_contacts contain all recipients but DC_CONTACT_ID_SELF
+ *
  * @memberof dc_chat_t
  * @param chat The chat object.
  * @return Chat type.
@@ -4744,6 +4781,11 @@ int64_t          dc_lot_get_timestamp     (const dc_lot_t* lot);
  */
 #define         DC_CHAT_TYPE_MAILINGLIST     140
 
+/**
+ * A broadcast list. See dc_chat_get_type() for details.
+ */
+#define         DC_CHAT_TYPE_BROADCAST       160
+
 /**
  * @}
  */
@@ -5967,6 +6009,11 @@ void dc_event_unref(dc_event_t* event);
 /// Used as a subtitle in quota context; can be plural always.
 #define DC_STR_MESSAGES                   114
 
+/// "Broadcast List"
+///
+/// Used as the default name for broadcast lists; a number may be added.
+#define DC_STR_BROADCAST_LIST             115
+
 /**
  * @}
  */

deltachat-ffi/src/lib.rs

@@ -1330,6 +1330,19 @@ pub unsafe extern "C" fn dc_create_group_chat(
     })
 }
 
+#[no_mangle]
+pub unsafe extern "C" fn dc_create_broadcast_list(context: *mut dc_context_t) -> u32 {
+    if context.is_null() {
+        eprintln!("ignoring careless call to dc_create_broadcast_list()");
+        return 0;
+    }
+    let ctx = &*context;
+    block_on(chat::create_broadcast_list(ctx))
+        .log_err(ctx, "Failed to create broadcast list")
+        .map(|id| id.to_u32())
+        .unwrap_or(0)
+}
+
 #[no_mangle]
 pub unsafe extern "C" fn dc_is_contact_in_chat(
     context: *mut dc_context_t,