Move crates under a sub folder to clean up the code

crates/index-scheduler/src/index_mapper/index_map.rs

@@ -0,0 +1,390 @@
+use std::collections::BTreeMap;
+use std::path::Path;
+use std::time::Duration;
+
+use meilisearch_types::heed::{EnvClosingEvent, EnvFlags, EnvOpenOptions};
+use meilisearch_types::milli::Index;
+use time::OffsetDateTime;
+use uuid::Uuid;
+
+use super::IndexStatus::{self, Available, BeingDeleted, Closing, Missing};
+use crate::lru::{InsertionOutcome, LruMap};
+use crate::{clamp_to_page_size, Result};
+
+/// Keep an internally consistent view of the open indexes in memory.
+///
+/// This view is made of an LRU cache that will evict the least frequently used indexes when new indexes are opened.
+/// Indexes that are being closed (for resizing or due to cache eviction) or deleted cannot be evicted from the cache and
+/// are stored separately.
+///
+/// This view provides operations to change the state of the index as it is known in memory:
+/// open an index (making it available for queries), close an index (specifying the new size it should be opened with),
+/// delete an index.
+///
+/// External consistency with the other bits of data of an index is provided by the `IndexMapper` parent structure.
+pub struct IndexMap {
+    /// A LRU map of indexes that are in the open state and available for queries.
+    available: LruMap<Uuid, Index>,
+    /// A map of indexes that are not available for queries, either because they are being deleted
+    /// or because they are being closed.
+    ///
+    /// If they are being deleted, the UUID points to `None`.
+    unavailable: BTreeMap<Uuid, Option<ClosingIndex>>,
+
+    /// A monotonically increasing generation number, used to differentiate between multiple successive index closing requests.
+    ///
+    /// Because multiple readers could be waiting on an index to close, the following could theoretically happen:
+    ///
+    /// 1. Multiple readers wait for the index closing to occur.
+    /// 2. One of them "wins the race", takes the lock and then removes the index that finished closing from the map.
+    /// 3. The index is reopened, but must be closed again (such as being resized again).
+    /// 4. One reader that "lost the race" in (2) wakes up and tries to take the lock and remove the index from the map.
+    ///
+    /// In that situation, the index may or may not have finished closing. The `generation` field allows to remember which
+    /// closing request was made, so the reader that "lost the race" has the old generation and will need to wait again for the index
+    /// to close.
+    generation: usize,
+}
+
+#[derive(Clone)]
+pub struct ClosingIndex {
+    uuid: Uuid,
+    closing_event: EnvClosingEvent,
+    enable_mdb_writemap: bool,
+    map_size: usize,
+    generation: usize,
+}
+
+impl ClosingIndex {
+    /// Waits for the index to be definitely closed.
+    ///
+    /// To avoid blocking, users should relinquish their locks to the IndexMap before calling this function.
+    ///
+    /// After the index is physically closed, the in memory map must still be updated to take this into account.
+    /// To do so, a `ReopenableIndex` is returned, that can be used to either definitely close or definitely open
+    /// the index without waiting anymore.
+    pub fn wait_timeout(self, timeout: Duration) -> Option<ReopenableIndex> {
+        self.closing_event.wait_timeout(timeout).then_some(ReopenableIndex {
+            uuid: self.uuid,
+            enable_mdb_writemap: self.enable_mdb_writemap,
+            map_size: self.map_size,
+            generation: self.generation,
+        })
+    }
+}
+
+pub struct ReopenableIndex {
+    uuid: Uuid,
+    enable_mdb_writemap: bool,
+    map_size: usize,
+    generation: usize,
+}
+
+impl ReopenableIndex {
+    /// Attempts to reopen the index, which can result in the index being reopened again or not
+    /// (e.g. if another thread already opened and closed the index again).
+    ///
+    /// Use get again on the IndexMap to get the updated status.
+    ///
+    /// Fails if the underlying index creation fails.
