From c7dc1a0173a14e9cb52adcb09ae566cf7eeb314a Mon Sep 17 00:00:00 2001
From: Hermes Agent <hermes@localhost>
Date: Wed, 12 Aug 2026 06:02:57 +0000
Subject: Protect companion manifests from overwrite

---
 src/build.rs     |  4 ++++
 src/manifest.rs  | 10 ++++++++--
 tests/build.rs   | 23 +++++++++++++++++++++++
 tests/compare.rs | 15 +++++++++++++++
 4 files changed, 50 insertions(+), 2 deletions(-)

diff --git a/src/build.rs b/src/build.rs
index 42c57ef..fcacfc0 100644
--- a/src/build.rs
+++ b/src/build.rs
@@ -42,6 +42,10 @@ impl<I: PackageInstaller, B: InitramfsBuilder> BuildExecutor<I, B> {
         if artifact.exists() {
             bail!("output artifact already exists; refusing to overwrite it: {}", artifact.display());
         }
+        let manifest = ArtifactManifest::path_beside(artifact)?;
+        if manifest.exists() {
+            bail!("output manifest already exists; refusing to overwrite it: {}", manifest.display());
+        }
         let parent = artifact
             .parent()
             .filter(|parent| !parent.as_os_str().is_empty())
diff --git a/src/manifest.rs b/src/manifest.rs
index 0848a98..4cd256c 100644
--- a/src/manifest.rs
+++ b/src/manifest.rs
@@ -1,6 +1,6 @@
 use std::collections::{BTreeMap, HashSet};
 use std::fs;
-use std::io::Read;
+use std::io::{Read, Write};
 use std::os::unix::fs::MetadataExt;
 use std::path::{Path, PathBuf};
 
@@ -51,7 +51,13 @@ impl ArtifactManifest {
     pub fn write_beside(&self, artifact: impl AsRef<Path>) -> Result<PathBuf> {
         let path = Self::path_beside(artifact)?;
         let text = toml::to_string_pretty(self).context("serialize artifact manifest")?;
-        fs::write(&path, text).with_context(|| format!("write artifact manifest {}", path.display()))?;
+        fs::OpenOptions::new()
+            .write(true)
+            .create_new(true)
+            .open(&path)
+            .with_context(|| format!("create artifact manifest {}", path.display()))?
+            .write_all(text.as_bytes())
+            .with_context(|| format!("write artifact manifest {}", path.display()))?;
         Ok(path)
     }
 
diff --git a/tests/build.rs b/tests/build.rs
index 6613cc7..c6ed70d 100644
--- a/tests/build.rs
+++ b/tests/build.rs
@@ -112,6 +112,29 @@ fn rejects_an_existing_artifact_before_creating_the_workspace_or_installing() {
     assert!(installer.requests.borrow().is_empty());
 }
 
+#[test]
+fn rejects_an_existing_companion_manifest_before_creating_the_workspace_or_installing() {
+    let fixture = tempdir().expect("fixture directory");
+    let spec = ImageSpec::load(std::path::Path::new("profiles/alt-controller.toml"))
+        .expect("load controller spec");
+    let plan = BuildPlan::compile(spec).expect("compile build plan");
+    let installer = FixtureInstaller::default();
+    let mut executor = BuildExecutor::new(&installer, FixtureInitramfsBuilder);
+    let workspace = fixture.path().join("work");
+    let artifact = fixture.path().join("controller.tar");
+    let companion = ArtifactManifest::path_beside(&artifact).expect("companion manifest path");
+    fs::write(&companion, "do not replace").expect("write pre-existing companion manifest");
+
+    let error = executor
+        .execute(&plan, &workspace, "profiles/apt.conf", &artifact)
+        .expect_err("pre-existing companion manifest must be rejected");
+
+    assert!(error.to_string().contains("output manifest already exists"));
+    assert_eq!(fs::read_to_string(&companion).expect("read companion manifest"), "do not replace");
+    assert!(!workspace.exists());
+    assert!(installer.requests.borrow().is_empty());
+}
+
 #[test]
 fn rejects_a_missing_apt_configuration_before_creating_the_workspace() {
     let fixture = tempdir().expect("fixture directory");
diff --git a/tests/compare.rs b/tests/compare.rs
index f7b3a04..4f48721 100644
--- a/tests/compare.rs
+++ b/tests/compare.rs
@@ -158,6 +158,21 @@ fn writes_distinct_manifests_for_artifacts_in_the_same_directory() {
     assert_eq!(ArtifactManifest::load(&native_path).expect("load native manifest"), native_manifest);
 }
 
+#[test]
+fn refuses_to_overwrite_an_existing_companion_manifest() {
+    let fixture = tempdir().expect("temporary directory");
+    let artifact = fixture.path().join("controller.tar");
+    let manifest_path = ArtifactManifest::path_beside(&artifact).expect("manifest path");
+    fs::write(&manifest_path, "preserve this manifest").expect("write existing manifest");
+
+    let error = baseline()
+        .write_beside(&artifact)
+        .expect_err("existing companion manifests must not be overwritten");
+
+    assert!(error.to_string().contains("create artifact manifest"));
+    assert_eq!(fs::read_to_string(&manifest_path).expect("read existing manifest"), "preserve this manifest");
+}
+
 #[test]
 fn rejects_duplicate_semantic_keys() {
     let error = ArtifactManifest::new(
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