Split _wait and _unref

This also removes BackupProvider::join in favour of implementing
Future directly.  I wondered about implementing a FusedFutre to make
this a little safer but it would introduce a dependency on the futures
crate in deltachat-ffi which did not exist yet, so I didn't do that.

deltachat-ffi/deltachat.h

@@ -2706,13 +2706,21 @@ char * dc_backup_provider_qr_svg (dc_context_t* context, const dc_backup_provide
 /**
  * Waits for the sending to finish and frees the backup provider object.
  *
- * @memberof dc_backup_sender_t
+ * This should only be called once.
+ *
+ * @memberof dc_backup_provider_t
  * @param context The context.
  * @param backup_provider The backup provider object as created by
  *    dc_backup_provider_new().  If NULL is given nothing is done.
  */
 void dc_backup_provider_wait (dc_context_t* context, dc_backup_provider_t* backup_provider);
 
+/**
+ * Frees a dc_backup_provider_t object.
+ *
+ * @memberof dc_backup_provider_t
+ */
+void dc_backup_prvider_unref (dc_backup_provider_t* backup_provider);
 
 /**
  * Gets a backup offered by a dc_backup_provider_t object on another device.

deltachat-ffi/src/lib.rs

@@ -4185,12 +4185,17 @@ pub unsafe extern "C" fn dc_backup_provider_wait(
         return;
     }
     let ctx = &*context;
-    let provider = Box::from_raw(provider);
-    block_on(provider.join())
+    let provider = &mut *provider;
+    block_on(provider)
         .log_err(ctx, "Failed to join provider")
         .ok();
 }
 
+#[no_mangle]
+pub unsafe extern "C" fn dc_backup_provider_unref(provider: *mut dc_backup_provider_t) {
+    drop(Box::from_raw(provider));
+}
+
 #[no_mangle]
 pub unsafe extern "C" fn dc_receive_backup(
     context: *mut dc_context_t,

deltachat-jsonrpc/src/api/mod.rs

@@ -1371,7 +1371,7 @@ impl CommandApi {
                 return Err(err);
             }
         };
-        let res = provider.join().await;
+        let res = provider.await;
         ctx.start_io().await;
         res
     }

deltachat-repl/src/cmdline.rs

@@ -492,7 +492,7 @@ pub async fn cmdline(context: Context, line: &str, chat_id: &mut ChatId) -> Resu
             let provider = BackupProvider::prepare(&context).await?;
             let qr = provider.qr();
             println!("QR code: {}", format_backup(&qr)?);
-            provider.join().await?;
+            provider.await?;
         }
         "receive-backup" => {
             ensure!(!arg1.is_empty(), "Argument <qr> is missing.");

src/imex/transfer.rs

@@ -22,7 +22,10 @@
 //! getter can not connect to an impersonated provider and the provider does not offer the
 //! download to an impersonated getter.
 
+use std::future::Future;
 use std::path::{Path, PathBuf};
+use std::pin::Pin;
+use std::task::Poll;
 
 use anyhow::{anyhow, bail, ensure, format_err, Context as _, Result};
 use async_channel::Receiver;
@@ -57,6 +60,9 @@ use super::{export_database, DeleteOnDrop, DBFILE_BACKUP_NAME};
 ///
 /// This starts a task which acquires the global "ongoing" mutex.  If you need to stop the
 /// task use the [`Context::stop_ongoing`] mechanism.
+///
+/// The task implements [`Future`] and awaiting it will complete once a transfer has been
+/// either completed or aborted.
 #[derive(Debug)]
 pub struct BackupProvider {
     /// The supervisor task, run by [`BackupProvider::watch_provider`].
@@ -244,15 +250,13 @@ impl BackupProvider {
             ticket: self.ticket.clone(),
         }
     }
+}
 
-    /// Awaits the [`BackupProvider`] until it is finished.
-    ///
-    /// This waits until someone connected to the sender and finished transferring a backup.
-    /// A failed transfer also counts as a finished transfer.  If the [`BackupProvider`]
-    /// task results in an error it will be returned here.
-    pub async fn join(self) -> Result<()> {
-        self.handle.await??;
-        Ok(())
+impl Future for BackupProvider {
+    type Output = Result<()>;
+
+    fn poll(mut self: Pin<&mut Self>, cx: &mut std::task::Context<'_>) -> Poll<Self::Output> {
+        Pin::new(&mut self.handle).poll(cx)?
     }
 }
 
@@ -507,7 +511,7 @@ mod tests {
         get_backup(&ctx1, provider.qr()).await.unwrap();
 
         // Make sure the provider finishes without an error.
-        tokio::time::timeout(Duration::from_secs(30), provider.join())
+        tokio::time::timeout(Duration::from_secs(30), provider)
             .await
             .expect("timed out")
             .expect("error in provider");