sys_util: add set_len() trait

Generalize file_sync into file_traits so that we can add another
wrapper, this time for the set_len() method implemented directly on
File.  This will also be implemented on QcowFile.

BUG=chromium:858815
TEST=build_test

Change-Id: I43fbd1968a844c8cac359973a63babcc26942204
Signed-off-by: Daniel Verkamp <dverkamp@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/1394148
Commit-Ready: ChromeOS CL Exonerator Bot <chromiumos-cl-exonerator@appspot.gserviceaccount.com>
Reviewed-by: David Tolnay <dtolnay@chromium.org>
Reviewed-by: Dylan Reid <dgreid@chromium.org>
Reviewed-by: Zach Reizner <zachr@chromium.org>

sys_util/src/file_traits.rs

@@ -19,3 +19,19 @@ impl FileSync for File {
         self.sync_all()
     }
 }
+
+/// A trait for setting the size of a file.
+/// This is equivalent to File's `set_len` method, but
+/// wrapped in a trait so that it can be implemented for
+/// other types.
+pub trait FileSetLen {
+    // Set the size of this file.
+    // This is the moral equivalent of `ftruncate()`.
+    fn set_len(&self, _len: u64) -> Result<()>;
+}
+
+impl FileSetLen for File {
+    fn set_len(&self, len: u64) -> Result<()> {
+        File::set_len(self, len)
+    }
+}

sys_util/src/lib.rs

@@ -21,7 +21,7 @@ pub mod syslog;
 mod errno;
 mod eventfd;
 mod file_flags;
-mod file_sync;
+mod file_traits;
 mod fork;
 mod guest_address;
 mod guest_memory;
@@ -62,7 +62,7 @@ pub use tempdir::*;
 pub use terminal::*;
 pub use timerfd::*;
 
-pub use file_sync::FileSync;
+pub use file_traits::{FileSetLen, FileSync};
 pub use guest_memory::Error as GuestMemoryError;
 pub use mmap::Error as MmapError;
 pub use seek_hole::SeekHole;