Merge #3061

3061: Name spawned threads r=irevoire a=dureuill

# Pull Request

## Related issue
None, this is to improve debuggability

## What does this PR do?
- This PR replaces the raw `thread::spawn(...)` calls by `thread::Builder::new().name(...).spawn(...).unwrap()` calls so that we can give meaningful names to threads. 
- This PR also setup the `rayon` thread pool to give a name to its threads.
- This improves debuggability, as the thread names are reported by debuggers:

<img width="411" alt="Capture d’écran 2022-11-16 à 10 26 27" src="https://user-images.githubusercontent.com/41078892/202141870-a88663aa-d2f8-494f-b4da-709fdbd072ba.png">

(screen showing vscode's debugger and its main/scheduler/indexing threads)

## PR checklist
Please check if your PR fulfills the following requirements:
- [x] Does this PR fix an existing issue, or have you listed the changes applied in the PR description (and why they are needed)?
- [x] Have you read the contributing guidelines?
- [x] Have you made sure that the title is accurate and descriptive of the changes?

Thank you so much for contributing to Meilisearch!


Co-authored-by: Louis Dureuil <louis@meilisearch.com>

index-scheduler/src/index_mapper.rs

@@ -126,7 +126,9 @@ impl IndexMapper {
         let index_map = self.index_map.clone();
         let index_path = self.base_path.join(uuid.to_string());
         let index_name = name.to_string();
-        thread::spawn(move || {
+        thread::Builder::new()
+            .name(String::from("index_deleter"))
+            .spawn(move || {
                 // We first wait to be sure that the previously opened index is effectively closed.
                 // This can take a lot of time, this is why we do that in a seperate thread.
                 if let Some(closing_event) = closing_event {
@@ -143,7 +145,8 @@ impl IndexMapper {
 
                 // Finally we remove the entry from the index map.
                 assert!(matches!(index_map.write().unwrap().remove(&uuid), Some(BeingDeleted)));
-        });
+            })
+            .unwrap();
 
         Ok(())
     }

index-scheduler/src/lib.rs

@@ -412,7 +412,9 @@ impl IndexScheduler {
     /// only once per index scheduler.
     fn run(&self) {
         let run = self.private_clone();
-        std::thread::spawn(move || loop {
+        std::thread::Builder::new()
+            .name(String::from("scheduler"))
+            .spawn(move || loop {
                 run.wake_up.wait();
 
                 match run.tick() {
@@ -433,7 +435,8 @@ impl IndexScheduler {
                         run.wake_up.signal();
                     }
                 }
-        });
+            })
+            .unwrap();
     }
 
     pub fn indexer_config(&self) -> &IndexerConfig {
@@ -925,7 +928,10 @@ impl IndexScheduler {
         // 2. Process the tasks
         let res = {
             let cloned_index_scheduler = self.private_clone();
-            let handle = std::thread::spawn(move || cloned_index_scheduler.process_batch(batch));
+            let handle = std::thread::Builder::new()
+                .name(String::from("batch-operation"))
+                .spawn(move || cloned_index_scheduler.process_batch(batch))
+                .unwrap();
             handle.join().unwrap_or(Err(Error::ProcessBatchPanicked))
         };
 

meilisearch-http/src/lib.rs

@@ -204,12 +204,15 @@ pub fn setup_meilisearch(opt: &Opt) -> anyhow::Result<(Arc<IndexScheduler>, Auth
     if opt.schedule_snapshot {
         let snapshot_delay = Duration::from_secs(opt.snapshot_interval_sec);
         let index_scheduler = index_scheduler.clone();
-        thread::spawn(move || loop {
+        thread::Builder::new()
+            .name(String::from("register-snapshot-tasks"))
+            .spawn(move || loop {
                 thread::sleep(snapshot_delay);
                 if let Err(e) = index_scheduler.register(KindWithContent::SnapshotCreation) {
                     error!("Error while registering snapshot: {}", e);
                 }
-        });
+            })
+            .unwrap();
     }
 
     Ok((index_scheduler, auth_controller))

meilisearch-http/src/option.rs

@@ -501,8 +501,10 @@ impl TryFrom<&IndexerOpts> for IndexerConfig {
     type Error = anyhow::Error;
 
     fn try_from(other: &IndexerOpts) -> Result<Self, Self::Error> {
-        let thread_pool =
-            rayon::ThreadPoolBuilder::new().num_threads(*other.max_indexing_threads).build()?;
+        let thread_pool = rayon::ThreadPoolBuilder::new()
+            .thread_name(|index| format!("indexing-thread:{index}"))
+            .num_threads(*other.max_indexing_threads)
+            .build()?;
 
         Ok(Self {
             log_every_n: Some(other.log_every_n),