indexedlog: extract empty mmap logic to a function

Summary:
This starts a series of changes to make indexedlog work in "in-memory-only"
mode. It is useful in 2 cases:

- buffering blackbox writes (short term)
- reused in Mononoke which does not want side-effect on disk (longer term)

Reviewed By: sfilipco

Differential Revision: D16044042

fbshipit-source-id: bdf35a9565e39e65c9ba4ddb949aa86412b4d496

lib/indexedlog/src/utils.rs

@@ -42,7 +42,7 @@ pub fn mmap_readonly(file: &File, len: Option<u64>) -> io::Result<(Mmap, u64)> {
     };
     let mmap = unsafe {
         if len == 0 {
-            MmapOptions::new().len(1).map_anon()?.make_read_only()?
+            mmap_empty()?
         } else {
             MmapOptions::new().len(len as usize).map(&file)?
         }
@@ -50,6 +50,11 @@ pub fn mmap_readonly(file: &File, len: Option<u64>) -> io::Result<(Mmap, u64)> {
     Ok((mmap, len))
 }
 
+/// Return a [`Mmap`] that is expected to be empty.
+pub fn mmap_empty() -> io::Result<Mmap> {
+    Ok(MmapOptions::new().len(1).map_anon()?.make_read_only()?)
+}
+
 /// Open a path. Usually for locking purpose.
 ///
 /// The path is assumed to be a directory. But this function does not do extra