Extract install_into_rootfs helper (U2)

Deduplicate the three near-identical write-temp→install blocks in
inject() into a single install_into_rootfs() helper.

src/inject.rs

@@ -87,46 +87,29 @@ pub fn build_exec_script(command: &str, env: &[String], working_dir: Option<&str
     lines
 }
 
+/// Write `content` to a temp file and install it into the mounted rootfs at
+/// `<mount>/slim/<name>` with the given mode.
+fn install_into_rootfs(mount: &str, name: &str, mode: &str, content: &str) -> Result<()> {
+    let temp = tempfile::NamedTempFile::new()?;
+    fs::write(temp.path(), content)?;
+    let src = temp.path().to_str().context("temp path is not UTF-8")?;
+    let dest = format!("{mount}/slim/{name}");
+    cmd(&[
+        "podman", "unshare", "--", "install", "-D", "-m", mode, src, &dest,
+    ])?;
+    Ok(())
+}
+
 /// Inject /slim/init, /slim/exec, and optionally /slim/user into a mounted
 /// container image rootfs.
 pub fn inject(mount_path: &Path, exec_script: &str, user: Option<&str>) -> Result<()> {
     let mount_str = mount_path.to_str().context("mount path is not UTF-8")?;
 
-    let init_temp = tempfile::NamedTempFile::new()?;
+    install_into_rootfs(mount_str, "init", "755", SLIM_INIT)?;
-    let exec_temp = tempfile::NamedTempFile::new()?;
+    install_into_rootfs(mount_str, "exec", "755", exec_script)?;
-    fs::write(init_temp.path(), SLIM_INIT)?;
-    fs::write(exec_temp.path(), exec_script)?;
-
-    let init_src = init_temp
-        .path()
-        .to_str()
-        .context("temp path is not UTF-8")?;
-    let exec_src = exec_temp
-        .path()
-        .to_str()
-        .context("temp path is not UTF-8")?;
-
-    let init_dest = format!("{mount_str}/slim/init");
-    let exec_dest = format!("{mount_str}/slim/exec");
-
-    cmd(&[
-        "podman", "unshare", "--", "install", "-D", "-m", "755", init_src, &init_dest,
-    ])?;
-    cmd(&[
-        "podman", "unshare", "--", "install", "-D", "-m", "755", exec_src, &exec_dest,
-    ])?;
 
     if let Some(user) = user.filter(|u| !u.is_empty()) {
-        let user_temp = tempfile::NamedTempFile::new()?;
+        install_into_rootfs(mount_str, "user", "644", user)?;
-        fs::write(user_temp.path(), user)?;
-        let user_src = user_temp
-            .path()
-            .to_str()
-            .context("temp path is not UTF-8")?;
-        let user_dest = format!("{mount_str}/slim/user");
-        cmd(&[
-            "podman", "unshare", "--", "install", "-D", "-m", "644", user_src, &user_dest,
-        ])?;
     }
 
     Ok(())