+    ///
+    /// # Status table
+    ///
+    /// | Previous Status | New Status                                   |
+    /// |-----------------|----------------------------------------------|
+    /// | Missing         | Missing                                      |
+    /// | BeingDeleted    | BeingDeleted                                 |
+    /// | Closing         | Available or Closing depending on generation |
+    /// | Available       | Available                                    |
+    ///
+    pub fn reopen(self, map: &mut IndexMap, path: &Path) -> Result<()> {
+        if let Closing(reopen) = map.get(&self.uuid) {
+            if reopen.generation != self.generation {
+                return Ok(());
+            }
+            map.unavailable.remove(&self.uuid);
+            map.create(&self.uuid, path, None, self.enable_mdb_writemap, self.map_size)?;
+        }
+        Ok(())
+    }
+
+    /// Attempts to close the index, which may or may not result in the index being closed
+    /// (e.g. if another thread already reopened the index again).
+    ///
+    /// Use get again on the IndexMap to get the updated status.
+    ///
+    /// # Status table
+    ///
+    /// | Previous Status | New Status                                 |
+    /// |-----------------|--------------------------------------------|
+    /// | Missing         | Missing                                    |
+    /// | BeingDeleted    | BeingDeleted                               |
+    /// | Closing         | Missing or Closing depending on generation |
+    /// | Available       | Available                                  |
+    pub fn close(self, map: &mut IndexMap) {
+        if let Closing(reopen) = map.get(&self.uuid) {
+            if reopen.generation != self.generation {
+                return;
+            }
+            map.unavailable.remove(&self.uuid);
+        }
+    }
+}
+
+impl IndexMap {
+    pub fn new(cap: usize) -> IndexMap {
+        Self { unavailable: Default::default(), available: LruMap::new(cap), generation: 0 }
+    }
+
+    /// Gets the current status of an index in the map.
+    ///
+    /// If the index is available it can be accessed from the returned status.
+    pub fn get(&self, uuid: &Uuid) -> IndexStatus {
+        self.available
+            .get(uuid)
+            .map(|index| Available(index.clone()))
+            .unwrap_or_else(|| self.get_unavailable(uuid))
+    }
+
+    fn get_unavailable(&self, uuid: &Uuid) -> IndexStatus {
+        match self.unavailable.get(uuid) {
+            Some(Some(reopen)) => Closing(reopen.clone()),
+            Some(None) => BeingDeleted,
+            None => Missing,
+        }
+    }
+
+    /// Attempts to create a new index that wasn't existing before.
+    ///
+    /// # Status table
+    ///
+    /// | Previous Status | New Status |
+    /// |-----------------|------------|
+    /// | Missing         | Available  |
+    /// | BeingDeleted    | panics     |
+    /// | Closing         | panics     |
+    /// | Available       | panics     |
+    ///
+    pub fn create(
+        &mut self,
+        uuid: &Uuid,
+        path: &Path,
+        date: Option<(OffsetDateTime, OffsetDateTime)>,
+        enable_mdb_writemap: bool,
+        map_size: usize,
+    ) -> Result<Index> {
+        if !matches!(self.get_unavailable(uuid), Missing) {
+            panic!("Attempt to open an index that was unavailable");
+        }
+        let index = create_or_open_index(path, date, enable_mdb_writemap, map_size)?;
+        match self.available.insert(*uuid, index.clone()) {
+            InsertionOutcome::InsertedNew => (),
+            InsertionOutcome::Evicted(evicted_uuid, evicted_index) => {
+                self.close(evicted_uuid, evicted_index, enable_mdb_writemap, 0);
+            }
+            InsertionOutcome::Replaced(_) => {
+                panic!("Attempt to open an index that was already opened")
+            }
+        }
+        Ok(index)
+    }
+
+    /// Increases the current generation. See documentation for this field.
+    ///
+    /// In the unlikely event that the 2^64 generations would have been exhausted, we simply wrap-around.
+    ///
+    /// For this to cause an issue, one should be able to stop a reader in time after it got a `ReopenableIndex` and before it takes the lock
+    /// to remove it from the unavailable map, and keep the reader in this frozen state for 2^64 closing of other indexes.
+    ///
+    /// This seems overwhelmingly impossible to achieve in practice.
+    fn next_generation(&mut self) -> usize {
+        self.generation = self.generation.wrapping_add(1);
+        self.generation
+    }
+
+    /// Attempts to close an index.
