Add check git binary and .git folder

mullvad-version/build.rs

@@ -66,8 +66,11 @@ fn compute_product_version(target: Target) -> String {
 
     // Rerun this build script on changes to the git ref that affects the build version.
     // NOTE: This must be kept up to date with the behavior of `git_rev_parse_commit_hash`.
-    rerun_if_git_ref_changed(&release_tag)
-        .expect("Failed to set 'cargo:rerun-if-changed' on git ref changes");
+    // If an error is returned, we are not in a repository and a proper release version cannot be
+    // built.
+    if let Err(()) = rerun_if_git_ref_changed(&release_tag) {
+        return format!("{release_tag}-dev-local");
+    };
 
     // Get the git commit hashes for the latest release and current HEAD
     let head_commit_hash = git_rev_parse_commit_hash("HEAD");
@@ -87,9 +90,21 @@ fn compute_product_version(target: Target) -> String {
 /// Trigger rebuild of `mullvad-version` on changing branch (`.git/HEAD`), on changes to the ref of
 /// the current branch (`.git/refs/heads/$current_branch`) and on changes to the ref of the current
 /// release tag (`.git/refs/tags/$current_release_tag`).
-fn rerun_if_git_ref_changed(release_tag: &str) -> std::io::Result<()> {
+///
+/// Returns an error if not in a git repository, or the git binary is not in `PATH`.
+fn rerun_if_git_ref_changed(release_tag: &str) -> Result<(), ()> {
     let git_dir = Path::new("..").join(".git");
 
+    if !git_dir.exists() {
+        return Err(());
+    }
+
+    // Check that `git` can be executed
+    match Command::new("git").arg("--version").status() {
+        Ok(status) if status.success() => {}
+        _ => return Err(()),
+    };
+
     // The `.git/HEAD` file contains the position of the current head. If in 'detached HEAD' state,
     // this will be the ref of the current commit. If on a branch it will just point to it, e.g.
     // `ref: refs/heads/main`. Tracking changes to this file will tell us if we change branch, or
@@ -104,7 +119,8 @@ fn rerun_if_git_ref_changed(release_tag: &str) -> std::io::Result<()> {
     let output = Command::new("git")
         .arg("branch")
         .arg("--show-current")
-        .output()?;
+        .output()
+        .expect("Failed to execute `git branch --show-current`");
 
     let current_branch = String::from_utf8(output.stdout).unwrap();
     let current_branch = current_branch.trim();