Introduce a stale readers clearing Env method

Author: Kerollmops

heed/src/env.rs

@@ -473,6 +473,17 @@ impl Env {
             }
         }
     }
+
+    /// Check for stale entries in the reader lock table and clear them.
+    ///
+    /// Returns the number of stale readers cleared.
+    pub fn clear_stale_readers(&self) -> Result<usize> {
+        let mut dead: i32 = 0;
+        unsafe { mdb_result(ffi::mdb_reader_check(self.0.env, &mut dead))? }
+        // safety: The reader_check function asks for an i32, initialize it to zero
+        //         and never decrements it. It is safe to use either an u32 or u64 (usize).
+        Ok(dead as usize)
+    }
 }
 
 #[derive(Clone)]

heed/src/mdb/lmdb_ffi.rs

@@ -2,9 +2,9 @@ pub use ffi::{
     mdb_cursor_close, mdb_cursor_del, mdb_cursor_get, mdb_cursor_open, mdb_cursor_put,
     mdb_dbi_open, mdb_del, mdb_drop, mdb_env_close, mdb_env_copyfd2 as mdb_env_copy2fd,
     mdb_env_create, mdb_env_open, mdb_env_set_mapsize, mdb_env_set_maxdbs, mdb_env_set_maxreaders,
-    mdb_env_stat, mdb_env_sync, mdb_filehandle_t, mdb_get, mdb_put, mdb_stat, mdb_txn_abort,
-    mdb_txn_begin, mdb_txn_commit, MDB_cursor, MDB_dbi, MDB_env, MDB_stat as MDB_Stat, MDB_txn,
-    MDB_APPEND, MDB_CP_COMPACT, MDB_CREATE, MDB_CURRENT, MDB_RDONLY,
+    mdb_env_stat, mdb_env_sync, mdb_filehandle_t, mdb_get, mdb_put, mdb_reader_check, mdb_stat,
+    mdb_txn_abort, mdb_txn_begin, mdb_txn_commit, MDB_cursor, MDB_dbi, MDB_env,
+    MDB_stat as MDB_Stat, MDB_txn, MDB_APPEND, MDB_CP_COMPACT, MDB_CREATE, MDB_CURRENT, MDB_RDONLY,
 };
 use lmdb_master3_sys as ffi;