download on demand (#2631)

* draft a download-api

* basic implementation

* allow partial downloads for protected chats

* use a separate column for download_state

* force a minimal timeout for delete_server_after in combination with partial messages

* add a warning if a possible download may expire by delete_server_after

* test load_imap_deletion_msgid()

* add a test for a partial download

* improve documentation and visibility

* let get_download_limit() return Result<Option>

* rusty getters

* apply MIN_DELETE_SERVER_AFTER to shown availability time

* move stub-creation to download.rs, use stock-strings, nicer logging

* make clippy happy (cargo clippy --tests)

* refine tests and comments

* fix typo

* remove superfluous closure in ffi

* respect partial_download for immediately scheduled DeleteMsgOnImap jobs

src/job.rs

@@ -102,6 +102,12 @@ pub enum Action {
     MoveMsg = 200,
     DeleteMsgOnImap = 210,
 
+    // This job will download partially downloaded messages completely
+    // and is added when download_full() is called.
+    // Most messages are downloaded automatically on fetch
+    // and do not go through this job.
+    DownloadMsg = 250,
+
     // UID synchronization is high-priority to make sure correct UIDs
     // are used by message moving/deletion.
     ResyncFolders = 300,
@@ -133,6 +139,7 @@ impl From<Action> for Thread {
             MarkseenMsgOnImap => Thread::Imap,
             MoveMsg => Thread::Imap,
             UpdateRecentQuota => Thread::Imap,
+            DownloadMsg => Thread::Imap,
 
             MaybeSendLocations => Thread::Smtp,
             MaybeSendLocationsEnded => Thread::Smtp,
@@ -1155,6 +1162,7 @@ async fn perform_job_action(
             Ok(status) => status,
             Err(err) => Status::Finished(Err(err)),
         },
+        Action::DownloadMsg => job.download_msg(context, connection.inbox()).await,
     };
 
     info!(context, "Finished immediate try {} of job {}", tries, job);
@@ -1219,7 +1227,8 @@ pub async fn add(context: &Context, job: Job) {
             | Action::MarkseenMsgOnImap
             | Action::FetchExistingMsgs
             | Action::MoveMsg
-            | Action::UpdateRecentQuota => {
+            | Action::UpdateRecentQuota
+            | Action::DownloadMsg => {
                 info!(context, "interrupt: imap");
                 context
                     .interrupt_inbox(InterruptInfo::new(false, None))