+    ///
+    /// # Status table
+    ///
+    /// | Previous Status | New Status    |
+    /// |-----------------|---------------|
+    /// | Missing         | Missing       |
+    /// | BeingDeleted    | BeingDeleted  |
+    /// | Closing         | Closing       |
+    /// | Available       | Closing       |
+    ///
+    pub fn close_for_resize(
+        &mut self,
+        uuid: &Uuid,
+        enable_mdb_writemap: bool,
+        map_size_growth: usize,
+    ) {
+        let Some(index) = self.available.remove(uuid) else {
+            return;
+        };
+        self.close(*uuid, index, enable_mdb_writemap, map_size_growth);
+    }
+
+    fn close(
+        &mut self,
+        uuid: Uuid,
+        index: Index,
+        enable_mdb_writemap: bool,
+        map_size_growth: usize,
+    ) {
+        let map_size = index.map_size() + map_size_growth;
+        let closing_event = index.prepare_for_closing();
+        let generation = self.next_generation();
+        self.unavailable.insert(
+            uuid,
+            Some(ClosingIndex { uuid, closing_event, enable_mdb_writemap, map_size, generation }),
+        );
+    }
+
+    /// Attempts to delete and index.
+    ///
+    ///  `end_deletion` must be called just after.
+    ///
+    /// # Status table
+    ///
+    /// | Previous Status | New Status   | Return value                |
+    /// |-----------------|--------------|-----------------------------|
+    /// | Missing         | BeingDeleted | Ok(None)                    |
+    /// | BeingDeleted    | BeingDeleted | Err(None)                   |
+    /// | Closing         | Closing      | Err(Some(reopen))           |
+    /// | Available       | BeingDeleted | Ok(Some(env_closing_event)) |
+    pub fn start_deletion(
+        &mut self,
+        uuid: &Uuid,
+    ) -> std::result::Result<Option<EnvClosingEvent>, Option<ClosingIndex>> {
+        if let Some(index) = self.available.remove(uuid) {
+            self.unavailable.insert(*uuid, None);
+            return Ok(Some(index.prepare_for_closing()));
+        }
+        match self.unavailable.remove(uuid) {
+            Some(Some(reopen)) => Err(Some(reopen)),
+            Some(None) => Err(None),
+            None => Ok(None),
+        }
+    }
+
+    /// Marks that an index deletion finished.
+    ///
+    /// Must be used after calling `start_deletion`.
+    ///
+    /// # Status table
+    ///
+    /// | Previous Status | New Status |
+    /// |-----------------|------------|
+    /// | Missing         | Missing    |
+    /// | BeingDeleted    | Missing    |
+    /// | Closing         | panics     |
+    /// | Available       | panics     |
+    pub fn end_deletion(&mut self, uuid: &Uuid) {
+        assert!(
+            self.available.get(uuid).is_none(),
+            "Attempt to finish deletion of an index that was not being deleted"
+        );
+        // Do not panic if the index was Missing or BeingDeleted
+        assert!(
+            !matches!(self.unavailable.remove(uuid), Some(Some(_))),
+            "Attempt to finish deletion of an index that was being closed"
+        );
+    }
+}
+
+/// Create or open an index in the specified path.
+/// The path *must* exist or an error will be thrown.
+fn create_or_open_index(
+    path: &Path,
+    date: Option<(OffsetDateTime, OffsetDateTime)>,
+    enable_mdb_writemap: bool,
+    map_size: usize,
+) -> Result<Index> {
+    let mut options = EnvOpenOptions::new();
+    options.map_size(clamp_to_page_size(map_size));
+    options.max_readers(1024);
+    if enable_mdb_writemap {
+        unsafe { options.flags(EnvFlags::WRITE_MAP) };
+    }
+
+    if let Some((created, updated)) = date {
+        Ok(Index::new_with_creation_dates(options, path, created, updated)?)
+    } else {
+        Ok(Index::new(options, path)?)
