initial layering attempt

implement per-package layering a la
https://grahamc.com/blog/nix-and-layered-docker-images/,
using nix's existing popularity algorithm

Author: tofay

.gitignore

@@ -3,3 +3,4 @@ tests/**/out
 out/
 vendor/
 tests/**/rpmoci.lock
+__pycache__

src/archive.rs

@@ -13,110 +13,13 @@
 //! You should have received a copy of the GNU General Public License
 //! along with this program.  If not, see <https://www.gnu.org/licenses/>.
 use anyhow::{Context, Result};
-use std::{
-    collections::{hash_map::Entry, HashMap},
-    io::Write,
-    os::unix::{
-        fs::MetadataExt,
-        prelude::{FileTypeExt, OsStrExt},
-    },
-    path::{Path, PathBuf},
-};
-use walkdir::WalkDir;
+use std::{io::Write, os::unix::prelude::OsStrExt, path::Path};
 
 // https://mgorny.pl/articles/portability-of-tar-features.html#id25
 const PAX_SCHILY_XATTR: &[u8; 13] = b"SCHILY.xattr.";
 
-/// custom implementation of tar-rs's append_dir_all that:
-/// - works around https://github.com/alexcrichton/tar-rs/issues/102 so that security capabilities are preserved
-/// - emulates tar's `--clamp-mtime` option so that any file/dir/symlink mtimes are no later than a specific value
-/// - supports hardlinks
-pub(super) fn append_dir_all_with_xattrs(
-    builder: &mut tar::Builder<impl Write>,
-    src_path: impl AsRef<Path>,
-    clamp_mtime: i64,
-) -> Result<()> {
-    let src_path = src_path.as_ref();
-    // Map (dev, inode) -> path for hardlinks
-    let mut hardlinks: HashMap<(u64, u64), PathBuf> = HashMap::new();
-
-    for entry in WalkDir::new(src_path)
-        .follow_links(false)
-        .sort_by_file_name()
-        .into_iter()
-    {
-        let entry = entry?;
-        let meta = entry.metadata()?;
-        // skip sockets as tar-rs errors when trying to archive them.
-        // For comparison, umoci also errors, whereas docker skips them
-        if meta.file_type().is_socket() {
-            continue;
-        }
-
-        let rel_path = pathdiff::diff_paths(entry.path(), src_path)
-            .expect("walkdir returns path inside of search root");
-        if rel_path == Path::new("") {
-            continue;
-        }
-
-        if entry.file_type().is_symlink() {
-            if meta.mtime() > clamp_mtime {
-                // Setting the mtime on a symlink is fiddly with tar-rs, so we use filetime to change
-                // the mtime before adding the symlink to the tar archive
-                let mtime = filetime::FileTime::from_unix_time(clamp_mtime, 0);
-                filetime::set_symlink_file_times(entry.path(), mtime, mtime)?;
-            }
-            add_pax_extension_header(entry.path(), builder)?;
-            builder.append_path_with_name(entry.path(), rel_path)?;
-        } else if entry.file_type().is_file() || entry.file_type().is_dir() {
-            add_pax_extension_header(entry.path(), builder)?;
-
-            // If this is a hardlink, add a link header instead of the file
-            // if this isn't the first time we've seen this inode
-            if meta.nlink() > 1 {
-                match hardlinks.entry((meta.dev(), meta.ino())) {
-                    Entry::Occupied(e) => {
-                        // Add link header and continue to next entry
-                        let mut header = tar::Header::new_gnu();
-                        header.set_metadata(&meta);
-                        if meta.mtime() > clamp_mtime {
-                            header.set_mtime(clamp_mtime as u64);
-                        }
-                        header.set_entry_type(tar::EntryType::Link);
-                        header.set_cksum();
-                        builder.append_link(&mut header, &rel_path, e.get())?;
-                        continue;
-                    }
-                    Entry::Vacant(e) => {
-                        // This is the first time we've seen this inode
-                        e.insert(rel_path.clone());
-                    }
-                }
-            }
-
-            let mut header = tar::Header::new_gnu();
-            header.set_size(meta.len());
-            header.set_metadata(&meta);
-            if meta.mtime() > clamp_mtime {
-                header.set_mtime(clamp_mtime as u64);
-            }
-            if entry.file_type().is_file() {
-                builder.append_data(
-                    &mut header,
-                    rel_path,
-                    &mut std::fs::File::open(entry.path())?,
-                )?;
-            } else {
-                builder.append_data(&mut header, rel_path, &mut std::io::empty())?;
-            };
-        }
-    }
-
-    Ok(())
-}
-
 // Convert any extended attributes on the specified path to a tar PAX extension header, and add it to the tar archive
-fn add_pax_extension_header(
+pub(crate) fn add_pax_extension_header(
     path: impl AsRef<Path>,
     builder: &mut tar::Builder<impl Write>,
 ) -> Result<(), anyhow::Error> {