fcntl adding apple F_PREALLOCATE flag. (#2393)

changelog/2393.added.md

@@ -0,0 +1 @@
+Add `fcntl`'s `F_PREALLOCATE` constant for Apple targets.

src/fcntl.rs

@@ -802,7 +802,12 @@ pub enum FcntlArg<'a> {
     F_RDADVISE(libc::radvisory),
     /// Turn read ahead off/on
     #[cfg(apple_targets)]
-    F_RDAHEAD(bool)
+    F_RDAHEAD(bool),
+    /// Pre-allocate storage with different policies on fd.
+    /// Note that we want a mutable reference for the OUT
+    /// fstore_t field fst_bytesalloc.
+    #[cfg(apple_targets)]
+    F_PREALLOCATE(&'a mut libc::fstore_t),
     // TODO: Rest of flags
 }
 
@@ -919,7 +924,11 @@ pub fn fcntl<Fd: std::os::fd::AsFd>(fd: Fd, arg: FcntlArg) -> Result<c_int> {
             F_RDAHEAD(on) => {
                 let val = if on { 1 } else { 0 };
                 libc::fcntl(fd, libc::F_RDAHEAD, val)
-            }
+            },
+            #[cfg(apple_targets)]
+            F_PREALLOCATE(st) => {
+                libc::fcntl(fd, libc::F_PREALLOCATE, st)
+            },
         }
     };
 

test/test_fcntl.rs

@@ -581,6 +581,24 @@ fn test_f_get_path() {
     );
 }
 
+#[cfg(apple_targets)]
+#[test]
+fn test_f_preallocate() {
+    use nix::fcntl::*;
+
+    let tmp = NamedTempFile::new().unwrap();
+    let mut st: libc::fstore_t = unsafe { std::mem::zeroed() };
+
+    st.fst_flags = libc::F_ALLOCATECONTIG as libc::c_uint;
+    st.fst_posmode = libc::F_PEOFPOSMODE;
+    st.fst_length = 1024;
+    let res = fcntl(tmp, FcntlArg::F_PREALLOCATE(&mut st))
+        .expect("preallocation failed");
+
+    assert_eq!(res, 0);
+    assert!(st.fst_bytesalloc > 0);
+}
+
 #[cfg(apple_targets)]
 #[test]
 fn test_f_get_path_nofirmlink() {