refactor(nix-compat/nar/reader): reuse prev_name allocation r/8095

We reuse the prev_name allocation for Entry, instead of allocating and
returning a separate Vec.

We encode the `prev_name: None` case as an empty vector, since we don't
allow empty names anyway, and the sorting is equivalent.

Change-Id: I975b37ff873805f5ff099bc82128706891052247
Reviewed-on: https://cl.tvl.fyi/c/depot/+/11607
Reviewed-by: Brian Olsen <me@griff.name>
Reviewed-by: flokli <flokli@flokli.de>
Tested-by: BuildkiteCI

2 files changed, 20 insertions, 23 deletions

diff --git a/tvix/nix-compat/src/nar/reader/async/mod.rs b/tvix/nix-compat/src/nar/reader/async/mod.rs
index 048283a67cb5..0808fba38c47 100644
--- a/tvix/nix-compat/src/nar/reader/async/mod.rs
+++ b/tvix/nix-compat/src/nar/reader/async/mod.rs
@@ -1,4 +1,5 @@
 use std::{
+    mem::MaybeUninit,
     pin::Pin,
     task::{self, Poll},
 };
@@ -110,11 +111,11 @@ pub struct DirReader<'a, 'r> {
     reader: &'a mut Reader<'r>,
     /// Previous directory entry name.
     /// We have to hang onto this to enforce name monotonicity.
-    prev_name: Option<Vec<u8>>,
+    prev_name: Vec<u8>,
 }
 
 pub struct Entry<'a, 'r> {
-    pub name: Vec<u8>,
+    pub name: &'a [u8],
     pub node: Node<'a, 'r>,
 }
 
@@ -122,7 +123,7 @@ impl<'a, 'r> DirReader<'a, 'r> {
     fn new(reader: &'a mut Reader<'r>) -> Self {
         Self {
             reader,
-            prev_name: None,
+            prev_name: vec![],
         }
     }
 
@@ -138,7 +139,7 @@ impl<'a, 'r> DirReader<'a, 'r> {
     pub async fn next(&mut self) -> io::Result<Option<Entry<'_, 'r>>> {
         // COME FROM the previous iteration: if we've already read an entry,
         // read its terminating TOK_PAR here.
-        if self.prev_name.is_some() {
+        if !self.prev_name.is_empty() {
             read::token(self.reader, &nar::wire::TOK_PAR).await?;
         }
 
@@ -146,30 +147,26 @@ impl<'a, 'r> DirReader<'a, 'r> {
             return Ok(None);
         }
 
-        let name = wire::read_bytes(self.reader, 1..=nar::wire::MAX_NAME_LEN).await?;
+        let mut name = [MaybeUninit::uninit(); nar::wire::MAX_NAME_LEN + 1];
+        let name =
+            wire::read_bytes_buf(self.reader, &mut name, 1..=nar::wire::MAX_NAME_LEN).await?;
 
         if name.contains(&0) || name.contains(&b'/') || name == b"." || name == b".." {
             return Err(InvalidData.into());
         }
 
         // Enforce strict monotonicity of directory entry names.
-        match &mut self.prev_name {
-            None => {
-                self.prev_name = Some(name.clone());
-            }
-            Some(prev_name) => {
-                if *prev_name >= name {
-                    return Err(InvalidData.into());
-                }
-
-                name[..].clone_into(prev_name);
-            }
+        if &self.prev_name[..] >= name {
+            return Err(InvalidData.into());
         }
 
+        self.prev_name.clear();
+        self.prev_name.extend_from_slice(name);
+
         read::token(self.reader, &nar::wire::TOK_NOD).await?;
 
         Ok(Some(Entry {
-            name,
+            name: &self.prev_name,
             node: Node::new(self.reader).await?,
         }))
     }
diff --git a/tvix/nix-compat/src/nar/reader/async/test.rs b/tvix/nix-compat/src/nar/reader/async/test.rs
index 58bb651fca21..7bc1f8942f50 100644
--- a/tvix/nix-compat/src/nar/reader/async/test.rs
+++ b/tvix/nix-compat/src/nar/reader/async/test.rs
@@ -80,7 +80,7 @@ async fn complicated() {
                     .expect("next must be some")
                     .expect("must be some");
 
-                assert_eq!(&b"keep"[..], &entry.name);
+                assert_eq!(b"keep", entry.name);
 
                 match entry.node {
                     nar::reader::Node::Directory(mut subdir_reader) => {
@@ -133,7 +133,7 @@ async fn file_read_abandoned() {
                     .expect("next must succeed")
                     .expect("must be some");
 
-                assert_eq!(&b".keep"[..], &entry.name);
+                assert_eq!(b".keep", entry.name);
                 // don't bother to finish reading it.
             };
 
@@ -183,7 +183,7 @@ async fn dir_read_abandoned() {
                     .expect("next must be some")
                     .expect("must be some");
 
-                assert_eq!(&b"keep"[..], &entry.name);
+                assert_eq!(b"keep", entry.name);
 
                 match entry.node {
                     nar::reader::Node::Directory(_) => {
@@ -239,7 +239,7 @@ async fn dir_read_after_none() {
                     .expect("next must be some")
                     .expect("must be some");
 
-                assert_eq!(&b"keep"[..], &entry.name);
+                assert_eq!(b"keep", entry.name);
 
                 match entry.node {
                     nar::reader::Node::Directory(mut subdir_reader) => {
@@ -280,7 +280,7 @@ async fn dir_read_after_none() {
 }
 
 async fn must_read_file(name: &'static str, entry: nar::reader::Entry<'_, '_>) {
-    assert_eq!(name.as_bytes(), &entry.name);
+    assert_eq!(name.as_bytes(), entry.name);
 
     match entry.node {
         nar::reader::Node::File {
@@ -299,7 +299,7 @@ fn must_be_symlink(
     exp_target: &'static str,
     entry: nar::reader::Entry<'_, '_>,
 ) {
-    assert_eq!(name.as_bytes(), &entry.name);
+    assert_eq!(name.as_bytes(), entry.name);
 
     match entry.node {
         nar::reader::Node::Symlink { target } => {