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);
 
@@ -6364,10 +6366,13 @@ 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.
  *
- * 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.
+ * 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 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))

deltachat-jsonrpc/src/api.rs

@@ -278,7 +278,8 @@ impl CommandApi {
 
     /// Performs a background fetch for all accounts in parallel with a timeout.
     ///
-    /// The `AccountsBackgroundFetchDone` event is emitted at the end even in case of timeout.
+    /// The `AccountsBackgroundFetchDone` event is emitted at the end even in case of timeout,
+    /// and immediately if another background fetch is already running.
     /// 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.
     async fn background_fetch(&self, timeout_in_seconds: f64) -> Result<()> {

deltachat-jsonrpc/src/api/types/events.rs

@@ -394,11 +394,15 @@ pub enum EventType {
         msg_id: u32,
     },
 
-    /// Tells that the Background fetch was completed (or timed out).
-    /// 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.
+    /// Tells that a background fetch call is done:
+    /// the fetch completed, timed out, was stopped or was not started.
     ///
-    /// This event is only emitted by the account manager
+    /// For the call that started the fetch, this event acts as a marker:
+    /// all events emitted during the fetch were processed once it is reached.
+    /// 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 is only emitted by the account manager.
     AccountsBackgroundFetchDone,
     /// Inform that set of chats or the order of the chats in the chatlist has changed.
     ///

src/accounts.rs

@@ -443,7 +443,12 @@ impl Accounts {
         interrupt_receiver: Option<Receiver<()>>,
     ) {
         let Some(interrupt_receiver) = interrupt_receiver else {
-            // Nothing to do if we got no interrupt receiver.
+            // Another background fetch is already running.
+            // Emit the event anyway so that a caller waiting for it does not hang.
+            events.emit(Event {
+                id: 0,
+                typ: EventType::AccountsBackgroundFetchDone,
+            });
             return;
         };
         if let Err(_err) = tokio::time::timeout(
@@ -480,6 +485,8 @@ impl Accounts {
     /// The `AccountsBackgroundFetchDone` event is emitted at the end,
     /// 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.
+    /// If another background fetch is already running,
+    /// nothing is fetched and the event is emitted immediately.
     ///
     /// Returns a future that resolves when background fetch is done,
     /// but does not capture `&self`.
@@ -1239,6 +1246,29 @@ mod tests {
         Ok(())
     }
 
+    #[tokio::test(flavor = "multi_thread", worker_threads = 2)]
+    async fn test_background_fetch_emits_done_when_already_running() -> Result<()> {
+        let dir = tempfile::tempdir()?;
+        let writable = true;
+        let accounts = Accounts::new(dir.path().join("accounts"), writable).await?;
+        let event_emitter = accounts.get_event_emitter();
+
+        let timeout = std::time::Duration::from_secs(3);
+        let first = accounts.background_fetch(timeout);
+        let second = accounts.background_fetch(timeout);
+        tokio::join!(first, second);
+
+        let mut done = 0;
+        while let Ok(event) = event_emitter.try_recv() {
+            if matches!(event.typ, EventType::AccountsBackgroundFetchDone) {
+                done += 1;
+            }
+        }
+        assert_eq!(done, 2);
+
+        Ok(())
+    }
+
     #[tokio::test(flavor = "multi_thread", worker_threads = 2)]
     async fn test_encrypted_account() -> Result<()> {
         let dir = tempfile::tempdir().context("failed to create tempdir")?;

src/events/payload.rs

@@ -357,11 +357,15 @@ pub enum EventType {
         msg_id: MsgId,
     },
 
-    /// Tells that the Background fetch was completed (or timed out).
-    /// 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.
+    /// Tells that a background fetch call is done:
+    /// the fetch completed, timed out, was stopped or was not started.
     ///
-    /// This event is only emitted by the account manager
+    /// For the call that started the fetch, this event acts as a marker:
+    /// all events emitted during the fetch were processed once it is reached.
+    /// 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 is only emitted by the account manager.
     AccountsBackgroundFetchDone,
     /// Inform that set of chats or the order of the chats in the chatlist has changed.
     ///