fcntl: adding macOs F_RDADVISE flag (#2480)

* fcntl: adding macOs F_RDADVISE flag

Hints to prefetch blocks of the file into memory when possible.
Can be useful to process large files.

* add changelog entry

* testing

Author: devnexen

changelog/2113.added.md

@@ -1 +1 @@
-Add socket option `IPV6_PKTINFO` for BSDs/Linux/Android, also `IPV6_RECVPKTINFO` for DragonFlyBSD
+Add socket option `IPV6_PKTINFO` for BSDs/Linux/Android, also `IPV6_RECVPKTINFO` for DragonFlyBSD

changelog/2480.added.md

@@ -0,0 +1 @@
+Add fcntl constant `F_RDADVISE` for Apple target

src/fcntl.rs

@@ -797,6 +797,9 @@ pub enum FcntlArg<'a> {
     /// Return the full path without firmlinks of the fd.
     #[cfg(apple_targets)]
     F_GETPATH_NOFIRMLINK(&'a mut PathBuf),
+    /// Issue an advisory read async with no copy to user
+    #[cfg(apple_targets)]
+    F_RDADVISE(libc::radvisory),
     // TODO: Rest of flags
 }
 
@@ -905,6 +908,10 @@ pub fn fcntl<Fd: std::os::fd::AsFd>(fd: Fd, arg: FcntlArg) -> Result<c_int> {
                 *path = PathBuf::from(OsString::from(optr.to_str().unwrap()));
                 return Ok(ok_res)
             },
+            #[cfg(apple_targets)]
+            F_RDADVISE(rad) => {
+                libc::fcntl(fd, libc::F_RDADVISE, &rad)
+            }
         }
     };
 

test/test_fcntl.rs

@@ -732,3 +732,23 @@ mod test_flock {
             .expect_err("Should not have been able to lock the file");
     }
 }
+
+#[cfg(apple_targets)]
+#[test]
+fn test_f_rdadvise() {
+    use nix::fcntl::*;
+
+    let contents = vec![1; 1024];
+    let mut buf = [0; 1024];
+    let mut tmp = NamedTempFile::new().unwrap();
+    tmp.write_all(&contents).unwrap();
+    let fd = open(tmp.path(), OFlag::empty(), Mode::empty()).unwrap();
+    let rad = libc::radvisory {
+        ra_offset: 0,
+        ra_count: contents.len() as _,
+    };
+    let res = fcntl(&tmp, FcntlArg::F_RDADVISE(rad)).expect("rdadivse failed");
+    assert_ne!(res, -1);
+    assert_eq!(contents.len(), read(&fd, &mut buf).unwrap());
+    assert_eq!(contents, &buf[0..contents.len()]);
+}