git: force push when not known to be a fast-forward

With this change, we no longer fail if the user moves a branch
sideways or backwards and then push.

The push should ideally only succeed if the remote branch is where we
thought it was (like `git push --force-with-lease`), but that requires
rust-lang/git2-rs#733 to be fixed first.

lib/src/git.rs

@@ -173,6 +173,10 @@ pub fn push_commit(
     target: &Commit,
     remote_name: &str,
     remote_branch: &str,
+    // TODO: We want this to be an Option<CommitId> for the expected current commit on the remote.
+    // It's a blunt "force" option instead until git2-rs supports the "push negotiation" callback
+    // (https://github.com/rust-lang/git2-rs/issues/733).
+    force: bool,
 ) -> Result<(), GitPushError> {
     // Create a temporary ref to work around https://github.com/libgit2/libgit2/issues/3178
     let temp_ref_name = format!("refs/jj/git-push/{}", target.id().hex());
@@ -184,7 +188,12 @@ pub fn push_commit(
     )?;
     // Need to add "refs/heads/" prefix due to https://github.com/libgit2/libgit2/issues/1125
     let qualified_remote_branch = format!("refs/heads/{}", remote_branch);
-    let refspec = format!("{}:{}", temp_ref_name, qualified_remote_branch);
+    let refspec = format!(
+        "{}{}:{}",
+        (if force { "+" } else { "" }),
+        temp_ref_name,
+        qualified_remote_branch
+    );
     let result = push_ref(git_repo, remote_name, &qualified_remote_branch, &refspec);
     // TODO: Figure out how to do the equivalent of absl::Cleanup for
     // temp_ref.delete().
@@ -196,6 +205,8 @@ pub fn delete_remote_branch(
     git_repo: &git2::Repository,
     remote_name: &str,
     remote_branch: &str,
+    /* TODO: Similar to push_commit(), we want an CommitId for the expected current commit on
+     * the remote. */
 ) -> Result<(), GitPushError> {
     // Need to add "refs/heads/" prefix due to https://github.com/libgit2/libgit2/issues/1125
     let qualified_remote_branch = format!("refs/heads/{}", remote_branch);

lib/tests/test_git.rs

@@ -353,7 +353,7 @@ fn test_push_commit_success() {
     let temp_dir = tempfile::tempdir().unwrap();
     let setup = set_up_push_repos(&settings, &temp_dir);
     let clone_repo = setup.jj_repo.store().git_repo().unwrap();
-    let result = git::push_commit(&clone_repo, &setup.new_commit, "origin", "main");
+    let result = git::push_commit(&clone_repo, &setup.new_commit, "origin", "main", false);
     assert_eq!(result, Ok(()));
 
     // Check that the ref got updated in the source repo
@@ -388,10 +388,38 @@ fn test_push_commit_not_fast_forward() {
         &new_commit,
         "origin",
         "main",
+        false,
     );
     assert_eq!(result, Err(GitPushError::NotFastForward));
 }
 
+#[test]
+fn test_push_commit_not_fast_forward_with_force() {
+    let settings = testutils::user_settings();
+    let temp_dir = tempfile::tempdir().unwrap();
+    let mut setup = set_up_push_repos(&settings, &temp_dir);
+    let new_commit = testutils::create_random_commit(&settings, &setup.jj_repo)
+        .write_to_new_transaction(&setup.jj_repo, "test");
+    setup.jj_repo = setup.jj_repo.reload();
+    let result = git::push_commit(
+        &setup.jj_repo.store().git_repo().unwrap(),
+        &new_commit,
+        "origin",
+        "main",
+        true,
+    );
+    assert_eq!(result, Ok(()));
+
+    // Check that the ref got updated in the source repo
+    let source_repo = git2::Repository::open(&setup.source_repo_dir).unwrap();
+    let new_target = source_repo
+        .find_reference("refs/heads/main")
+        .unwrap()
+        .target();
+    let new_oid = Oid::from_bytes(&new_commit.id().0).unwrap();
+    assert_eq!(new_target, Some(new_oid));
+}
+
 #[test]
 fn test_push_commit_no_such_remote() {
     let settings = testutils::user_settings();
@@ -402,6 +430,7 @@ fn test_push_commit_no_such_remote() {
         &setup.new_commit,
         "invalid-remote",
         "main",
+        false,
     );
     assert!(matches!(result, Err(GitPushError::NoSuchRemote(_))));
 }
@@ -416,6 +445,7 @@ fn test_push_commit_invalid_remote() {
         &setup.new_commit,
         "http://invalid-remote",
         "main",
+        false,
     );
     assert!(matches!(result, Err(GitPushError::NoSuchRemote(_))));
 }

src/commands.rs

@@ -2585,7 +2585,8 @@ fn cmd_git_push(
     }
 
     let branch_target = maybe_branch_target.unwrap();
-    if branch_target.local_target.as_ref() == branch_target.remote_targets.get(remote_name) {
+    let maybe_remote_target = branch_target.remote_targets.get(remote_name);
+    if branch_target.local_target.as_ref() == maybe_remote_target {
         writeln!(
             ui,
             "Branch {}@{} already matches {}",
@@ -2610,8 +2611,27 @@ fn cmd_git_push(
                         "Won't push open commit".to_string(),
                     ));
                 }
-                git::push_commit(&git_repo, &new_target_commit, remote_name, branch_name)
-                    .map_err(|err| CommandError::UserError(err.to_string()))?;
+                let force = match maybe_remote_target {
+                    None => false,
+                    Some(RefTarget::Conflict { .. }) => {
+                        return Err(CommandError::UserError(format!(
+                            "Branch {}@{} is conflicted",
+                            branch_name, remote_name
+                        )));
+                    }
+                    Some(RefTarget::Normal(old_target_id)) => {
+                        !repo.index().is_ancestor(old_target_id, new_target_id)
+                    }
+                };
+
+                git::push_commit(
+                    &git_repo,
+                    &new_target_commit,
+                    remote_name,
+                    branch_name,
+                    force,
+                )
+                .map_err(|err| CommandError::UserError(err.to_string()))?;
             }
         }
     } else {