Make Rust bindings work on MacOS

Summary:
# Context

At the source control team we are benchmarking various IO workflows on file systems like APFS, BTRFS, and NTFS, as well as using DBs for those workflows instead of native file system of the OS (e.g, RocksDB and LMDB).

See the
```
buck run fbcode/mode/opt fbcode//eden/fs/facebook/prototypes/fsiomicrobench:main
```
tool for more details.

# This diff

The tool is written in Rust and we are adding the LMDB benchmark. We found that the LMDB Rust binding does not compile on MacOS. The issue is that the mode_t is 16 bit on MacOS and is 32 bit on Linux. The Rust bindings always assumed u32 for the `perms` argument.

I have changed the type to u16 as 16 bits can fully represent permissions anyway and converting it to u32 is always safe. While the opposite approach of keeping u32 and and converting to u16 requires partial functions that may fail at run time. That is `.into()` is always safer than `try_into()`.

Welcome alternative suggestions, if there is a better way of doing this.

Reviewed By: nt591

Differential Revision: D68388014

fbshipit-source-id: d50035f398ae7186fef6b96a9858771c15e3d8f4

hphp/hack/src/utils/lmdb/lmdb.rs

@@ -148,15 +148,15 @@ impl Options {
     /// SAFETY: Only one instance per canonical path may exist process-wide.
     /// Do not have open an LMDB database twice in the same process at the same time.
     /// Not even from a plain open() call - close()ing it breaks flock() advisory locking.
-    pub fn create_file(self, path: impl AsRef<Path>, perms: u32) -> Result<Env> {
+    pub fn create_file(self, path: impl AsRef<Path>, perms: u16) -> Result<Env> {
         let Self { env, flags } = self;
         unsafe {
             let path = CString::new(path.as_ref().as_os_str().as_bytes())?;
             check(mdb_env_open(
                 env,
                 path.as_ptr(),
                 flags | MDB_NOSUBDIR,
-                perms,
+                perms.into(),
             ))?;
             std::mem::forget(self); // don't close our MDB_env
             Ok(Env {
@@ -166,15 +166,15 @@ impl Options {
     }
 
     /// Open a database environment
-    pub fn open_file(self, path: impl AsRef<Path>, perms: u32) -> Result<Env> {
+    pub fn open_file(self, path: impl AsRef<Path>, perms: u16) -> Result<Env> {
         let Self { env, flags } = self;
         unsafe {
             let path = CString::new(path.as_ref().as_os_str().as_bytes())?;
             check(mdb_env_open(
                 env,
                 path.as_ptr(),
                 flags | MDB_RDONLY | MDB_NOSUBDIR,
-                perms,
+                perms.into(),
             ))?;
             std::mem::forget(self);
             Ok(Env {
@@ -307,11 +307,11 @@ impl Env {
     }
 
     /// Change unix permissions on env and lock file
-    pub fn chmod(&self, mode: u32) -> Result<()> {
+    pub fn chmod(&self, mode: u16) -> Result<()> {
         use std::os::unix::fs::PermissionsExt;
         let (path, lockpath) = self.paths()?;
-        std::fs::set_permissions(path, PermissionsExt::from_mode(mode))?;
-        std::fs::set_permissions(lockpath, PermissionsExt::from_mode(mode))?;
+        std::fs::set_permissions(path, PermissionsExt::from_mode(mode.into()))?;
+        std::fs::set_permissions(lockpath, PermissionsExt::from_mode(mode.into()))?;
         Ok(())
     }