+    }
+}
+
+/// Putting the tests of the LRU down there so we have access to the cache's private members
+#[cfg(test)]
+mod tests {
+
+    use meilisearch_types::heed::Env;
+    use meilisearch_types::Index;
+    use uuid::Uuid;
+
+    use super::super::IndexMapper;
+    use crate::tests::IndexSchedulerHandle;
+    use crate::utils::clamp_to_page_size;
+    use crate::IndexScheduler;
+
+    impl IndexMapper {
+        fn test() -> (Self, Env, IndexSchedulerHandle) {
+            let (index_scheduler, handle) = IndexScheduler::test(true, vec![]);
+            (index_scheduler.index_mapper, index_scheduler.env, handle)
+        }
+    }
+
+    fn check_first_unavailable(mapper: &IndexMapper, expected_uuid: Uuid, is_closing: bool) {
+        let index_map = mapper.index_map.read().unwrap();
+        let (uuid, state) = index_map.unavailable.first_key_value().unwrap();
+        assert_eq!(uuid, &expected_uuid);
+        assert_eq!(state.is_some(), is_closing);
+    }
+
+    #[test]
+    fn evict_indexes() {
+        let (mapper, env, _handle) = IndexMapper::test();
+        let mut uuids = vec![];
+        // LRU cap + 1
+        for i in 0..(5 + 1) {
+            let index_name = format!("index-{i}");
+            let wtxn = env.write_txn().unwrap();
+            mapper.create_index(wtxn, &index_name, None).unwrap();
+            let txn = env.read_txn().unwrap();
+            uuids.push(mapper.index_mapping.get(&txn, &index_name).unwrap().unwrap());
+        }
+        // index-0 was evicted
+        check_first_unavailable(&mapper, uuids[0], true);
+
+        // get back the evicted index
+        let wtxn = env.write_txn().unwrap();
+        mapper.create_index(wtxn, "index-0", None).unwrap();
+
+        // Least recently used is now index-1
+        check_first_unavailable(&mapper, uuids[1], true);
+    }
+
+    #[test]
+    fn resize_index() {
+        let (mapper, env, _handle) = IndexMapper::test();
+        let index = mapper.create_index(env.write_txn().unwrap(), "index", None).unwrap();
+        assert_index_size(index, mapper.index_base_map_size);
+
+        mapper.resize_index(&env.read_txn().unwrap(), "index").unwrap();
+
+        let index = mapper.create_index(env.write_txn().unwrap(), "index", None).unwrap();
+        assert_index_size(index, mapper.index_base_map_size + mapper.index_growth_amount);
+
+        mapper.resize_index(&env.read_txn().unwrap(), "index").unwrap();
+
+        let index = mapper.create_index(env.write_txn().unwrap(), "index", None).unwrap();
+        assert_index_size(index, mapper.index_base_map_size + mapper.index_growth_amount * 2);
+    }
+
+    fn assert_index_size(index: Index, expected: usize) {
+        let expected = clamp_to_page_size(expected);
+        let index_map_size = index.map_size();
+        assert_eq!(index_map_size, expected);
+    }
+}

crates/index-scheduler/src/index_mapper/mod.rs

@@ -0,0 +1,496 @@
+use std::path::PathBuf;
+use std::sync::{Arc, RwLock};
+use std::time::Duration;
+use std::{fs, thread};
+
+use meilisearch_types::heed::types::{SerdeJson, Str};
+use meilisearch_types::heed::{Database, Env, RoTxn, RwTxn};
+use meilisearch_types::milli::update::IndexerConfig;
+use meilisearch_types::milli::{FieldDistribution, Index};
+use serde::{Deserialize, Serialize};
+use time::OffsetDateTime;
+use tracing::error;
+use uuid::Uuid;
+
+use self::index_map::IndexMap;
+use self::IndexStatus::{Available, BeingDeleted, Closing, Missing};
+use crate::uuid_codec::UuidCodec;
+use crate::{Error, Result};
+
+mod index_map;
+
+const INDEX_MAPPING: &str = "index-mapping";
+const INDEX_STATS: &str = "index-stats";
+
+/// Structure managing meilisearch's indexes.
+///
+/// It is responsible for:
+/// 1. Creating new indexes
+/// 2. Opening indexes and storing references to these opened indexes
+/// 3. Accessing indexes through their uuid
+/// 4. Mapping a user-defined name to each index uuid.
+///
+/// # Implementation notes
+///
+/// An index exists as 3 bits of data:
+/// 1. The index data on disk, that can exist in 3 states: Missing, Present, or BeingDeleted.
+/// 2. The persistent database containing the association between the index' name and its UUID,
+///    that can exist in 2 states: Missing or Present.
