repository: Atomically replace refs/ symlinks

Naming a new version of an image the same as the old version is
pretty standard, and this is currenlty giving EEXIST errors. We
fix this by doing an atomic symlink + rename operation to replace
the symlink.

Signed-off-by: Alexander Larsson <[email protected]>

Author: alexlarsson

crates/composefs/Cargo.toml

@@ -29,6 +29,7 @@ tempfile = { version = "3.8.0", optional = true, default-features = false }
 xxhash-rust = { version = "0.8.2", default-features = false, features = ["xxh32"] }
 zerocopy = { version = "0.8.0", default-features = false, features = ["derive", "std"] }
 zstd = { version = "0.13.0", default-features = false }
+rand = { version = "0.9.1", default-features = true }
 
 [dev-dependencies]
 insta = "1.42.2"

crates/composefs/src/repository.rs

@@ -10,6 +10,7 @@ use std::{
 
 use anyhow::{bail, ensure, Context, Result};
 use once_cell::sync::OnceCell;
+use rand::{distr::Alphanumeric, Rng};
 use rustix::{
     fs::{
         fdatasync, flock, linkat, mkdirat, open, openat, readlinkat, symlinkat, AtFlags, Dir,
@@ -432,7 +433,30 @@ impl<ObjectID: FsVerityHashValue> Repository<ObjectID> {
             relative.push(target_component);
         }
 
-        symlinkat(relative, &self.repository, name)
+        // Atomically replace existing symlink
+
+        let mut count = 0;
+        let tmp_name = loop {
+            let rand_string: String = rand::rng()
+                .sample_iter(&Alphanumeric)
+                .take(12)
+                .map(char::from)
+                .collect();
+            let tmp_name = format!(".symlink{}", rand_string);
+            match symlinkat(&relative, &self.repository, &tmp_name) {
+                Ok(_) => break tmp_name,
+                Err(Errno::EXIST) => {
+                    if count > 10 {
+                        Err(Errno::EXIST)?
+                    }
+                    count += 1;
+                    continue;
+                }
+                Err(other) => Err(other)?,
+            }
+        };
+
+        renameat(&self.repository, &tmp_name, &self.repository, name)
     }
 
     pub fn ensure_symlink<P: AsRef<Path>>(&self, name: P, target: &str) -> ErrnoResult<()> {