nydus-image: enhance unpack command to support extracting files to dir

When --output is specified as a directory, the nydus-image unpack
command will return an error, which is not convenient for users.

This patch enhances this command to allow --output to be passed into a
dir. In this case, nydus-image will extract all files into this dir.

Signed-off-by: Qinqi Qu <quqinqi@linux.alibaba.com>

src/bin/nydus-image/main.rs

@@ -720,7 +720,7 @@ fn prepare_cmd_args(bti_string: &'static str) -> App {
             .arg(
                 Arg::new("output")
                     .long("output")
-                    .help("path for output tar file")
+                    .help("Path for output tar file, If it is a directory, extract all files to there.")
                     .required(true),
             ),
     )

src/bin/nydus-image/unpack/mod.rs

@@ -3,7 +3,7 @@
 //
 // SPDX-License-Identifier: Apache-2.0
 use std::collections::HashMap;
-use std::fs::{File, OpenOptions};
+use std::fs::{create_dir_all, remove_file, File, OpenOptions};
 use std::io::Read;
 use std::path::{Path, PathBuf};
 use std::rc::Rc;
@@ -18,7 +18,7 @@ use nydus_rafs::{
 };
 use nydus_storage::backend::BlobBackend;
 use nydus_storage::device::BlobInfo;
-use tar::{Builder, Header};
+use tar::{Archive, Builder, Header};
 
 use self::pax::{
     OCIBlockBuilder, OCICharBuilder, OCIDirBuilder, OCIFifoBuilder, OCILinkBuilder, OCIRegBuilder,
@@ -32,7 +32,7 @@ pub trait Unpacker {
     fn unpack(&self, config: Arc<ConfigV2>) -> Result<()>;
 }
 
-///  A unpacker with the ability to convert bootstrap file and blob file to tar
+///  A unpacker with the ability to convert bootstrap file and blob file to tar or dir.
 pub struct OCIUnpacker {
     bootstrap: PathBuf,
     blob_backend: Option<Arc<dyn BlobBackend + Send + Sync>>,
@@ -69,20 +69,45 @@ impl OCIUnpacker {
 impl Unpacker for OCIUnpacker {
     fn unpack(&self, config: Arc<ConfigV2>) -> Result<()> {
         debug!(
-            "oci unpacker, bootstrap file: {:?}, output file: {:?}",
+            "oci unpacker, bootstrap file: {:?}, output path: {:?}",
             self.bootstrap, self.output
         );
 
         let rafs = self.load_rafs(config)?;
 
+        // If self.output ends with path separator, then it is a dir.
+        // Output a tar file first, and untar it later.
+        let is_dir = self.output.to_string_lossy().ends_with(std::path::MAIN_SEPARATOR);
+        let mut tar_path = self.output.clone();
+        if is_dir {
+            if !self.output.exists() {
+                create_dir_all(&self.output)?;
+            }
+            let mut base_name = self
+                .bootstrap
+                .file_name()
+                .unwrap_or_default()
+                .to_os_string();
+            base_name.push(".tar");
+            tar_path = PathBuf::from(base_name);
+        }
+
         let mut builder = self
             .builder_factory
-            .create(&rafs, &self.blob_backend, &self.output)?;
+            .create(&rafs, &self.blob_backend, &tar_path)?;
 
         for (node, path) in RafsIterator::new(&rafs) {
             builder.append(node, &path)?;
         }
 
+        // untar this tar file to self.output dir
+        if is_dir {
+            let file = File::open(&tar_path)?;
+            let mut tar = Archive::new(file);
+            tar.unpack(&self.output)?;
+            remove_file(&tar_path)?;
+        }
+
         Ok(())
     }
 }