cli/payload: Add support for unpacking skeleton images

This is useful for getting properly sized (but empty) image files when
unpacking delta payloads.

Discussion: #582

Signed-off-by: Andrew Gunnerson <accounts+github@chiller3.com>

README.extra.md

@@ -340,7 +340,7 @@ avbroot payload unpack -i <input payload>
 
 This subcommand unpacks the payload header information to `payload.toml` and the partition images to the `payload_images` directory.
 
-Only full payload binaries can be unpacked. Delta payload binaries from incremental OTAs are not supported.
+Data can only be unpacked from full payload binaries. For delta payloads, either use `--no-output-images` to skip image extraction or use `--skeleton` to create properly sized sparse files with no data.
 
 ### Packing a payload binary
 

avbroot/src/cli/payload.rs

@@ -108,6 +108,7 @@ pub fn unpack_payload(
     output_info: &Path,
     output_images: &Path,
     no_output_images: bool,
+    skeleton: bool,
     reader: &File,
     payload_offset: u64,
     payload_size: u64,
@@ -123,13 +124,30 @@ pub fn unpack_payload(
     write_info(output_info, &header)?;
 
     if !no_output_images {
+        fs::create_dir_all(output_images)
+            .with_context(|| format!("Failed to create directory: {output_images:?}"))?;
+
+        if skeleton {
+            for partition in &header.manifest.partitions {
+                let name = &partition.partition_name;
+                let path = util::path_join_single(output_images, format!("{name}.img"))?;
+                let size = partition
+                    .new_partition_info
+                    .as_ref()
+                    .and_then(|info| info.size)
+                    .ok_or_else(|| anyhow!("Size not found for partition: {name}"))?;
+
+                let file = File::create(&path)
+                    .with_context(|| format!("Failed to create file: {path:?}"))?;
+
+                file.set_len(size)
+                    .with_context(|| format!("Failed to truncate file: {path:?}"))?;
+            }
+        } else {
             if !header.is_full_ota() {
                 bail!("Cannot extract images from a delta payload");
             }
 
-        fs::create_dir_all(output_images)
-            .with_context(|| format!("Failed to create directory: {output_images:?}"))?;
-
             ota::extract_payload(
                 reader,
                 output_images,
@@ -146,6 +164,7 @@ pub fn unpack_payload(
                 cancel_signal,
             )?;
         }
+    }
 
     Ok(())
 }
@@ -165,6 +184,7 @@ fn unpack_subcommand(
         &cli.output_info,
         &cli.output_images,
         cli.no_output_images,
+        cli.skeleton,
         &reader,
         0,
         payload_size,
@@ -448,8 +468,12 @@ struct UnpackCli {
     output_images: PathBuf,
 
     /// Do not output images.
-    #[arg(long, conflicts_with = "output_images")]
+    #[arg(long, conflicts_with_all = ["output_images", "skeleton"])]
     no_output_images: bool,
+
+    /// Only create empty sparse files for output images.
+    #[arg(long)]
+    skeleton: bool,
 }
 
 /// Pack a payload binary.

avbroot/src/cli/zip.rs

@@ -303,6 +303,7 @@ fn unpack_subcommand(zip_cli: &ZipCli, cli: &UnpackCli, cancel_signal: &AtomicBo
                 &cli.output_payload_info,
                 &cli.output_payload_images,
                 cli.no_output_payload_images,
+                cli.payload_skeleton,
                 &zip_reader.get_ref().0,
                 payload_offset,
                 payload_size,
@@ -625,8 +626,16 @@ struct UnpackCli {
     output_payload_images: PathBuf,
 
     /// Do not output payload images.
-    #[arg(long, conflicts_with = "output_payload_images", requires = "payload")]
+    #[arg(
+        long,
+        conflicts_with_all = ["output_payload_images", "payload_skeleton"],
+        requires = "payload"
+    )]
     no_output_payload_images: bool,
+
+    /// Only create empty sparse files for output payload images.
+    #[arg(long, requires = "payload")]
+    payload_skeleton: bool,
 }
 
 /// Pack an OTA zip.