fix(core): resolve git metadata in worktrees

Signed-off-by: Matthew Grossman <mgrossman@nvidia.com>

crates/openshell-core/build.rs

@@ -4,16 +4,20 @@
 use std::env;
 use std::path::{Path, PathBuf};
 
+#[path = "build_support/git.rs"]
+mod git;
+
 const PROTO_REL: &str = "../../proto";
 
 fn main() -> Result<(), Box<dyn std::error::Error>> {
+    let manifest_dir = PathBuf::from(env::var("CARGO_MANIFEST_DIR")?);
+
     // --- Git-derived version ---
     // Compute a version from `git describe` for local builds. In Docker/CI
     // builds where .git is absent, this silently does nothing and the binary
     // falls back to CARGO_PKG_VERSION (which is already sed-patched by the
     // build pipeline).
-    println!("cargo:rerun-if-changed=../../.git/HEAD");
-    println!("cargo:rerun-if-changed=../../.git/refs/tags");
+    git::emit_rerun_if_changed(&manifest_dir);
 
     if let Some(version) = git_version() {
         println!("cargo:rustc-env=OPENSHELL_GIT_VERSION={version}");
@@ -33,7 +37,6 @@ fn main() -> Result<(), Box<dyn std::error::Error>> {
         env::set_var("PROTOC", protobuf_src::protoc());
     }
 
-    let manifest_dir = PathBuf::from(env::var("CARGO_MANIFEST_DIR")?);
     let proto_root = manifest_dir.join(PROTO_REL);
 
     let mut proto_files = Vec::new();

crates/openshell-core/build_support/git.rs

@@ -0,0 +1,45 @@
+// SPDX-FileCopyrightText: Copyright (c) 2025-2026 NVIDIA CORPORATION & AFFILIATES. All rights reserved.
+// SPDX-License-Identifier: Apache-2.0
+
+use std::collections::BTreeSet;
+use std::path::{Path, PathBuf};
+use std::process::Command;
+
+const WATCH_PATHS: [&str; 2] = ["HEAD", "refs/tags"];
+
+#[cfg(not(test))]
+pub fn emit_rerun_if_changed(manifest_dir: &Path) {
+    for path in watch_paths(manifest_dir) {
+        println!("cargo:rerun-if-changed={}", path.display());
+    }
+}
+
+pub fn watch_paths(manifest_dir: &Path) -> Vec<PathBuf> {
+    WATCH_PATHS
+        .into_iter()
+        .filter_map(|path| resolve_existing_git_path(manifest_dir, path))
+        .collect::<BTreeSet<_>>()
+        .into_iter()
+        .collect()
+}
+
+fn resolve_existing_git_path(manifest_dir: &Path, path: &str) -> Option<PathBuf> {
+    let output = Command::new("git")
+        .args(["rev-parse", "--git-path", path])
+        .current_dir(manifest_dir)
+        .output()
+        .ok()?;
+
+    if !output.status.success() {
+        return None;
+    }
+
+    let path = PathBuf::from(String::from_utf8(output.stdout).ok()?.trim());
+    let path = if path.is_absolute() {
+        path
+    } else {
+        manifest_dir.join(path)
+    };
+
+    path.exists().then_some(path)
+}

crates/openshell-core/tests/git_watch_paths.rs

@@ -0,0 +1,92 @@
+// SPDX-FileCopyrightText: Copyright (c) 2025-2026 NVIDIA CORPORATION & AFFILIATES. All rights reserved.
+// SPDX-License-Identifier: Apache-2.0
+
+#[path = "../build_support/git.rs"]
+mod git;
+
+use std::fs;
+use std::path::{Path, PathBuf};
+use std::process::Command;
+
+fn run_git(repo: &Path, args: &[&str]) -> String {
+    let output = Command::new("git")
+        .arg("-C")
+        .arg(repo)
+        .args(args)
+        .output()
+        .expect("run git");
+    assert!(
+        output.status.success(),
+        "git {args:?} failed: {}",
+        String::from_utf8_lossy(&output.stderr)
+    );
+    String::from_utf8(output.stdout)
+        .expect("git output is UTF-8")
+        .trim()
+        .to_owned()
+}
+
+fn init_repository() -> (tempfile::TempDir, PathBuf) {
+    let temp = tempfile::tempdir().expect("create temp directory");
+    let repo = temp.path().join("repo");
+    fs::create_dir(&repo).expect("create repository directory");
+
+    run_git(&repo, &["init", "--quiet"]);
+    run_git(&repo, &["config", "user.name", "OpenShell Test"]);
+    run_git(
+        &repo,
+        &["config", "user.email", "openshell@example.invalid"],
+    );
+    fs::write(repo.join("README.md"), "test\n").expect("write tracked file");
+    run_git(&repo, &["add", "README.md"]);
+    run_git(&repo, &["commit", "--quiet", "-m", "initial"]);
+    run_git(&repo, &["tag", "v0.0.1"]);
+
+    (temp, repo)
+}
+
+fn git_path(repo: &Path, path: &str) -> PathBuf {
+    let path = PathBuf::from(run_git(repo, &["rev-parse", "--git-path", path]));
+    if path.is_absolute() {
+        path
+    } else {
+        repo.join(path)
+    }
+}
+
+#[test]
+fn resolves_metadata_paths_in_normal_checkout() {
+    let (_temp, repo) = init_repository();
+
+    let paths = git::watch_paths(&repo);
+
+    assert!(paths.contains(&git_path(&repo, "HEAD")));
+    assert!(paths.contains(&git_path(&repo, "refs/tags")));
+    assert!(paths.iter().all(|path| path.exists()));
+}
+
+#[test]
+fn resolves_metadata_paths_in_linked_worktree() {
+    let (temp, repo) = init_repository();
+    let worktree = temp.path().join("linked");
+    let worktree_arg = worktree.to_str().expect("worktree path is UTF-8");
+    run_git(
+        &repo,
+        &["worktree", "add", "--quiet", "-b", "linked", worktree_arg],
+    );
+
+    let paths = git::watch_paths(&worktree);
+    let head = git_path(&worktree, "HEAD");
+
+    assert!(paths.contains(&head));
+    assert_ne!(head, worktree.join(".git/HEAD"));
+    assert!(paths.contains(&git_path(&worktree, "refs/tags")));
+    assert!(paths.iter().all(|path| path.exists()));
+}
+
+#[test]
+fn ignores_source_tree_without_git_metadata() {
+    let temp = tempfile::tempdir().expect("create temp directory");
+
+    assert!(git::watch_paths(temp.path()).is_empty());
+}