Accept the max readers param by env var and increase it

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

@@ -1,4 +1,5 @@
 use std::collections::BTreeMap;
+use std::env::VarError;
 use std::path::Path;
 use std::time::Duration;
 
@@ -300,9 +301,19 @@ fn create_or_open_index(
     enable_mdb_writemap: bool,
     map_size: usize,
 ) -> Result<Index> {
+    use std::str::FromStr;
+
     let mut options = EnvOpenOptions::new();
     options.map_size(clamp_to_page_size(map_size));
-    options.max_readers(1024);
+
+    let max_readers = match std::env::var("MEILI_INDEX_MAX_READERS") {
+        Ok(value) => u32::from_str(&value).unwrap(),
+        Err(VarError::NotPresent) => 100 * 1024,
+        Err(VarError::NotUnicode(value)) => {
+            panic!("Invalid unicode for the `MEILI_INDEX_MAX_READERS` env var: {value:?}")
+        }
+    };
+    options.max_readers(max_readers);
     if enable_mdb_writemap {
         unsafe { options.flags(EnvFlags::WRITE_MAP) };
     }