cli: fix "workspace add --at-op" to set up new working copy properly

Because new workspace is created for the new operation forked from the --at-op
operation, the new working copy should be writable.

cli/src/cli_util.rs

@@ -378,7 +378,8 @@ impl CommandHelper {
         workspace: Workspace,
         repo: Arc<ReadonlyRepo>,
     ) -> Result<WorkspaceCommandHelper, CommandError> {
-        WorkspaceCommandHelper::new(ui, self, workspace, repo)
+        let loaded_at_head = self.global_args.at_operation == "@";
+        WorkspaceCommandHelper::new(ui, self, workspace, repo, loaded_at_head)
     }
 }
 
@@ -499,13 +500,13 @@ impl WorkspaceCommandHelper {
         command: &CommandHelper,
         workspace: Workspace,
         repo: Arc<ReadonlyRepo>,
+        loaded_at_head: bool,
     ) -> Result<Self, CommandError> {
         let settings = command.settings.clone();
         let commit_summary_template_text =
             settings.config().get_string("templates.commit_summary")?;
         let revset_aliases_map = revset_util::load_revset_aliases(ui, &command.layered_configs)?;
         let template_aliases_map = command.load_template_aliases(ui)?;
-        let loaded_at_head = command.global_args.at_operation == "@";
         let may_update_working_copy = loaded_at_head && !command.global_args.ignore_working_copy;
         let working_copy_shared_with_git = is_colocated_git_workspace(&workspace, &repo);
         let path_converter = RepoPathUiConverter::Fs {

cli/src/commands/workspace.rs

@@ -173,7 +173,9 @@ fn cmd_workspace_add(
     )?;
 
     // Copy sparse patterns from workspace where the command was run
-    let mut new_workspace_command = WorkspaceCommandHelper::new(ui, command, new_workspace, repo)?;
+    let loaded_at_head = true;
+    let mut new_workspace_command =
+        WorkspaceCommandHelper::new(ui, command, new_workspace, repo, loaded_at_head)?;
     let (mut locked_ws, _wc_commit) = new_workspace_command.start_working_copy_mutation()?;
     let sparse_patterns = old_workspace_command
         .working_copy()

cli/tests/test_workspaces.rs

@@ -168,17 +168,18 @@ fn test_workspaces_add_at_operation() {
     Parent commit      : rlvkpnrz 0dbaa19a 2
     "###);
 
-    // FIXME: --at-op should disable snapshot in the main workspace, but the
-    // newly created workspace should still be writable.
+    // --at-op should disable snapshot in the main workspace, but the newly
+    // created workspace should still be writable.
     std::fs::write(main_path.join("file3"), "").unwrap();
-    let stderr = test_env.jj_cmd_failure(
+    let (_stdout, stderr) = test_env.jj_cmd_ok(
         &main_path,
         &["workspace", "add", "--at-op=@-", "../secondary"],
     );
     insta::assert_snapshot!(stderr.replace('\\', "/"), @r###"
     Created workspace in "../secondary"
-    Error: This command must be able to update the working copy.
-    Hint: Don't use --at-op.
+    Working copy now at: rzvqmyuk a4d1cbc9 (empty) (no description set)
+    Parent commit      : qpvuntsm 3364a7ed 1
+    Added 1 files, modified 0 files, removed 0 files
     "###);
     let secondary_path = test_env.env_root().join("secondary");
 
@@ -188,8 +189,8 @@ fn test_workspaces_add_at_operation() {
     insta::assert_snapshot!(stdout, @r###"
     Working copy changes:
     A file4
-    Working copy : zsuskuln 7df5cfc8 (no description set)
-    Parent commit: zzzzzzzz 00000000 (empty) (no description set)
+    Working copy : rzvqmyuk 2ba74f85 (no description set)
+    Parent commit: qpvuntsm 3364a7ed 1
     "###);
     insta::assert_snapshot!(stderr, @r###"
     Concurrent modification detected, resolving automatically.
@@ -201,6 +202,7 @@ fn test_workspaces_add_at_operation() {
     ○    resolve concurrent operations
     ├─╮
     ○ │  commit cd06097124e3e5860867e35c2bb105902c28ea38
+    │ ○  create initial working-copy commit in workspace secondary
     │ ○  add workspace 'secondary'
     ├─╯
     ○  snapshot working copy