+/// 3. The state of the index in the in-memory `IndexMap`, that can exist in multiple states:
+///   - Missing
+///   - Available
+///   - Closing (because an index needs resizing or was evicted from the cache)
+///   - BeingDeleted
+///
+/// All of this data should be kept consistent between index operations, which is achieved by the `IndexMapper`
+/// with the use of the following primitives:
+/// - A RwLock on the `IndexMap`.
+/// - Transactions on the association database.
+/// - ClosingEvent signals emitted when closing an environment.
+#[derive(Clone)]
+pub struct IndexMapper {
+    /// Keep track of the opened indexes. Used mainly by the index resolver.
+    index_map: Arc<RwLock<IndexMap>>,
+
+    /// Map an index name with an index uuid currently available on disk.
+    pub(crate) index_mapping: Database<Str, UuidCodec>,
+    /// Map an index UUID with the cached stats associated to the index.
+    ///
+    /// Using an UUID forces to use the index_mapping table to recover the index behind a name, ensuring
+    /// consistency wrt index swapping.
+    pub(crate) index_stats: Database<UuidCodec, SerdeJson<IndexStats>>,
+
+    /// Path to the folder where the LMDB environments of each index are.
+    base_path: PathBuf,
+    /// The map size an index is opened with on the first time.
+    index_base_map_size: usize,
+    /// The quantity by which the map size of an index is incremented upon reopening, in bytes.
+    index_growth_amount: usize,
+    /// Whether we open a meilisearch index with the MDB_WRITEMAP option or not.
+    enable_mdb_writemap: bool,
+    pub indexer_config: Arc<IndexerConfig>,
+
+    /// A few types of long running batches of tasks that act on a single index set this field
+    /// so that a handle to the index is available from other threads (search) in an optimized manner.
+    currently_updating_index: Arc<RwLock<Option<(String, Index)>>>,
+}
+
+/// Whether the index is available for use or is forbidden to be inserted back in the index map
+#[allow(clippy::large_enum_variant)]
+#[derive(Clone)]
+pub enum IndexStatus {
+    /// Not currently in the index map.
+    Missing,
+    /// Do not insert it back in the index map as it is currently being deleted.
+    BeingDeleted,
+    /// Temporarily do not insert the index in the index map as it is currently being resized/evicted from the map.
+    Closing(index_map::ClosingIndex),
+    /// You can use the index without worrying about anything.
+    Available(Index),
+}
+
+/// The statistics that can be computed from an `Index` object.
+#[derive(Serialize, Deserialize, Debug)]
+pub struct IndexStats {
+    /// Number of documents in the index.
+    pub number_of_documents: u64,
+    /// Size taken up by the index' DB, in bytes.
+    ///
+    /// This includes the size taken by both the used and free pages of the DB, and as the free pages
+    /// are not returned to the disk after a deletion, this number is typically larger than
+    /// `used_database_size` that only includes the size of the used pages.
+    pub database_size: u64,
+    /// Size taken by the used pages of the index' DB, in bytes.
+    ///
+    /// As the DB backend does not return to the disk the pages that are not currently used by the DB,
+    /// this value is typically smaller than `database_size`.
+    pub used_database_size: u64,
+    /// Association of every field name with the number of times it occurs in the documents.
+    pub field_distribution: FieldDistribution,
+    /// Creation date of the index.
+    #[serde(with = "time::serde::rfc3339")]
+    pub created_at: OffsetDateTime,
+    /// Date of the last update of the index.
+    #[serde(with = "time::serde::rfc3339")]
+    pub updated_at: OffsetDateTime,
+}
+
+impl IndexStats {
+    /// Compute the stats of an index
+    ///
+    /// # Parameters
+    ///
+    /// - rtxn: a RO transaction for the index, obtained from `Index::read_txn()`.
