disk: Turn Android Sparse format into a feature

This is a follow-up to
https://chromium-review.googlesource.com/c/crosvm/crosvm/+/3864926,
which implements this for QCOW.

Same rationale: allow to turn off support for disk formats that projects
do not need/want.

BUG=None
TEST=./tools/presubmit; cd crosvm-fuzz && cargo build

Change-Id: I401bad4d4ccb1a00a303ed86f1156f153b70b562
Reviewed-on: https://chromium-review.googlesource.com/c/crosvm/crosvm/+/3872278
Commit-Queue: Christian Blichmann <cblichmann@google.com>
Reviewed-by: David Stevens <stevensd@chromium.org>
Tested-by: Christian Blichmann <cblichmann@google.com>

disk/Cargo.toml

@@ -8,6 +8,7 @@ edition = "2021"
 path = "src/disk.rs"
 
 [features]
+android-sparse = []
 composite-disk = ["crc32fast", "protos", "protobuf", "uuid"]
 qcow = []
 

disk/src/asynchronous.rs

@@ -34,6 +34,7 @@ pub struct AsyncDiskFileWrapper<T: DiskFile + Send> {
 }
 
 impl<T: DiskFile + Send> AsyncDiskFileWrapper<T> {
+    #[allow(dead_code)] // Only used if qcow or android-sparse features are enabled
     pub fn new(disk_file: T, _ex: &Executor) -> Self {
         Self {
             blocking_pool: BlockingPool::new(1, Duration::from_secs(10)),

disk/src/disk.rs

@@ -31,6 +31,7 @@ use cros_async::IoSourceExt;
 use thiserror::Error as ThisError;
 
 mod asynchronous;
+#[allow(unused)]
 pub(crate) use asynchronous::AsyncDiskFileWrapper;
 #[cfg(feature = "qcow")]
 mod qcow;
@@ -63,8 +64,11 @@ pub use composite::PartitionInfo;
 #[cfg(feature = "composite-disk")]
 pub use gpt::Error as GptError;
 
+#[cfg(feature = "android-sparse")]
 mod android_sparse;
+#[cfg(feature = "android-sparse")]
 use android_sparse::AndroidSparse;
+#[cfg(feature = "android-sparse")]
 use android_sparse::SPARSE_HEADER_MAGIC;
 
 /// Nesting depth limit for disk formats that can open other disk files.
@@ -76,6 +80,7 @@ pub enum Error {
     BlockDeviceNew(base::Error),
     #[error("requested file conversion not supported")]
     ConversionNotSupported,
+    #[cfg(feature = "android-sparse")]
     #[error("failure in android sparse disk: {0}")]
     CreateAndroidSparseDisk(android_sparse::Error),
     #[cfg(feature = "composite-disk")]
@@ -242,11 +247,13 @@ pub fn detect_image_type(file: &File) -> Result<ImageType> {
         }
     }
 
+    #[allow(unused_variables)] // magic4 is only used with the qcow or android-sparse features.
     if let Some(magic4) = magic.data.get(0..4) {
         #[cfg(feature = "qcow")]
         if magic4 == QCOW_MAGIC.to_be_bytes() {
             return Ok(ImageType::Qcow2);
         }
+        #[cfg(feature = "android-sparse")]
         if magic4 == SPARSE_HEADER_MAGIC.to_le_bytes() {
             return Ok(ImageType::AndroidSparse);
         }
@@ -296,6 +303,7 @@ pub fn create_disk_file(
                 .map_err(Error::CreateCompositeDisk)?,
             ) as Box<dyn DiskFile>
         }
+        #[cfg(feature = "android-sparse")]
         ImageType::AndroidSparse => {
             Box::new(AndroidSparse::from_file(raw_image).map_err(Error::CreateAndroidSparseDisk)?)
                 as Box<dyn DiskFile>

disk/src/sys/unix.rs

@@ -80,6 +80,7 @@ mod tests {
     }
 
     #[test]
+    #[cfg(feature = "android-sparse")]
     fn detect_image_type_android_sparse() {
         let mut t = tempfile::tempfile().unwrap();
         // Write the Android sparse magic signature. The rest of the header is not filled in, so if