Add sparse disk file to not require image to be sized initially.

src/disk/sparse.rs

@@ -0,0 +1,48 @@
+use std::fs::{File, OpenOptions};
+use std::io::{Read, Write, Seek, SeekFrom};
+use std::u64;
+use syscall::error::{Error, Result, EIO};
+
+use BLOCK_SIZE;
+use disk::Disk;
+
+macro_rules! try_disk {
+    ($expr:expr) => (match $expr {
+        Ok(val) => val,
+        Err(err) => {
+            eprintln!("Disk I/O Error: {}", err);
+            return Err(Error::new(EIO));
+        }
+    })
+}
+
+pub struct DiskSparse {
+    pub file: File,
+}
+
+impl DiskSparse {
+    pub fn create(path: &str) -> Result<DiskSparse> {
+        let file = try_disk!(OpenOptions::new().read(true).write(true).create(true).open(path));
+        Ok(DiskSparse {
+            file
+        })
+    }
+}
+
+impl Disk for DiskSparse {
+    fn read_at(&mut self, block: u64, buffer: &mut [u8]) -> Result<usize> {
+        try_disk!(self.file.seek(SeekFrom::Start(block * BLOCK_SIZE)));
+        let count = try_disk!(self.file.read(buffer));
+        Ok(count)
+    }
+
+    fn write_at(&mut self, block: u64, buffer: &[u8]) -> Result<usize> {
+        try_disk!(self.file.seek(SeekFrom::Start(block * BLOCK_SIZE)));
+        let count = try_disk!(self.file.write(buffer));
+        Ok(count)
+    }
+
+    fn size(&mut self) -> Result<u64> {
+        Ok(u64::MAX)
+    }
+}