Add config option to mlock the mmap'ed headers of the shard

Cargo.toml

@@ -18,6 +18,7 @@ parking_lot = "0.12.3"
 uuid = { version = "1.10.0" }
 rand = "0.8.5"
 fslock = "0.2.1"
+libc = "0.2.158"
 
 [features]
 whitebox_testing = []

src/lib.rs

@@ -108,6 +108,7 @@ pub struct Config {
     pub max_concurrent_list_ops: u32, // number of keyed locks for concurrent list ops
     pub truncate_up: bool, // whether or not to truncate up shard files to their max size (spare files)
     pub clear_on_unsupported_version: bool, // whether or not to clear the DB if the version is unsupported
+    pub mlock_headers: bool, // whether or not to mlock the shard headers to RAM (POSIX only)
 }
 
 impl Default for Config {
@@ -121,6 +122,7 @@ impl Default for Config {
             max_concurrent_list_ops: 64,
             truncate_up: true,
             clear_on_unsupported_version: false,
+            mlock_headers: false,
         }
     }
 }

src/shard.rs

@@ -272,6 +272,10 @@ impl Shard {
 
         let mut mmap = unsafe { MmapOptions::new().len(HEADER_SIZE as usize).map_mut(&file) }?;
 
+        if cfg!(target_family = "unix") {
+            unsafe { libc::mlock(mmap.as_ptr() as *const _, mmap.len()) };
+        }
+
         let header = unsafe { &mut *(mmap.as_mut_ptr() as *mut ShardHeader) };
         header.metadata.magic = SHARD_FILE_MAGIC;
         header.metadata.version = SHARD_FILE_VERSION;