image: change inline data from Vec<u8> to Box<[u8]>

This makes more sense, and we don't risk accidentally carrying around
spare capacity.  Do the same for splitstream as well.  This requires a
couple of adjustments around the tree, but nothing major.

Take the chance to make better use of the new Buffer API in rustix to
avoid a copy (from a slice subset of a vector back into a vector) when
reading the file data from the filesystem.

Signed-off-by: Allison Karlitskaya <[email protected]>

Author: allisonkarlitskaya

src/fs.rs

@@ -13,6 +13,7 @@ use std::{
 
 use anyhow::{ensure, Result};
 use rustix::{
+    buffer::spare_capacity,
     fd::{AsFd, OwnedFd},
     fs::{
         fstat, getxattr, linkat, listxattr, mkdirat, mknodat, openat, readlinkat, symlinkat,
@@ -184,18 +185,19 @@ impl FilesystemReader<'_> {
         let content = match FileType::from_raw_mode(buf.st_mode) {
             FileType::Directory | FileType::Unknown => unreachable!(),
             FileType::RegularFile => {
-                let mut buffer = vec![MaybeUninit::uninit(); buf.st_size as usize];
-                let (data, _) = read(fd, &mut buffer)?;
+                let mut buffer = Vec::with_capacity(buf.st_size as usize);
+                read(fd, spare_capacity(&mut buffer))?;
+                let buffer = Box::from(buffer);
 
                 if buf.st_size > INLINE_CONTENT_MAX as i64 {
                     let id = if let Some(repo) = self.repo {
-                        repo.ensure_object(data)?
+                        repo.ensure_object(&buffer)?
                     } else {
-                        FsVerityHasher::hash(data)
+                        FsVerityHasher::hash(&buffer)
                     };
                     LeafContent::Regular(RegularFile::External(id, buf.st_size as u64))
                 } else {
-                    LeafContent::Regular(RegularFile::Inline(Vec::from(data)))
+                    LeafContent::Regular(RegularFile::Inline(buffer))
                 }
             }
             FileType::Symlink => {

src/image.rs

@@ -22,7 +22,7 @@ pub struct Stat {
 
 #[derive(Debug)]
 pub enum RegularFile {
-    Inline(Vec<u8>),
+    Inline(Box<[u8]>),
     External(Sha256HashValue, u64),
 }
 

src/oci/image.rs

@@ -122,7 +122,7 @@ fn file_entry(path: &str) -> oci::tar::TarEntry {
             st_mtim_sec: 0,
             xattrs: RefCell::new(BTreeMap::new()),
         },
-        item: oci::tar::TarItem::Leaf(LeafContent::Regular(RegularFile::Inline(vec![]))),
+        item: oci::tar::TarItem::Leaf(LeafContent::Regular(RegularFile::Inline([].into()))),
     }
 }
 

src/selabel.rs

@@ -116,7 +116,7 @@ pub fn openat<'a>(
 
     match &dir.entries[idx].inode {
         Inode::Leaf(leaf) => match &leaf.content {
-            LeafContent::Regular(RegularFile::Inline(data)) => Ok(Some(Box::new(data.as_slice()))),
+            LeafContent::Regular(RegularFile::Inline(data)) => Ok(Some(Box::new(&**data))),
             LeafContent::Regular(RegularFile::External(id, ..)) => {
                 Ok(Some(Box::new(File::from(repo.open_object(id)?))))
             }

src/splitstream.rs

@@ -168,7 +168,7 @@ impl SplitStreamWriter<'_> {
 
 #[derive(Debug)]
 pub enum SplitStreamData {
-    Inline(Vec<u8>),
+    Inline(Box<[u8]>),
     External(Sha256HashValue),
 }
 
@@ -315,7 +315,7 @@ impl<R: Read> SplitStreamReader<R> {
             read_into_vec(&mut self.decoder, &mut content, stored_size)?;
             content.truncate(actual_size);
             self.inline_bytes -= stored_size;
-            Ok(SplitStreamData::Inline(content))
+            Ok(SplitStreamData::Inline(content.into()))
         }
     }
 

tests/mkfs.rs

@@ -54,7 +54,7 @@ fn simple(fs: &mut FileSystem) {
     add_leaf(
         &mut fs.root,
         "regular-inline",
-        LeafContent::Regular(RegularFile::Inline(b"hihi".to_vec())),
+        LeafContent::Regular(RegularFile::Inline((*b"hihi").into())),
     );
     add_leaf(
         &mut fs.root,