fix: always emit AccountsBackgroundFetchDone

A second background_fetch() while one is already running returned
without emitting the event, and the FFI returned 1 for it,
so a UI waiting for an event hangs dc_get_next_event().
Emit the event in any case, so waiting for it is safe.

deltachat-ffi/deltachat.h

@@ -3193,15 +3193,17 @@ void           dc_accounts_maybe_network_lost    (dc_accounts_t* accounts);
  *
  * dc_accounts_background_fetch() was created for the iOS Background fetch.
  *
- * The `DC_EVENT_ACCOUNTS_BACKGROUND_FETCH_DONE` event is emitted at the end
- * even in case of timeout, unless the function fails and returns 0.
+ * The `DC_EVENT_ACCOUNTS_BACKGROUND_FETCH_DONE` event is emitted at the end,
+ * also on timeout, when another background fetch is already running
+ * and when the call is ignored because the timeout is too small,
+ * so it is safe to wait for the event whenever `accounts` is not NULL.
  * Process all events until you get this one and you can safely return to the background
  * without forgetting to create notifications caused by timing race conditions.
  *
  * @memberof dc_accounts_t
  * @param accounts The account manager as created by dc_accounts_new().
  * @param timeout The timeout in seconds
- * @return Return 1 if DC_EVENT_ACCOUNTS_BACKGROUND_FETCH_DONE was emitted and 0 otherwise.
+ * @return Return 0 if the call was ignored because `accounts` is NULL or the timeout is too small, 1 otherwise.
  */
 int            dc_accounts_background_fetch    (dc_accounts_t* accounts, uint64_t timeout);
 
@@ -6364,11 +6366,14 @@ void dc_event_unref(dc_event_t* event);
 #define DC_EVENT_WEBXDC_REALTIME_ADVERTISEMENT    2151
 
 /**
- * Tells that the Background fetch was completed (or timed out).
+ * Tells that a call to dc_accounts_background_fetch() is done:
+ * the fetch completed, timed out, was stopped or was not started.
+ *
+ * For the call that started the fetch, this event acts as a marker:
+ * when you reach it, all events emitted during the fetch were processed.
+ * A call made while another background fetch is running gets the event immediately,
+ * and the running fetch keeps emitting events until its own marker.
  *
- * This event acts as a marker, when you reach this event you can be sure
- * that all events emitted during the background fetch were processed.
- * 
  * This event is only emitted by the account manager
  */
 

deltachat-ffi/src/lib.rs

@@ -4750,12 +4750,17 @@ pub unsafe extern "C" fn dc_accounts_background_fetch(
     accounts: *const dc_accounts_t,
     timeout_in_seconds: u64,
 ) -> libc::c_int {
-    if accounts.is_null() || timeout_in_seconds <= 2 {
+    if accounts.is_null() {
         eprintln!("ignoring careless call to dc_accounts_background_fetch()");
         return 0;
     }
 
     let accounts = unsafe { &*accounts };
+    if timeout_in_seconds <= 2 {
+        eprintln!("ignoring careless call to dc_accounts_background_fetch(): timeout too small");
+        block_on(accounts.read()).emit_event(EventType::AccountsBackgroundFetchDone);
+        return 0;
+    }
     let background_fetch_future = {
         let lock = block_on(accounts.read());
         lock.background_fetch(Duration::from_secs(timeout_in_seconds))