job: generate IMAP deletion jobs dynamically

This prevents generation of a large number of jobs when IMAP deletion
setting is enabled for the first time. Writing jobs to the database
locks it for readers and may cause UI freezing, because chatlist and
messages can't be read until all jobs are written.

Note that on failure job will be written to the database, to make sure
it is postponed instead of being retried immediately.

src/job.rs

@@ -993,39 +993,34 @@ pub fn job_send_msg(context: &Context, msg_id: MsgId) -> Result<()> {
     Ok(())
 }
 
-fn add_imap_deletion_jobs(context: &Context) -> sql::Result<()> {
+fn load_imap_deletion_msgid(context: &Context) -> sql::Result<Option<MsgId>> {
     if let Some(delete_server_after) = context.get_config_delete_server_after() {
         let threshold_timestamp = time() - delete_server_after;
 
-        // Select all expired messages which don't have a
-        // corresponding message deletion job yet.
-        let msg_ids = context.sql.query_map(
+        context.sql.query_row_optional(
             "SELECT id FROM msgs \
              WHERE timestamp < ? \
-             AND server_uid != 0 \
-             AND NOT EXISTS (SELECT 1 FROM jobs WHERE foreign_id = msgs.id \
-             AND action = ?)",
-            params![threshold_timestamp, Action::DeleteMsgOnImap],
+             AND server_uid != 0",
+            params![threshold_timestamp],
             |row| row.get::<_, MsgId>(0),
-            |ids| {
-                ids.collect::<std::result::Result<Vec<_>, _>>()
-                    .map_err(Into::into)
-            },
-        )?;
-
-        // Schedule IMAP deletion for expired messages.
-        for msg_id in msg_ids {
-            job_add(
-                context,
-                Action::DeleteMsgOnImap,
-                msg_id.to_u32() as i32,
-                Params::new(),
-                0,
-            )
-        }
+        )
+    } else {
+        Ok(None)
     }
+}
 
-    Ok(())
+fn load_imap_deletion_job(context: &Context) -> sql::Result<Option<Job>> {
+    let res = if let Some(msg_id) = load_imap_deletion_msgid(context)? {
+        Some(Job::new(
+            Action::DeleteMsgOnImap,
+            msg_id.to_u32(),
+            Params::new(),
+            0,
+        ))
+    } else {
+        None
+    };
+    Ok(res)
 }
 
 pub fn perform_inbox_jobs(context: &Context) {
@@ -1035,9 +1030,6 @@ pub fn perform_inbox_jobs(context: &Context) {
     *context.probe_imap_network.write().unwrap() = false;
     *context.perform_inbox_jobs_needed.write().unwrap() = false;
 
-    if let Err(err) = add_imap_deletion_jobs(context) {
-        warn!(context, "Can't add IMAP message deletion jobs: {}", err);
-    }
     job_perform(context, Thread::Imap, probe_imap_network);
     info!(context, "dc_perform_inbox_jobs ended.",);
 }
@@ -1328,7 +1320,7 @@ fn load_next_job(context: &Context, thread: Thread, probe_network: bool) -> Opti
         params_probe
     };
 
-    context
+    let job = context
         .sql
         .query_map(
             query,
@@ -1357,7 +1349,23 @@ fn load_next_job(context: &Context, thread: Thread, probe_network: bool) -> Opti
                 Ok(None)
             },
         )
-        .unwrap_or_default()
+        .unwrap_or_default();
+
+    if thread == Thread::Imap {
+        if let Some(job) = job {
+            if job.action < Action::DeleteMsgOnImap {
+                load_imap_deletion_job(context)
+                    .unwrap_or_default()
+                    .or(Some(job))
+            } else {
+                Some(job)
+            }
+        } else {
+            load_imap_deletion_job(context).unwrap_or_default()
+        }
+    } else {
+        job
+    }
 }
 
 #[cfg(test)]