+    pub fn new(index: &Index, rtxn: &RoTxn) -> Result<Self> {
+        Ok(IndexStats {
+            number_of_documents: index.number_of_documents(rtxn)?,
+            database_size: index.on_disk_size()?,
+            used_database_size: index.used_size()?,
+            field_distribution: index.field_distribution(rtxn)?,
+            created_at: index.created_at(rtxn)?,
+            updated_at: index.updated_at(rtxn)?,
+        })
+    }
+}
+
+impl IndexMapper {
+    pub fn new(
+        env: &Env,
+        base_path: PathBuf,
+        index_base_map_size: usize,
+        index_growth_amount: usize,
+        index_count: usize,
+        enable_mdb_writemap: bool,
+        indexer_config: IndexerConfig,
+    ) -> Result<Self> {
+        let mut wtxn = env.write_txn()?;
+        let index_mapping = env.create_database(&mut wtxn, Some(INDEX_MAPPING))?;
+        let index_stats = env.create_database(&mut wtxn, Some(INDEX_STATS))?;
+        wtxn.commit()?;
+
+        Ok(Self {
+            index_map: Arc::new(RwLock::new(IndexMap::new(index_count))),
+            index_mapping,
+            index_stats,
+            base_path,
+            index_base_map_size,
+            index_growth_amount,
+            enable_mdb_writemap,
+            indexer_config: Arc::new(indexer_config),
+            currently_updating_index: Default::default(),
+        })
+    }
+
+    /// Get or create the index.
+    pub fn create_index(
+        &self,
+        mut wtxn: RwTxn,
+        name: &str,
+        date: Option<(OffsetDateTime, OffsetDateTime)>,
+    ) -> Result<Index> {
+        match self.index(&wtxn, name) {
+            Ok(index) => {
+                wtxn.commit()?;
+                Ok(index)
+            }
+            Err(Error::IndexNotFound(_)) => {
+                let uuid = Uuid::new_v4();
+                self.index_mapping.put(&mut wtxn, name, &uuid)?;
+
+                let index_path = self.base_path.join(uuid.to_string());
+                fs::create_dir_all(&index_path)?;
+
+                // Error if the UUIDv4 somehow already exists in the map, since it should be fresh.
+                // This is very unlikely to happen in practice.
+                // TODO: it would be better to lazily create the index. But we need an Index::open function for milli.
+                let index = self.index_map.write().unwrap().create(
+                    &uuid,
+                    &index_path,
+                    date,
+                    self.enable_mdb_writemap,
+                    self.index_base_map_size,
+                )?;
+
+                wtxn.commit()?;
+
+                Ok(index)
+            }
+            error => error,
+        }
+    }
+
+    /// Removes the index from the mapping table and the in-memory index map
+    /// but keeps the associated tasks.
+    pub fn delete_index(&self, mut wtxn: RwTxn, name: &str) -> Result<()> {
+        let uuid = self
+            .index_mapping
+            .get(&wtxn, name)?
+            .ok_or_else(|| Error::IndexNotFound(name.to_string()))?;
+
+        // Not an error if the index had no stats in cache.
+        self.index_stats.delete(&mut wtxn, &uuid)?;
+
+        // Once we retrieved the UUID of the index we remove it from the mapping table.
+        assert!(self.index_mapping.delete(&mut wtxn, name)?);
+
+        wtxn.commit()?;
+
+        let mut tries = 0;
+        // Attempts to remove the index from the in-memory index map in a loop.
+        //
+        // If the index is currently being closed, we will wait for it to be closed and retry getting it in a subsequent
+        // loop iteration.
+        //
+        // We make 100 attempts before giving up.
+        // This could happen in the following situations:
+        //
+        // 1. There is a bug preventing the index from being correctly closed, or us from detecting this.
+        // 2. A user of the index is keeping it open for more than 600 seconds. This could happen e.g. during a pathological search.
+        //    This can not be caused by indexation because deleting an index happens in the scheduler itself, so cannot be concurrent with indexation.
+        //
+        // In these situations, reporting the error through a panic is in order.
+        let closing_event = loop {
+            let mut lock = self.index_map.write().unwrap();
+            match lock.start_deletion(&uuid) {
+                Ok(env_closing) => break env_closing,
+                Err(Some(reopen)) => {
+                    // drop the lock here so that we don't synchronously wait for the index to close.
+                    drop(lock);
+                    tries += 1;
+                    if tries >= 100 {
+                        panic!("Too many attempts to close index {name} prior to deletion.")
+                    }
+                    let reopen = if let Some(reopen) = reopen.wait_timeout(Duration::from_secs(6)) {
+                        reopen
+                    } else {
+                        continue;
+                    };
+                    reopen.close(&mut self.index_map.write().unwrap());
+                    continue;
+                }
+                Err(None) => return Ok(()),
+            }
+        };
+
+        let index_map = self.index_map.clone();
+        let index_path = self.base_path.join(uuid.to_string());
+        let index_name = name.to_string();
+        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 separate thread.
