From c4dc58a3b45ab56f09c7baef3c679c1a80ee4ee9 Mon Sep 17 00:00:00 2001
From: Seongmin Lee <git@boltless.me>
Date: Tue, 29 Sep 2026 04:53:46 +0900
Subject: [PATCH] slop: nix,gix-pack: welcome back mmap!

Signed-off-by: Seongmin Lee <git@boltless.me>
---
 Cargo.lock                                 |  1 +
 knot2/third_party/gix-pack/Cargo.toml      |  3 ++
 knot2/third_party/gix-pack/src/data/mod.rs |  6 +++-
 knot2/third_party/gix-pack/src/lib.rs      | 33 ++++++----------------
 nix/Cargo.nix                              |  4 +++
 5 files changed, 21 insertions(+), 26 deletions(-)

diff --git a/Cargo.lock b/Cargo.lock
index b0b83d979..758c7d407 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -3689,6 +3689,7 @@ dependencies = [
  "gix-path",
  "gix-tempfile",
  "gix-traverse",
+ "memmap2",
  "parking_lot",
  "smallvec",
  "thiserror 2.0.18",
diff --git a/knot2/third_party/gix-pack/Cargo.toml b/knot2/third_party/gix-pack/Cargo.toml
index e482f053b..dd0ebff6b 100644
--- a/knot2/third_party/gix-pack/Cargo.toml
+++ b/knot2/third_party/gix-pack/Cargo.toml
@@ -121,6 +121,9 @@ version = "^0.12.1"
 version = "^0.58.0"
 optional = true
 
+[dependencies.memmap2]
+version = "0.9.10"
+
 [dependencies.parking_lot]
 version = "0.12.4"
 optional = true
diff --git a/knot2/third_party/gix-pack/src/data/mod.rs b/knot2/third_party/gix-pack/src/data/mod.rs
index f2058ee75..3bbb37f0c 100644
--- a/knot2/third_party/gix-pack/src/data/mod.rs
+++ b/knot2/third_party/gix-pack/src/data/mod.rs
@@ -143,7 +143,11 @@ impl File {
     }
 
     pub(crate) fn materialized(&self) -> std::io::Result<crate::MMap> {
-        crate::MMap::map(&self.file)
+        // SAFETY: we have to take the risk of somebody changing the file underneath. Git never writes into the same file.
+        #[expect(unsafe_code)]
+        unsafe {
+            memmap2::MmapOptions::new().map_copy_read_only(&self.file)
+        }
     }
 
     /// Returns the pack data at the given slice if its range is contained in the pack data.
diff --git a/knot2/third_party/gix-pack/src/lib.rs b/knot2/third_party/gix-pack/src/lib.rs
index 478f85c8b..9b7f4a7c8 100755
--- a/knot2/third_party/gix-pack/src/lib.rs
+++ b/knot2/third_party/gix-pack/src/lib.rs
@@ -20,29 +20,7 @@
 
 use std::{borrow::Cow, ops::Deref, path::Path};
 
-/// The default in-memory backing store for index and multi-index files.
-#[allow(missing_docs)]
-pub struct MMap(Vec<u8>);
-
-impl MMap {
-    #[allow(missing_docs)]
-    pub fn map(file: &std::fs::File) -> std::io::Result<Self> {
-        use std::os::unix::fs::FileExt;
-        let len = usize::try_from(file.metadata()?.len())
-            .map_err(|_| std::io::Error::other("file too large to load into memory"))?;
-        let mut bytes = vec![0u8; len];
-        file.read_exact_at(&mut bytes, 0)?;
-        Ok(MMap(bytes))
-    }
-}
-
-impl Deref for MMap {
-    type Target = [u8];
-
-    fn deref(&self) -> &[u8] {
-        &self.0
-    }
-}
+pub use memmap2::Mmap as MMap;
 
 /// A byte-oriented backing store for pack data and indices.
 pub trait FileData: Deref<Target = [u8]> {}
@@ -81,8 +59,13 @@ pub mod verify;
 mod mmap {
     use std::path::Path;
 
-    pub fn read_only(path: &Path) -> std::io::Result<super::MMap> {
-        Ok(super::MMap(std::fs::read(path)?))
+    pub fn read_only(path: &Path) -> std::io::Result<memmap2::Mmap> {
+        let file = std::fs::File::open(path)?;
+        // SAFETY: we have to take the risk of somebody changing the file underneath. Git never writes into the same file.
+        #[expect(unsafe_code)]
+        unsafe {
+            memmap2::MmapOptions::new().map_copy_read_only(&file)
+        }
     }
 }
 
diff --git a/nix/Cargo.nix b/nix/Cargo.nix
index 10cfcdef3..3b43cac70 100644
--- a/nix/Cargo.nix
+++ b/nix/Cargo.nix
@@ -13236,6 +13236,10 @@ rec {
             packageId = "gix-traverse";
             optional = true;
           }
+          {
+            name = "memmap2";
+            packageId = "memmap2";
+          }
           {
             name = "parking_lot";
             packageId = "parking_lot";
-- 
2.51.2