Prevent archive overwrite races

2 files changed, 28 insertions, 2 deletions

diff --git a/src/archive.rs b/src/archive.rs
index ad424c4..c0a2195 100644
--- a/src/archive.rs
+++ b/src/archive.rs
@@ -27,6 +27,9 @@ impl NativeTarWriter {
 
         let output = output.as_ref();
         reject_output_inside_rootfs(rootfs, output)?;
+        if output.exists() {
+            bail!("archive output already exists; refusing to overwrite it: {}", output.display());
+        }
         let temporary = temporary_output_path(output)?;
         let result = (|| {
             let file = OpenOptions::new()
@@ -39,13 +42,18 @@ impl NativeTarWriter {
             archive
                 .finish()
                 .with_context(|| format!("finish archive {}", temporary.display()))?;
-            fs::rename(&temporary, output).with_context(|| {
+            // `rename` replaces an existing destination on Unix. Publishing via
+            // a hard link instead gives this same-directory temporary file an
+            // atomic no-replace final name.
+            fs::hard_link(&temporary, output).with_context(|| {
                 format!(
                     "publish completed archive {} as {}",
                     temporary.display(),
                     output.display()
                 )
-            })
+            })?;
+            fs::remove_file(&temporary)
+                .with_context(|| format!("remove temporary archive {}", temporary.display()))
         })();
         if result.is_err() {
             let _ = fs::remove_file(&temporary);
diff --git a/tests/archive.rs b/tests/archive.rs
index 6f803ef..6e59837 100644
--- a/tests/archive.rs
+++ b/tests/archive.rs
@@ -125,6 +125,24 @@ fn rejects_an_archive_output_inside_the_rootfs() {
 }
 
 #[test]
+fn refuses_to_replace_an_existing_archive() {
+    let fixture = tempdir().expect("fixture directory");
+    let rootfs = fixture.path().join("rootfs");
+    fs::create_dir(&rootfs).expect("create rootfs");
+    fs::write(rootfs.join("controller.conf"), "controller=true\n").expect("write rootfs file");
+    let artifact = fixture.path().join("controller.tar");
+    fs::write(&artifact, "existing artifact").expect("write existing artifact");
+
+    let error = NativeTarWriter::new()
+        .write(&rootfs, &artifact)
+        .expect_err("an existing archive must not be replaced");
+
+    assert!(error.to_string().contains("archive output already exists"));
+    assert_eq!(fs::read_to_string(&artifact).expect("read existing artifact"), "existing artifact");
+    assert!(!fixture.path().join("controller.tar.partial").exists());
+}
+
+#[test]
 fn removes_partial_output_when_an_unsupported_entry_stops_packaging() {
     let fixture = tempdir().expect("fixture directory");
     let rootfs = fixture.path().join("rootfs");