cli: make jj workspace add preserve all parents of current workspace

CHANGELOG.md

@@ -69,6 +69,9 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 * The new template keywords `local_branches`/`remote_branches` are added to show
   only local/remote branches.
 
+* `jj workspace add` now preserves all parents of the old working-copy commit
+  instead of just the first one.
+
 ### Fixed bugs
 
 * Updating the working copy to a commit where a file that's currently ignored

cli/src/commands/mod.rs

@@ -1698,23 +1698,30 @@ fn cmd_workspace_add(
         &name
     ));
 
-    let new_wc_commit = if let Some(specific_rev) = &args.revision {
-        old_workspace_command.resolve_single_rev(specific_rev, ui)?
+    let parents = if let Some(specific_rev) = &args.revision {
+        vec![old_workspace_command.resolve_single_rev(specific_rev, ui)?]
     } else {
-        // Check out a parent of the current workspace's working-copy commit, or the
+        // Check out parents of the current workspace's working-copy commit, or the
         // root if there is no working-copy commit in the current workspace.
         if let Some(old_wc_commit_id) = tx
             .base_repo()
             .view()
             .get_wc_commit_id(old_workspace_command.workspace_id())
         {
-            tx.repo().store().get_commit(old_wc_commit_id)?.parents()[0].clone()
+            tx.repo().store().get_commit(old_wc_commit_id)?.parents()
         } else {
-            tx.repo().store().root_commit()
+            vec![tx.repo().store().root_commit()]
         }
     };
 
-    tx.check_out(&new_wc_commit)?;
+    let tree = merge_commit_trees(tx.repo(), &parents)?;
+    let parent_ids = parents.iter().map(|c| c.id().clone()).collect_vec();
+    let new_wc_commit = tx
+        .mut_repo()
+        .new_commit(command.settings(), parent_ids, tree.id())
+        .write()?;
+
+    tx.edit(&new_wc_commit)?;
     tx.finish(ui)?;
     Ok(())
 }

cli/tests/test_workspaces.rs

@@ -71,6 +71,41 @@ fn test_workspaces_add_second_workspace() {
     "###);
 }
 
+/// Test adding a second workspace while the current workspace is editing a
+/// merge
+#[test]
+fn test_workspaces_add_second_workspace_on_merge() {
+    let test_env = TestEnvironment::default();
+    test_env.jj_cmd_ok(test_env.env_root(), &["init", "--git", "main"]);
+    let main_path = test_env.env_root().join("main");
+
+    test_env.jj_cmd_ok(&main_path, &["describe", "-m=left"]);
+    test_env.jj_cmd_ok(&main_path, &["new", "@-", "-m=right"]);
+    test_env.jj_cmd_ok(&main_path, &["new", "all:@-+", "-m=merge"]);
+
+    let stdout = test_env.jj_cmd_success(&main_path, &["workspace", "list"]);
+    insta::assert_snapshot!(stdout, @r###"
+    default: zsuskuln 21a0ea6d (empty) merge
+    "###);
+
+    test_env.jj_cmd_ok(
+        &main_path,
+        &["workspace", "add", "--name", "second", "../secondary"],
+    );
+
+    // The new workspace's working-copy commit shares all parents with the old one.
+    insta::assert_snapshot!(get_log_output(&test_env, &main_path), @r###"
+    ◉    6d4c2b8ab610148410b6a346d900cb7ad6298ede second@
+    ├─╮
+    │ │ @  21a0ea6d1c86c413cb30d7f7d216a74754b148c4 default@
+    ╭─┬─╯
+    │ ◉  09ba8d9dfa21b8572c2361e6947ca024d8f0a198
+    ◉ │  1694f2ddf8ecf9e55ca3cd9554bc0654186b07e0
+    ├─╯
+    ◉  0000000000000000000000000000000000000000
+    "###);
+}
+
 /// Test adding a workspace, but at a specific revision using '-r'
 #[test]
 fn test_workspaces_add_workspace_at_revision() {