radicle: Detect current repository using `jj`

When using Jujutsu and a non-colocated Git repository, the detection
using `git2` directly fails (just as `git` in a shell would fail)
because there is no `.git` directory found by traversing up the
filesystem hierarchy.

Add an invocation of `jj git root` as a fallback.

crates/radicle-cli/examples/jj-init-bare.md

@@ -0,0 +1,19 @@
+We initialize Jujutusu for our repository for use with a bare Git repo.
+
+```(stderr)
+$ jj git init --git-repo heartwood heartwood.jj
+Done importing changes from the underlying Git repo.
+Working copy  (@) now at: lvxkkpmk 9ec513df (empty) (no description set)
+Parent commit (@-)      : xpnzuzwn f2de534b master | Second commit
+Added 1 files, modified 0 files, removed 0 files
+Initialized repo in "heartwood.jj"
+```
+
+```
+$ cd heartwood.jj
+```
+
+```
+$ rad .
+rad:z42hL2jL4XNk6K8oHQaSWfMgCL7ji
+```

crates/radicle-cli/tests/commands.rs

@@ -858,6 +858,37 @@ fn rad_patch() {
     Environment::alice(["rad-init", "rad-patch"]);
 }
 
+#[test]
+fn rad_jj_bare() {
+    // We test whether `jj` is installed, and have this test succeed if it is not.
+    // Programmatic skipping of tests is not supported as of 2024-08.
+    if !program_reports_version("jj") {
+        return;
+    }
+
+    let mut environment = Environment::new();
+    let mut profile = environment.node("alice");
+    let rid = profile.project("heartwood", "Radicle Heartwood Protocol & Stack");
+
+    test(
+        "examples/rad-init-existing-bare.md",
+        environment.work(&profile),
+        Some(&profile.home),
+        [(
+            "URL",
+            git::url::File::new(profile.storage.path())
+                .rid(rid)
+                .to_string()
+                .as_str(),
+        )],
+    )
+    .unwrap();
+
+    environment
+        .tests(["jj-config", "jj-init-bare"], &profile)
+        .unwrap();
+}
+
 #[test]
 fn rad_jj_colocated_patch() {
     // We test whether `jj` is installed, and have this test succeed if it is not.

crates/radicle/src/rad.rs

@@ -372,6 +372,18 @@ pub fn remove_remote(repo: &git2::Repository) -> Result<(), RemoteError> {
     Ok(())
 }
 
+#[derive(Error, Debug)]
+pub enum CwdError {
+    #[error(transparent)]
+    Remote(#[from] RemoteError),
+
+    #[error("Detection failed (git: '{git}', jj: '{jj}')")]
+    Detection {
+        git: git2::Error,
+        jj: JujutsuGitRootError,
+    },
+}
+
 /// Get the RID of the repository in current working directory
 ///
 /// It will atempt to search parent directories if `path` did not find
@@ -382,10 +394,11 @@ pub fn remove_remote(repo: &git2::Repository) -> Result<(), RemoteError> {
 /// This function should only perform read operations since we do not
 /// want to modify the wrong repository in the case that it found a
 /// Git repository that is not a Radicle repository.
-pub fn cwd() -> Result<(git2::Repository, RepoId), RemoteError> {
-    let repo = repo()?;
-    let (_, id) = remote(&repo)?;
+pub fn cwd() -> Result<(git2::Repository, RepoId), CwdError> {
+    let repo =
+        repo().or_else(|git| repo_jj_git_root().map_err(|jj| CwdError::Detection { git, jj }))?;
 
+    let (_, id) = remote(&repo)?;
     Ok((repo, id))
 }
 
@@ -411,6 +424,34 @@ pub fn repo() -> Result<git2::Repository, git2::Error> {
     Ok(repo)
 }
 
+#[derive(Error, Debug)]
+pub enum JujutsuGitRootError {
+    #[error("git: {0}")]
+    Git(#[from] git2::Error),
+
+    #[error("i/o: {0}")]
+    Io(#[from] io::Error),
+
+    #[error("exited with status {status}")]
+    CommandFailure { status: std::process::ExitStatus },
+}
+
+/// Get the Git repo underlying the current Jujutsu repository.
+pub fn repo_jj_git_root() -> Result<git2::Repository, JujutsuGitRootError> {
+    let output = std::process::Command::new("jj")
+        .args(["git", "root"])
+        .output()?;
+
+    if !output.status.success() {
+        return Err(JujutsuGitRootError::CommandFailure {
+            status: output.status,
+        });
+    }
+
+    let path = std::path::PathBuf::from(String::from_utf8_lossy(&output.stdout).to_string().trim());
+    Ok(git2::Repository::open(path)?)
+}
+
 /// Setup patch upstream branch such that `git push` updates the patch.
 pub fn setup_patch_upstream<'a>(
     patch: &ObjectId,