sys_util: add FileAllocate trait

This trait provides a generic interface for allocating space on the
filesystem within a given file.  It is equivalent to the fallocate(2)
system call with the default mode (mode = 0).

BUG=chromium:858815
TEST=cargo build --features=composite-disk

Change-Id: I2f4e8aceb4878790e8dec2e3d539071915efd205
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/crosvm/+/2015828
Reviewed-by: Dylan Reid <dgreid@chromium.org>
Tested-by: kokoro <noreply+kokoro@google.com>
Commit-Queue: Daniel Verkamp <dverkamp@chromium.org>

2 files changed, 20 insertions, 1 deletions

diff --git a/sys_util/src/file_traits.rs b/sys_util/src/file_traits.rs
index 7e615d6..54e710f 100644
--- a/sys_util/src/file_traits.rs
+++ b/sys_util/src/file_traits.rs
@@ -8,6 +8,8 @@ use std::os::unix::io::{AsRawFd, RawFd};
 
 use data_model::VolatileSlice;
 
+use crate::{fallocate, FallocateMode};
+
 /// A trait for flushing the contents of a file to disk.
 /// This is equivalent to File's `sync_all` method, but
 /// wrapped in a trait so that it can be implemented for
@@ -54,6 +56,20 @@ impl FileGetLen for File {
     }
 }
 
+/// A trait for allocating disk space in a sparse file.
+/// This is equivalent to fallocate() with no special flags.
+pub trait FileAllocate {
+    /// Allocate storage for the region of the file starting at `offset` and extending `len` bytes.
+    fn allocate(&mut self, offset: u64, len: u64) -> Result<()>;
+}
+
+impl FileAllocate for File {
+    fn allocate(&mut self, offset: u64, len: u64) -> Result<()> {
+        fallocate(self, FallocateMode::Allocate, true, offset, len)
+            .map_err(|e| Error::from_raw_os_error(e.errno()))
+    }
+}
+
 /// A trait similar to `Read` and `Write`, but uses volatile memory as buffers.
 pub trait FileReadWriteVolatile {
     /// Read bytes from this file into the given slice, returning the number of bytes read on
diff --git a/sys_util/src/lib.rs b/sys_util/src/lib.rs
index bde5eb2..357e6b4 100644
--- a/sys_util/src/lib.rs
+++ b/sys_util/src/lib.rs
@@ -64,7 +64,8 @@ pub use crate::timerfd::*;
 pub use poll_token_derive::*;
 
 pub use crate::file_traits::{
-    AsRawFds, FileGetLen, FileReadWriteAtVolatile, FileReadWriteVolatile, FileSetLen, FileSync,
+    AsRawFds, FileAllocate, FileGetLen, FileReadWriteAtVolatile, FileReadWriteVolatile, FileSetLen,
+    FileSync,
 };
 pub use crate::guest_memory::Error as GuestMemoryError;
 pub use crate::mmap::Error as MmapError;
@@ -175,6 +176,7 @@ pub fn flock(file: &dyn AsRawFd, op: FlockOperation, nonblocking: bool) -> Resul
 pub enum FallocateMode {
     PunchHole,
     ZeroRange,
+    Allocate,
 }
 
 /// Safe wrapper for `fallocate()`.
@@ -200,6 +202,7 @@ pub fn fallocate(
     let mut mode = match mode {
         FallocateMode::PunchHole => libc::FALLOC_FL_PUNCH_HOLE,
         FallocateMode::ZeroRange => libc::FALLOC_FL_ZERO_RANGE,
+        FallocateMode::Allocate => 0,
     };
 
     if keep_size {