Merge #3889

3889: Display the total number of tasks matching a filter/query r=dureuill a=Kerollmops

This PR returns a new field on the `/tasks` routes. The `total` field exposes the total number of tasks that matches the given filter/query. It is useful to display information on a user interface and can help understand when progress is made in processing tasks, i.e., the total number of tasks on `/tasks?statuses=succeeded` will increase over time.

Fixes #3888.

- [ ] Update the specs fo the `/tasks` route.

## How have I implemented it?

I found it much easier to run two times the task filtering system. Once with the original `from` and `limit` parameters and a second time without. The second call will return the total number of tasks that match the query, not only the number of tasks on the current page.

So far, in terms of performance, there doesn't seem to be any issue. I tried different filters with something like 250k tasks. Note that there is a limit of 1M tasks in the queue.

Co-authored-by: Clément Renault <clement@meilisearch.com>

meilisearch/src/routes/tasks.rs

@@ -325,7 +325,7 @@ async fn cancel_tasks(
 
     let query = params.into_query();
 
-    let tasks = index_scheduler.get_task_ids_from_authorized_indexes(
+    let (tasks, _) = index_scheduler.get_task_ids_from_authorized_indexes(
         &index_scheduler.read_txn()?,
         &query,
         index_scheduler.filters(),
@@ -370,7 +370,7 @@ async fn delete_tasks(
     );
     let query = params.into_query();
 
-    let tasks = index_scheduler.get_task_ids_from_authorized_indexes(
+    let (tasks, _) = index_scheduler.get_task_ids_from_authorized_indexes(
         &index_scheduler.read_txn()?,
         &query,
         index_scheduler.filters(),
@@ -387,6 +387,7 @@ async fn delete_tasks(
 #[derive(Debug, Serialize)]
 pub struct AllTasks {
     results: Vec<TaskView>,
+    total: u64,
     limit: u32,
     from: Option<u32>,
     next: Option<u32>,
@@ -406,23 +407,17 @@ async fn get_tasks(
     let limit = params.limit.0;
     let query = params.into_query();
 
-    let mut tasks_results: Vec<TaskView> = index_scheduler
-        .get_tasks_from_authorized_indexes(query, index_scheduler.filters())?
-        .into_iter()
-        .map(|t| TaskView::from_task(&t))
-        .collect();
+    let filters = index_scheduler.filters();
+    let (tasks, total) = index_scheduler.get_tasks_from_authorized_indexes(query, filters)?;
+    let mut results: Vec<_> = tasks.iter().map(TaskView::from_task).collect();
 
     // If we were able to fetch the number +1 tasks we asked
     // it means that there is more to come.
-    let next = if tasks_results.len() == limit as usize {
-        tasks_results.pop().map(|t| t.uid)
-    } else {
-        None
-    };
+    let next = if results.len() == limit as usize { results.pop().map(|t| t.uid) } else { None };
 
-    let from = tasks_results.first().map(|t| t.uid);
+    let from = results.first().map(|t| t.uid);
+    let tasks = AllTasks { results, limit: limit.saturating_sub(1), total, from, next };
 
-    let tasks = AllTasks { results: tasks_results, limit: limit.saturating_sub(1), from, next };
     Ok(HttpResponse::Ok().json(tasks))
 }
 
@@ -444,10 +439,10 @@ async fn get_task(
     analytics.publish("Tasks Seen".to_string(), json!({ "per_task_uid": true }), Some(&req));
 
     let query = index_scheduler::Query { uids: Some(vec![task_uid]), ..Query::default() };
+    let filters = index_scheduler.filters();
+    let (tasks, _) = index_scheduler.get_tasks_from_authorized_indexes(query, filters)?;
 
-    if let Some(task) =
-        index_scheduler.get_tasks_from_authorized_indexes(query, index_scheduler.filters())?.first()
-    {
+    if let Some(task) = tasks.first() {
         let task_view = TaskView::from_task(task);
         Ok(HttpResponse::Ok().json(task_view))
     } else {