+                if let Some(closing_event) = closing_event {
+                    closing_event.wait();
+                }
+
+                // Then we remove the content from disk.
+                if let Err(e) = fs::remove_dir_all(&index_path) {
+                    error!(
+                        "An error happened when deleting the index {} ({}): {}",
+                        index_name, uuid, e
+                    );
+                }
+
+                // Finally we remove the entry from the index map.
+                index_map.write().unwrap().end_deletion(&uuid);
+            })
+            .unwrap();
+
+        Ok(())
+    }
+
+    pub fn exists(&self, rtxn: &RoTxn, name: &str) -> Result<bool> {
+        Ok(self.index_mapping.get(rtxn, name)?.is_some())
+    }
+
+    /// Resizes the maximum size of the specified index to the double of its current maximum size.
+    ///
+    /// This operation involves closing the underlying environment and so can take a long time to complete.
+    ///
+    /// # Panics
+    ///
+    /// - If the Index corresponding to the passed name is concurrently being deleted/resized or cannot be found in the
+    ///   in memory hash map.
+    pub fn resize_index(&self, rtxn: &RoTxn, name: &str) -> Result<()> {
+        let uuid = self
+            .index_mapping
+            .get(rtxn, name)?
+            .ok_or_else(|| Error::IndexNotFound(name.to_string()))?;
+
+        // We remove the index from the in-memory index map.
+        self.index_map.write().unwrap().close_for_resize(
+            &uuid,
+            self.enable_mdb_writemap,
+            self.index_growth_amount,
+        );
+
+        Ok(())
+    }
+
+    /// Return an index, may open it if it wasn't already opened.
+    pub fn index(&self, rtxn: &RoTxn, name: &str) -> Result<Index> {
+        if let Some((current_name, current_index)) =
+            self.currently_updating_index.read().unwrap().as_ref()
+        {
+            if current_name == name {
+                return Ok(current_index.clone());
+            }
+        }
+
+        let uuid = self
+            .index_mapping
+            .get(rtxn, name)?
+            .ok_or_else(|| Error::IndexNotFound(name.to_string()))?;
+
+        let mut tries = 0;
+        // attempts to open the index in a loop.
+        //
+        // If the index is currently being closed, we will wait for it to be closed and retry getting it in a subsequent
+        // loop iteration.
+        //
+        // We make 100 attempts before giving up.
+        // This could happen in the following situations:
+        //
+        // 1. There is a bug preventing the index from being correctly closed, or us from detecting it was.
+        // 2. A user of the index is keeping it open for more than 600 seconds. This could happen e.g. during a long indexation,
+        //    a pathological search, and so on.
+        //
+        // In these situations, reporting the error through a panic is in order.
+        let index = loop {
+            tries += 1;
+            if tries > 100 {
+                panic!("Too many spurious wake ups while trying to open the index {name}");
+            }
+
+            // we get the index here to drop the lock before entering the match
+            let index = self.index_map.read().unwrap().get(&uuid);
+
+            match index {
+                Available(index) => break index,
+                Closing(reopen) => {
+                    // Avoiding deadlocks: no lock taken while doing this operation.
+                    let reopen = if let Some(reopen) = reopen.wait_timeout(Duration::from_secs(6)) {
+                        reopen
+                    } else {
+                        continue;
+                    };
+                    let index_path = self.base_path.join(uuid.to_string());
+                    // take the lock to reopen the environment.
+                    reopen.reopen(&mut self.index_map.write().unwrap(), &index_path)?;
+                    continue;
+                }
+                BeingDeleted => return Err(Error::IndexNotFound(name.to_string())),
+                // since we're lazy, it's possible that the index has not been opened yet.
+                Missing => {
+                    let mut index_map = self.index_map.write().unwrap();
+                    // between the read lock and the write lock it's not impossible
+                    // that someone already opened the index (eg if two searches happen
+                    // at the same time), thus before opening it we check a second time
+                    // if it's not already there.
