Warn user about the working copy when configuring the author

CHANGELOG.md

@@ -124,6 +124,8 @@ to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 
 * `jj describe` can now update the description of multiple commits.
 
+* When reconfiguring the author, warn that the working copy won't be updated
+
 ### Fixed bugs
 
 * `jj status` will show different messages in a conflicted tree, depending

cli/src/commands/config/set.rs

@@ -12,10 +12,12 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
+use jj_lib::commit::Commit;
+use jj_lib::repo::Repo;
 use tracing::instrument;
 
 use super::ConfigLevelArgs;
-use crate::cli_util::{get_new_config_file_path, CommandHelper};
+use crate::cli_util::{get_new_config_file_path, CommandHelper, WorkspaceCommandHelper};
 use crate::command_error::{user_error, CommandError};
 use crate::config::{
     parse_toml_value_or_bare_string, write_config_value_to_file, ConfigNamePathBuf,
@@ -33,9 +35,15 @@ pub struct ConfigSetArgs {
     level: ConfigLevelArgs,
 }
 
+/// Denotes a type of author change
+enum AuthorChange {
+    Name,
+    Email,
+}
+
 #[instrument(skip_all)]
 pub fn cmd_config_set(
-    _ui: &mut Ui,
+    ui: &mut Ui,
     command: &CommandHelper,
     args: &ConfigSetArgs,
 ) -> Result<(), CommandError> {
@@ -46,7 +54,63 @@ pub fn cmd_config_set(
             path = config_path.display()
         )));
     }
+
     // TODO(#531): Infer types based on schema (w/ --type arg to override).
     let value = parse_toml_value_or_bare_string(&args.value);
+
+    // If the user is trying to change the author config, we should warn them that
+    // it won't affect the working copy author
+    if args.name == ConfigNamePathBuf::from_iter(vec!["user", "name"]) {
+        check_wc_author(command, ui, &value, AuthorChange::Name)?;
+    } else if args.name == ConfigNamePathBuf::from_iter(vec!["user", "email"]) {
+        check_wc_author(command, ui, &value, AuthorChange::Email)?;
+    };
+
     write_config_value_to_file(&args.name, value, &config_path)
 }
+
+/// Returns the commit of the working copy if it exists.
+fn maybe_wc_commit(helper: &WorkspaceCommandHelper) -> Option<Commit> {
+    let repo = helper.repo();
+    let maybe_wc_commit = helper
+        .get_wc_commit_id()
+        .map(|id| repo.store().get_commit(id))
+        .transpose()
+        .unwrap();
+    maybe_wc_commit
+}
+
+/// Check if the working copy author name matches the user's config value
+/// If it doesn't, print a warning message
+fn check_wc_author(
+    command: &CommandHelper,
+    ui: &mut Ui,
+    new_value: &toml_edit::Value,
+    author_change: AuthorChange,
+) -> Result<(), CommandError> {
+    let helper = match command.workspace_helper(ui) {
+        Ok(helper) => helper,
+        Err(_) => return Ok(()), // config set should work even if cwd isn't a jj repo
+    };
+    if let Some(wc_commit) = maybe_wc_commit(&helper) {
+        let author = wc_commit.author();
+        let orig_value = match author_change {
+            AuthorChange::Name => &author.name,
+            AuthorChange::Email => &author.email,
+        };
+        if new_value.as_str() != Some(orig_value) {
+            warn_wc_author(&author.name, &author.email, ui)?
+        }
+    }
+    Ok(())
+}
+
+/// Prints a warning message about the working copy to the user
+fn warn_wc_author(user_name: &str, user_email: &str, ui: &mut Ui) -> Result<(), CommandError> {
+    Ok(writeln!(
+        ui.warning_default(),
+        "This setting will only impact future commits.\nThe author of the working copy will stay \
+         \"{user_name} <{user_email}>\".\nTo change the working copy author, use \"jj describe \
+         --reset-author --no-edit\""
+    )?)
+}

cli/tests/test_config_command.rs

@@ -857,6 +857,32 @@ fn test_config_show_paths() {
     "###);
 }
 
+#[test]
+fn test_config_author_change_warning() {
+    let test_env = TestEnvironment::default();
+    test_env.jj_cmd_ok(test_env.env_root(), &["git", "init", "repo"]);
+    let repo_path = test_env.env_root().join("repo");
+    let (stdout, stderr) = test_env.jj_cmd_ok(
+        &repo_path,
+        &["config", "set", "--repo", "user.email", "'Foo'"],
+    );
+    insta::assert_snapshot!(stdout, @"");
+    insta::assert_snapshot!(stderr, @r###"
+    Warning: This setting will only impact future commits.
+    The author of the working copy will stay "Test User <[email protected]>".
+    To change the working copy author, use "jj describe --reset-author --no-edit"
+    "###);
+
+    // test_env.jj_cmd resets state for every invocation
+    // for this test, the state (user.email) is needed
+    let mut log_cmd = test_env.jj_cmd(&repo_path, &["describe", "--reset-author", "--no-edit"]);
+    log_cmd.env_remove("JJ_EMAIL");
+    log_cmd.assert().success();
+
+    let (stdout, _) = test_env.jj_cmd_ok(&repo_path, &["log"]);
+    assert!(stdout.contains("Foo"));
+}
+
 fn find_stdout_lines(keyname_pattern: &str, stdout: &str) -> String {
     let key_line_re = Regex::new(&format!(r"(?m)^{keyname_pattern} = .*$")).unwrap();
     key_line_re