Extract `rerun-if` logic to separate fn

1 files changed, 57 insertions, 43 deletions

diff --git a/mullvad-version/build.rs b/mullvad-version/build.rs
index 008ec8d104..fc7eb9edbe 100644
--- a/mullvad-version/build.rs
+++ b/mullvad-version/build.rs
@@ -52,15 +52,26 @@ fn get_product_version(target: Target) -> String {
         Target::Desktop => DESKTOP_VERSION_FILE_PATH,
     };
     println!("cargo:rerun-if-changed={version_file_path}");
-    let version = fs::read_to_string(version_file_path)
+    let product_version = fs::read_to_string(version_file_path)
         .unwrap_or_else(|_| panic!("Failed to read {version_file_path}"))
         .trim()
         .to_owned();
 
-    if let Some(dev_suffix) = get_dev_suffix(target, &version) {
-        format!("{version}{dev_suffix}")
+    // Compute the expected tag name for the release named `product_version`
+    let release_tag = match target {
+        Target::Android => format!("android/{product_version}"),
+        Target::Desktop => product_version.to_owned(),
+    };
+
+    // 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 let Some(dev_suffix) = get_dev_suffix(&product_version) {
+        format!("{product_version}{dev_suffix}")
     } else {
-        version
+        product_version
     }
 }
 
@@ -68,13 +79,26 @@ fn get_product_version(target: Target) -> String {
 /// suffix if the build is not done on a git tag named `product_version`.
 /// This also returns `None` if the `git` command can't run, or the code does
 /// not live in a git repository.
-fn get_dev_suffix(target: Target, product_version: &str) -> Option<String> {
-    // Compute the expected tag name for the release named `product_version`
-    let release_tag = match target {
-        Target::Android => format!("android/{product_version}"),
-        Target::Desktop => product_version.to_owned(),
-    };
+fn get_dev_suffix(release_tag: &str) -> Option<String> {
+    // Get the git commit hashes for the latest release and current HEAD
+    // Return `None` if unable to find the hash for HEAD.
+    let head_commit_hash = git_rev_parse_commit_hash("HEAD")?;
+    let product_version_commit_hash = git_rev_parse_commit_hash(release_tag);
+
+    // If we are currently building the release tag, there is no dev suffix
+    if Some(&head_commit_hash) == product_version_commit_hash.as_ref() {
+        return None;
+    }
+    Some(format!(
+        "-dev-{}",
+        &head_commit_hash[..GIT_HASH_DEV_SUFFIX_LEN]
+    ))
+}
 
+/// 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<()> {
     let git_dir = Path::new("..").join(".git");
 
     // If we build our output on information about HEAD we need to re-run if HEAD moves
@@ -83,45 +107,35 @@ fn get_dev_suffix(target: Target, product_version: &str) -> Option<String> {
         println!("cargo:rerun-if-changed={}", head_path.display());
     }
 
+    // If we build our output on information about the ref of the current branch, we need to re-run
+    // on changes to it
     let output = Command::new("git")
         .arg("branch")
         .arg("--show-current")
-        .output()
-        .ok()?;
-    let current_branch = String::from_utf8(output.stdout).unwrap();
-    // If we build our output on information about a git reference, we need to re-run
-    // if it moves. Instead of trying to be smart, just re-run if any git reference moves.
-    let git_current_branch_ref = git_dir
-        .join("refs")
-        .join("heads")
-        .join(current_branch.trim());
-    if git_current_branch_ref.exists() {
-        println!(
-            "cargo:rerun-if-changed={}",
-            git_current_branch_ref.display()
-        );
-    }
-    let git_current_branch_ref = git_dir.join("refs").join("tags").join(&release_tag);
-    if git_current_branch_ref.exists() {
-        println!(
-            "cargo:rerun-if-changed={}",
-            git_current_branch_ref.display()
-        );
-    }
+        .output()?;
 
-    // Get the git commit hashes for the latest release and current HEAD
-    // Return `None` if unable to find the hash for HEAD.
-    let head_commit_hash = git_rev_parse_commit_hash("HEAD")?;
-    let product_version_commit_hash = git_rev_parse_commit_hash(&release_tag);
+    let current_branch = String::from_utf8(output.stdout).unwrap();
+    let current_branch = current_branch.trim();
 
-    // If we are currently building the release tag, there is no dev suffix
-    if Some(&head_commit_hash) == product_version_commit_hash.as_ref() {
-        return None;
+    // When in 'detached HEAD' state, the output will be empty. However, in that case we already get
+    // the ref from `.git/HEAD`, so we can safely skip this part.
+    if !current_branch.is_empty() {
+        let git_current_branch_ref = git_dir.join("refs").join("heads").join(current_branch);
+        if git_current_branch_ref.exists() {
+            println!(
+                "cargo:rerun-if-changed={}",
+                git_current_branch_ref.display()
+            );
+        }
     }
-    Some(format!(
-        "-dev-{}",
-        &head_commit_hash[..GIT_HASH_DEV_SUFFIX_LEN]
-    ))
+
+    // Since the product version depends on if the build is done on the commit with the
+    // corresponding release tag or not, we must track creation of/changes to said tag
+    let git_release_tag_ref = git_dir.join("refs").join("tags").join(release_tag);
+    if git_release_tag_ref.exists() {
+        println!("cargo:rerun-if-changed={}", git_release_tag_ref.display());
+    };
+    Some(())
 }
 
 /// Returns the commit hash for the commit that `git_ref` is pointing to.