debcargo: Replacing subprocess crate with git2 crate

In my previous post I talked about using subprocess crate to extract beginning and ending year from git repository for generating debian/copyright file. In this post I'm going to talk on how I replaced subprocess with native git2 crate and achieved the same result in much cleaner and safer way.

git2 is a native Rust crate which provides access to Git repository internals. git2 does not involve any unsafe invocation as it is built against libgit2-sys which is actually using Rust FFI to directly bind to underlying libgit library. Below is the new copyright_fromgit function with git2 crate.

fn copyright_fromgit(repo_url: &str) -> Result<String> {
   let tempdir = TempDir::new_in(".", "debcargo")?;
   let repo = Repository::clone(repo_url, tempdir.path())?;

   let mut revwalker = repo.revwalk()?;
   revwalker.push_head()?;

   // Get the latest and first commit id. This is bit ugly
   let latest_id = revwalker.next().unwrap()?;
   let first_id = revwalker.last().unwrap()?; // revwalker ends here is consumed by last

   let first_commit = repo.find_commit(first_id)?;
   let latest_commit = repo.find_commit(latest_id)?;

   let first_year =
       DateTime::<Utc>::from_utc(
               NaiveDateTime::from_timestamp(first_commit.time().seconds(), 0),
               Utc).year();

   let latest_year =
       DateTime::<Utc>::from_utc(
             NaiveDateTime::from_timestamp(latest_commit.time().seconds(), 0),
             Utc).year();

   let notice = match first_year.cmp(&latest_year) {
       Ordering::Equal => format!("{}", first_year),
       _ => format!("{}-{},", first_year, latest_year),
   };

   Ok(notice)
}

So here is what I'm doing

1. Use git2::Repository::clone to clone the given URL. We are thus avoiding exec of git clone command.
2. Get a revision walker object. git2::RevWalk implements Iterator trait and allows walking through the git history. This is what we are using to avoid exec of git log command.
3. revwalker.push_head() is important because we want to tell revwalker from where we want to walk the history. In this case we are asking it to walk history from repository HEAD. Without this line next line will not work. (Learned it in hard way :-) ).
4. Then we extract git2::Oid which is we can say similar to commit hash and can be used to lookup a particular commit. We take latest commit hash using RevWalk::next call and the first commit using Revwalk::last, note the order this is because Revwalk::last consumes the revwalker so doing it in reverse order will make borrow checker unhappy :-). This replaces exec of head -n1 command.
5. Look up the git2::Commit objects using git2::Repository::find_commit
6. Then convert the git2::Time to chrono::DateTime and take out the years.

After this change I found an obvious error which went unnoticed in previous version, that is if there was no repository key in Cargo.toml. When there was no repo URL git clone exec did not error out and our shell commands happily extracted year from the debcargo repository!. Well since I was testing code from debcargo repository It never failed, but when I executed from non-git repository folder git threw an error but that was git log and not git clone. This error was spotted right away because git2 threw me an error that I gave it empty URL.

When it comes to performance I see that debcargo is faster compared to previous version. This makes sense because previously it was doing 5 fork and exec system calls and now that is avoided.