devices: block: require traits not a file

Allow block to take anything that satisfies the traits it uses. This
will allow for different backends to be used in addition to normal
files. The first new backend will be qcow2 added in subsequent commits.

Change-Id: I571ffc8ad4b5d56f286e134f47a60261d639b160
Signed-off-by: Dylan Reid <dgreid@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/862627
Reviewed-by: Zach Reizner <zachr@chromium.org>

devices/src/virtio/block.rs

@@ -3,7 +3,6 @@
 // found in the LICENSE file.
 
 use std::cmp;
-use std::fs::File;
 use std::io::{self, Seek, SeekFrom, Read, Write};
 use std::os::unix::io::{AsRawFd, RawFd};
 use std::result;
@@ -29,6 +28,9 @@ const VIRTIO_BLK_S_OK: u8 = 0;
 const VIRTIO_BLK_S_IOERR: u8 = 1;
 const VIRTIO_BLK_S_UNSUPP: u8 = 2;
 
+pub trait DiskFile: Read + Seek + Write {}
+impl<D: Read + Seek + Write> DiskFile for D {}
+
 #[derive(PartialEq)]
 enum RequestType {
     In,
@@ -150,7 +152,7 @@ impl Request {
            })
     }
 
-    fn execute<T: Seek + Read + Write>(&self,
+    fn execute<T: DiskFile>(&self,
                                        disk: &mut T,
                                        mem: &GuestMemory)
                                        -> result::Result<u32, ExecuteError> {
@@ -173,15 +175,15 @@ impl Request {
     }
 }
 
-struct Worker {
+struct Worker<T: DiskFile> {
     queues: Vec<Queue>,
     mem: GuestMemory,
-    disk_image: File,
+    disk_image: T,
     interrupt_status: Arc<AtomicUsize>,
     interrupt_evt: EventFd,
 }
 
-impl Worker {
+impl<T: DiskFile> Worker<T> {
     fn process_queue(&mut self, queue_index: usize) -> bool {
         let queue = &mut self.queues[queue_index];
 
@@ -265,9 +267,9 @@ impl Worker {
 }
 
 /// Virtio device for exposing block level read/write operations on a host file.
-pub struct Block {
+pub struct Block<T: DiskFile> {
     kill_evt: Option<EventFd>,
-    disk_image: Option<File>,
+    disk_image: Option<T>,
     config_space: Vec<u8>,
 }
 
@@ -283,11 +285,11 @@ fn build_config_space(disk_size: u64) -> Vec<u8> {
     config
 }
 
-impl Block {
+impl<T: DiskFile> Block<T> {
     /// Create a new virtio block device that operates on the given file.
     ///
     /// The given file must be seekable and sizable.
-    pub fn new(mut disk_image: File) -> SysResult<Block> {
+    pub fn new(mut disk_image: T) -> SysResult<Block<T>> {
         let disk_size = disk_image.seek(SeekFrom::End(0))? as u64;
         if disk_size % SECTOR_SIZE != 0 {
             warn!("Disk size {} is not a multiple of sector size {}; \
@@ -303,7 +305,7 @@ impl Block {
     }
 }
 
-impl Drop for Block {
+impl<T: DiskFile> Drop for Block<T> {
     fn drop(&mut self) {
         if let Some(kill_evt) = self.kill_evt.take() {
             // Ignore the result because there is nothing we can do about it.
@@ -312,7 +314,7 @@ impl Drop for Block {
     }
 }
 
-impl VirtioDevice for Block {
+impl<T: 'static + AsRawFd + DiskFile + Send> VirtioDevice for Block<T> {
     fn keep_fds(&self) -> Vec<RawFd> {
         let mut keep_fds = Vec::new();
 
@@ -387,6 +389,7 @@ impl VirtioDevice for Block {
 
 #[cfg(test)]
 mod tests {
+    use std::fs::File;
     use std::path::PathBuf;
     use sys_util::TempDir;