Add a --siblings option to the jj split command

CHANGELOG.md

@@ -60,6 +60,9 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 * There is a new global `--quiet` flag to silence commands' non-primary output.
 
+* `jj split` now supports a `--siblings/-s` option that splits the target
+  revision into siblings with the same parents and children.
+
 ### Fixed bugs
 
 ## [0.15.1] - 2024-03-06

cli/src/commands/rebase.rs

@@ -12,6 +12,7 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
+use std::borrow::Borrow;
 use std::collections::HashMap;
 use std::io::Write;
 use std::sync::Arc;
@@ -30,7 +31,7 @@ use tracing::instrument;
 
 use crate::cli_util::{
     resolve_multiple_nonempty_revsets_default_single, short_commit_hash, CommandHelper,
-    RevisionArg, WorkspaceCommandHelper,
+    RevisionArg, WorkspaceCommandHelper, WorkspaceCommandTransaction,
 };
 use crate::command_error::{user_error, CommandError};
 use crate::formatter::Formatter;
@@ -222,7 +223,7 @@ Please use `jj rebase -d 'all:x|y'` instead of `jj rebase --allow-large-revsets
     } else if !args.source.is_empty() {
         let source_commits =
             resolve_multiple_nonempty_revsets_default_single(&workspace_command, &args.source)?;
-        rebase_descendants(
+        rebase_descendants_transaction(
             ui,
             command.settings(),
             &mut workspace_command,
@@ -273,7 +274,7 @@ fn rebase_branch(
         .iter()
         .commits(workspace_command.repo().store())
         .try_collect()?;
-    rebase_descendants(
+    rebase_descendants_transaction(
         ui,
         settings,
         workspace_command,
@@ -283,7 +284,30 @@ fn rebase_branch(
     )
 }
 
-fn rebase_descendants(
+/// Rebases `old_commits` onto `new_parents`.
+pub fn rebase_descendants(
+    tx: &mut WorkspaceCommandTransaction,
+    settings: &UserSettings,
+    new_parents: &[Commit],
+    old_commits: &[impl Borrow<Commit>],
+    rebase_options: RebaseOptions,
+) -> Result<usize, CommandError> {
+    for old_commit in old_commits.iter() {
+        rebase_commit_with_options(
+            settings,
+            tx.mut_repo(),
+            old_commit.borrow(),
+            new_parents,
+            &rebase_options,
+        )?;
+    }
+    let num_rebased = old_commits.len()
+        + tx.mut_repo()
+            .rebase_descendants_with_options(settings, rebase_options)?;
+    Ok(num_rebased)
+}
+
+fn rebase_descendants_transaction(
     ui: &mut Ui,
     settings: &UserSettings,
     workspace_command: &mut WorkspaceCommandHelper,
@@ -307,20 +331,8 @@ fn rebase_descendants(
         check_rebase_destinations(workspace_command.repo(), new_parents, old_commit)?;
     }
     let mut tx = workspace_command.start_transaction();
-    // `rebase_descendants` takes care of sorting in reverse topological order, so
-    // no need to do it here.
-    for old_commit in old_commits.iter() {
-        rebase_commit_with_options(
-            settings,
-            tx.mut_repo(),
-            old_commit,
-            new_parents,
-            &rebase_options,
-        )?;
-    }
-    let num_rebased = old_commits.len()
-        + tx.mut_repo()
-            .rebase_descendants_with_options(settings, rebase_options)?;
+    let num_rebased =
+        rebase_descendants(&mut tx, settings, new_parents, &old_commits, rebase_options)?;
     writeln!(ui.status(), "Rebased {num_rebased} commits")?;
     let tx_message = if old_commits.len() == 1 {
         format!(

cli/src/commands/split.rs

@@ -13,13 +13,17 @@
 // limitations under the License.
 use std::io::Write;
 
+use itertools::Itertools;
+use jj_lib::commit::Commit;
 use jj_lib::object_id::ObjectId;
 use jj_lib::repo::Repo;
+use jj_lib::revset::{RevsetExpression, RevsetIteratorExt};
 use jj_lib::rewrite::merge_commit_trees;
 use tracing::instrument;
 
 use crate::cli_util::{CommandHelper, RevisionArg};
 use crate::command_error::CommandError;
+use crate::commands::rebase::rebase_descendants;
 use crate::description_util::{description_template_for_commit, edit_description};
 use crate::ui::Ui;
 
@@ -47,6 +51,9 @@ pub(crate) struct SplitArgs {
     /// The revision to split
     #[arg(long, short, default_value = "@")]
     revision: RevisionArg,
+    /// Split the revision into two siblings instead of a parent and child.
+    #[arg(long, short)]
+    siblings: bool,
     /// Put these paths in the first commit
     #[arg(value_hint = clap::ValueHint::AnyPath)]
     paths: Vec<String>,
@@ -72,13 +79,13 @@ pub(crate) fn cmd_split(
     let base_tree = merge_commit_trees(tx.repo(), &commit.parents())?;
     let instructions = format!(
         "\
-You are splitting a commit in two: {}
+You are splitting a commit into two: {}
 
 The diff initially shows the changes in the commit you're splitting.
 
