sys_util: add trait to fsync File and QcowFile

File exposes sync_all() and sync_data() functions, which map to fsync()
and fdatasync(), but these functions are not in a trait (they are just
implemented directly on File), so they can't be implemented and used in
a generic way for QcowFile.

Add a new trait, FileSync, that exposes a fsync() function that may be
used in the virtio block model.  Previously, we were translating a block
flush request into a call to File's flush() function, but this just
flushes internal Rust library buffers to the file descriptor; it didn't
actually result in a fsync() call.  Using the new trait, we can cause an
actual fsync() to occur for raw files, as intended.  QcowFile was
already safe, since its flush() function actually calls sync_all() under
the hood.

BUG=None
TEST=sync with raw disk and verify fsync() in strace output

Change-Id: I9bee2c0d2df3747aac1e7d9ec7d9b46a7862dc48
Signed-off-by: Daniel Verkamp <dverkamp@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/1297839
Commit-Ready: ChromeOS CL Exonerator Bot <chromiumos-cl-exonerator@appspot.gserviceaccount.com>
Reviewed-by: Zach Reizner <zachr@chromium.org>

1 files changed, 7 insertions, 1 deletions

diff --git a/qcow/src/qcow.rs b/qcow/src/qcow.rs
index 22d022c..2be4a4c 100644
--- a/qcow/src/qcow.rs
+++ b/qcow/src/qcow.rs
@@ -23,7 +23,7 @@ use std::io::{self, Read, Seek, SeekFrom, Write};
 use std::mem::size_of;
 use std::os::unix::io::{AsRawFd, RawFd};
 
-use sys_util::{PunchHole, SeekHole, WriteZeroes};
+use sys_util::{FileSync, PunchHole, SeekHole, WriteZeroes};
 
 #[derive(Debug)]
 pub enum Error {
@@ -1069,6 +1069,12 @@ impl Write for QcowFile {
     }
 }
 
+impl FileSync for QcowFile {
+    fn fsync(&mut self) -> std::io::Result<()> {
+        self.flush()
+    }
+}
+
 impl PunchHole for QcowFile {
     fn punch_hole(&mut self, offset: u64, length: u64) -> std::io::Result<()> {
         let mut remaining = length;