block_util: Preserve ordering of sync file backend completions

For the synchronous backends efficiently preserve the order for
completion requests through the use of VecDequeue. Preserving the order
is not required but is beneficial as it matches the existing
optimisation that looks to match completions and requests.

Signed-off-by: Rob Bradford <robert.bradford@intel.com>

block_util/src/raw_sync.rs

@@ -5,6 +5,7 @@
 use crate::async_io::{
     AsyncIo, AsyncIoError, AsyncIoResult, DiskFile, DiskFileError, DiskFileResult, DiskTopology,
 };
+use std::collections::VecDeque;
 use std::fs::File;
 use std::io::{Seek, SeekFrom};
 use std::os::unix::io::{AsRawFd, RawFd};
@@ -44,7 +45,7 @@ impl DiskFile for RawFileDiskSync {
 pub struct RawFileSync {
     fd: RawFd,
     eventfd: EventFd,
-    completion_list: Vec<(u64, i32)>,
+    completion_list: VecDeque<(u64, i32)>,
 }
 
 impl RawFileSync {
@@ -52,7 +53,7 @@ impl RawFileSync {
         RawFileSync {
             fd,
             eventfd: EventFd::new(libc::EFD_NONBLOCK).expect("Failed creating EventFd for RawFile"),
-            completion_list: Vec::new(),
+            completion_list: VecDeque::new(),
         }
     }
 }
@@ -81,7 +82,7 @@ impl AsyncIo for RawFileSync {
             return Err(AsyncIoError::ReadVectored(std::io::Error::last_os_error()));
         }
 
-        self.completion_list.push((user_data, result as i32));
+        self.completion_list.push_back((user_data, result as i32));
         self.eventfd.write(1).unwrap();
 
         Ok(())
@@ -106,7 +107,7 @@ impl AsyncIo for RawFileSync {
             return Err(AsyncIoError::WriteVectored(std::io::Error::last_os_error()));
         }
 
-        self.completion_list.push((user_data, result as i32));
+        self.completion_list.push_back((user_data, result as i32));
         self.eventfd.write(1).unwrap();
 
         Ok(())
@@ -120,7 +121,7 @@ impl AsyncIo for RawFileSync {
         }
 
         if let Some(user_data) = user_data {
-            self.completion_list.push((user_data, result));
+            self.completion_list.push_back((user_data, result));
             self.eventfd.write(1).unwrap();
         }
 
@@ -128,6 +129,6 @@ impl AsyncIo for RawFileSync {
     }
 
     fn next_completed_request(&mut self) -> Option<(u64, i32)> {
-        self.completion_list.pop()
+        self.completion_list.pop_front()
     }
 }