-Adjust the right side until it shows the contents you want for the first
-(parent) commit. The remainder will be in the second commit. If you
-don't make any changes, then the operation will be aborted.
+Adjust the right side until it shows the contents you want for the first commit.
+The remainder will be in the second commit. If you don't make any changes, then
+the operation will be aborted.
 ",
         tx.format_commit_summary(&commit)
     );
@@ -106,7 +113,7 @@ don't make any changes, then the operation will be aborted.
         ui,
         command.settings(),
         tx.base_workspace_helper(),
-        "Enter commit description for the first part (parent).",
+        "Enter a description for the first commit.",
         commit.description(),
         &base_tree,
         &selected_tree,
@@ -119,7 +126,21 @@ don't make any changes, then the operation will be aborted.
         .set_description(first_description)
         .write()?;
 
-    // Create the second commit, which includes everything the user didn't select.
+    // Create the second commit, which includes everything the user didn't
+    // select.
+    let (second_tree, second_base_tree) = if args.siblings {
+        // Merge the original commit tree with its parent using the tree
+        // containing the user selected changes as the base for the merge.
+        // This results in a tree with the changes the user didn't select.
+        (end_tree.merge(&selected_tree, &base_tree)?, &base_tree)
+    } else {
+        (end_tree, &selected_tree)
+    };
+    let second_commit_parents = if args.siblings {
+        commit.parent_ids().to_vec()
+    } else {
+        vec![first_commit.id().clone()]
+    };
     let second_description = if commit.description().is_empty() {
         // If there was no description before, don't ask for one for the second commit.
         "".to_string()
@@ -128,27 +149,50 @@ don't make any changes, then the operation will be aborted.
             ui,
             command.settings(),
             tx.base_workspace_helper(),
-            "Enter commit description for the second part (child).",
+            "Enter a description for the second commit.",
             commit.description(),
-            &selected_tree,
-            &end_tree,
+            second_base_tree,
+            &second_tree,
         )?;
         edit_description(tx.base_repo(), &second_template, command.settings())?
     };
     let second_commit = tx
         .mut_repo()
         .rewrite_commit(command.settings(), &commit)
-        .set_parents(vec![first_commit.id().clone()])
-        .set_tree_id(commit.tree_id().clone())
+        .set_parents(second_commit_parents)
+        .set_tree_id(second_tree.id())
+        // Generate a new change id so that the commit being split doesn't
+        // become divergent.
         .generate_new_change_id()
         .set_description(second_description)
         .write()?;
 
-    // We want only the `second_commit` to inherit `commit`'s branches and
-    // descendants.
+    // Mark the commit being split as rewritten to the second commit. As a
+    // result, if @ points to the commit being split, it will point to the
+    // second commit after the command finishes. This also means that any
+    // branches pointing to the commit being split are moved to the second
+    // commit.
     tx.mut_repo()
         .set_rewritten_commit(commit.id().clone(), second_commit.id().clone());
-    let num_rebased = tx.mut_repo().rebase_descendants(command.settings())?;
+    // Rebase descendants of the commit being split.
+    let new_parents = if args.siblings {
+        vec![first_commit.clone(), second_commit.clone()]
+    } else {
+        vec![second_commit.clone()]
+    };
+    let children: Vec<Commit> = RevsetExpression::commit(commit.id().clone())
+        .children()
+        .evaluate_programmatic(tx.base_repo().as_ref())?
+        .iter()
+        .commits(tx.base_repo().store())
+        .try_collect()?;
+    let num_rebased = rebase_descendants(
+        &mut tx,
+        command.settings(),
+        &new_parents,
+        &children,
+        Default::default(),
+    )?;
     if let Some(mut formatter) = ui.status_formatter() {
         if num_rebased > 0 {
             writeln!(formatter, "Rebased {num_rebased} descendant commits")?;

cli/tests/cli-reference@.md.snap

@@ -1690,6 +1690,10 @@ If the change you split had a description, you will be asked to enter a change d
 * `-r`, `--revision <REVISION>` — The revision to split
 
   Default value: `@`
+* `-s`, `--siblings` — Split the revision into two siblings instead of a parent and child
+
+  Possible values: `true`, `false`
+