+                    match index_map.get(&uuid) {
+                        Missing => {
+                            let index_path = self.base_path.join(uuid.to_string());
+
+                            break index_map.create(
+                                &uuid,
+                                &index_path,
+                                None,
+                                self.enable_mdb_writemap,
+                                self.index_base_map_size,
+                            )?;
+                        }
+                        Available(index) => break index,
+                        Closing(_) => {
+                            // the reopening will be handled in the next loop operation
+                            continue;
+                        }
+                        BeingDeleted => return Err(Error::IndexNotFound(name.to_string())),
+                    }
+                }
+            }
+        };
+
+        Ok(index)
+    }
+
+    /// Attempts `f` for each index that exists in the index mapper.
+    ///
+    /// It is preferable to use this function rather than a loop that opens all indexes, as a way to avoid having all indexes opened,
+    /// which is unsupported in general.
+    ///
+    /// Since `f` is allowed to return a result, and `Index` is cloneable, it is still possible to wrongly build e.g. a vector of
+    /// all the indexes, but this function makes it harder and so less likely to do accidentally.
+    pub fn try_for_each_index<U, V>(
+        &self,
+        rtxn: &RoTxn,
+        mut f: impl FnMut(&str, &Index) -> Result<U>,
+    ) -> Result<V>
+    where
+        V: FromIterator<U>,
+    {
+        self.index_mapping
+            .iter(rtxn)?
+            .map(|res| {
+                res.map_err(Error::from)
+                    .and_then(|(name, _)| self.index(rtxn, name).and_then(|index| f(name, &index)))
+            })
+            .collect()
+    }
+
+    /// Return the name of all indexes without opening them.
+    pub fn index_names(&self, rtxn: &RoTxn) -> Result<Vec<String>> {
+        self.index_mapping
+            .iter(rtxn)?
+            .map(|res| res.map_err(Error::from).map(|(name, _)| name.to_string()))
+            .collect()
+    }
+
+    /// Swap two index names.
+    pub fn swap(&self, wtxn: &mut RwTxn, lhs: &str, rhs: &str) -> Result<()> {
+        let lhs_uuid = self
+            .index_mapping
+            .get(wtxn, lhs)?
+            .ok_or_else(|| Error::IndexNotFound(lhs.to_string()))?;
+        let rhs_uuid = self
+            .index_mapping
+            .get(wtxn, rhs)?
+            .ok_or_else(|| Error::IndexNotFound(rhs.to_string()))?;
+
+        self.index_mapping.put(wtxn, lhs, &rhs_uuid)?;
+        self.index_mapping.put(wtxn, rhs, &lhs_uuid)?;
+
+        Ok(())
+    }
+
+    /// The stats of an index.
+    ///
+    /// If available in the cache, they are directly returned.
+    /// Otherwise, the `Index` is opened to compute the stats on the fly (the result is not cached).
+    /// The stats for an index are cached after each `Index` update.
+    pub fn stats_of(&self, rtxn: &RoTxn, index_uid: &str) -> Result<IndexStats> {
+        let uuid = self
+            .index_mapping
+            .get(rtxn, index_uid)?
+            .ok_or_else(|| Error::IndexNotFound(index_uid.to_string()))?;
+
+        match self.index_stats.get(rtxn, &uuid)? {
+            Some(stats) => Ok(stats),
+            None => {
+                let index = self.index(rtxn, index_uid)?;
+                let index_rtxn = index.read_txn()?;
+                IndexStats::new(&index, &index_rtxn)
+            }
+        }
+    }
+
+    /// Stores the new stats for an index.
+    ///
+    /// Expected usage is to compute the stats the index using `IndexStats::new`, the pass it to this function.
+    pub fn store_stats_of(
+        &self,
+        wtxn: &mut RwTxn,
+        index_uid: &str,
+        stats: &IndexStats,
+    ) -> Result<()> {
+        let uuid = self
+            .index_mapping
+            .get(wtxn, index_uid)?
+            .ok_or_else(|| Error::IndexNotFound(index_uid.to_string()))?;
+
+        self.index_stats.put(wtxn, &uuid, stats)?;
+        Ok(())
+    }
+
+    pub fn index_exists(&self, rtxn: &RoTxn, name: &str) -> Result<bool> {
+        Ok(self.index_mapping.get(rtxn, name)?.is_some())
+    }
+
+    pub fn indexer_config(&self) -> &IndexerConfig {
+        &self.indexer_config
+    }
+
+    pub fn set_currently_updating_index(&self, index: Option<(String, Index)>) {
+        *self.currently_updating_index.write().unwrap() = index;
+    }
+}