feat: implement --new flag

The new `--new` or `-n` flag allows to create a new revision after the
single revision being described. Running `jj-cz --new` is the
equivalent of running `jj-cz @ && jj new`. Running `jj-cz --new xs` is
the equivalent of running `jj-cz xs && jj new xs`.

Passing several revisions to `jj-cz` with the `--new` flag will result
in an error.

Refs: #6

src/cli/mod.rs

@@ -18,6 +18,10 @@ pub struct Cli {
     /// The revision(s) whose description to edit (default: @)
     #[arg(value_name = "REVSETS")]
     revsets: Vec<String>,
+
+    /// Create a new child revision after editing the description
+    #[arg(short, long)]
+    new: bool,
 }
 
 impl Cli {
@@ -29,4 +33,95 @@ impl Cli {
             self.revsets.iter().map(|s| s.as_str()).collect()
         }
     }
+
+    pub fn create_new(&self) -> bool {
+        self.new
+    }
+
+    pub fn validate(&self) -> Result<(), jj_cz::Error> {
+        if self.new && self.revsets().len() > 1 {
+            Err(jj_cz::Error::NewFlagWithMultipleRevisions)
+        } else {
+            Ok(())
+        }
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use clap::Parser;
+
+    #[test]
+    fn revsets_defaults_to_at() {
+        let cli = Cli::parse_from(["jj-cz"]);
+        assert_eq!(cli.revsets(), vec!["@"]);
+    }
+
+    #[test]
+    fn revsets_returns_provided_values() {
+        let cli = Cli::parse_from(["jj-cz", "abc", "def"]);
+        let revsets = cli.revsets();
+        assert_eq!(revsets, vec!["abc", "def"]);
+    }
+
+    #[test]
+    fn revsets_single_revset() {
+        let cli = Cli::parse_from(["jj-cz", "xyz"]);
+        assert_eq!(cli.revsets(), vec!["xyz"]);
+    }
+
+    #[test]
+    fn create_new_returns_false_by_default() {
+        let cli = Cli::parse_from(["jj-cz"]);
+        assert!(!cli.create_new());
+    }
+
+    #[test]
+    fn create_new_returns_true_with_flag() {
+        let cli = Cli::parse_from(["jj-cz", "--new"]);
+        assert!(cli.create_new());
+    }
+
+    #[test]
+    fn create_new_returns_true_with_short_flag() {
+        let cli = Cli::parse_from(["jj-cz", "-n"]);
+        assert!(cli.create_new());
+    }
+
+    #[test]
+    fn validate_ok_with_no_args() {
+        let cli = Cli::parse_from(["jj-cz"]);
+        assert!(cli.validate().is_ok());
+    }
+
+    #[test]
+    fn validate_ok_with_new_and_single_revset() {
+        let cli = Cli::parse_from(["jj-cz", "--new", "@"]);
+        assert!(cli.validate().is_ok());
+    }
+
+    #[test]
+    fn validate_ok_with_multiple_revsets_no_new() {
+        let cli = Cli::parse_from(["jj-cz", "abc", "def"]);
+        assert!(cli.validate().is_ok());
+    }
+
+    #[test]
+    fn validate_err_with_new_and_multiple_revsets() {
+        let cli = Cli::parse_from(["jj-cz", "--new", "abc", "def"]);
+        let result = cli.validate();
+        assert!(result.is_err());
+        assert!(matches!(
+            result.unwrap_err(),
+            jj_cz::Error::NewFlagWithMultipleRevisions
+        ));
+    }
+
+    #[test]
+    fn cli_derives_debug() {
+        let cli = Cli::parse_from(["jj-cz", "--new", "@"]);
+        let debug = format!("{:?}", cli);
+        assert!(debug.contains("Cli"));
+    }
 }

src/error.rs

@@ -31,6 +31,8 @@ pub enum Error {
     RevsetResolutionError { revset: String, context: String },
     #[error("Revision set '{revset}' resolves to multiple commits; specify a single revision")]
     MultipleRevisions { revset: String },
+    #[error("--new cannot be used with multiple revisions")]
+    NewFlagWithMultipleRevisions,
 }
 
 impl From<ScopeError> for Error {

src/jj/lib_executor.rs

@@ -263,6 +263,33 @@ impl JjExecutor for JjLib {
             })?;
         Ok(commit.description().trim_end().to_string())
     }
+
+    async fn new_revision(&self, revset: &str) -> Result<(), Error> {
+        let commit_id = self.get_commit_id(revset).await?;
+        let repo = self.repo.lock()?.clone();
+        let mut tx = repo.start_transaction();
+        let parent_commit =
+            tx.repo()
+                .store()
+                .get_commit(&commit_id)
+                .map_err(|e| Error::JjOperation {
+                    context: e.to_string(),
+                })?;
+        tx.repo_mut()
+            .check_out(self.workspace_name.clone(), &parent_commit)
+            .await
+            .map_err(|e| Error::JjOperation {
+                context: e.to_string(),
+            })?;
+        let new_repo =
+            tx.commit("jj-cz: create new revision")
+                .await
+                .map_err(|e| Error::JjOperation {
+                    context: e.to_string(),
+                })?;
+        *self.repo.lock()? = new_repo;
+        Ok(())
+    }
 }
 
 #[cfg(test)]

src/jj/mock.rs

@@ -11,18 +11,22 @@ use std::sync::{Mutex, atomic::AtomicBool};
 /// Mock implementation of JjExecutor for testing
 #[derive(Debug)]
 pub struct MockJjExecutor {
-    /// Response to return from is_repository()
+    /// Response to return from `is_repository()`
     is_repo_response: Result<bool, Error>,
-    /// Response to return from describe()
+    /// Response to return from `describe()`
     describe_response: Result<(), Error>,
     /// Track described revsets
     described_revsets: Mutex<Vec<String>>,
-    /// Track response to return from get_description()
+    /// Track response to return from `get_description()`
     get_description_response: Result<String, Error>,
-    /// Track calls to is_repository()
+    /// Track calls to `is_repository()`
     is_repo_called: AtomicBool,
-    /// Track calls to describe() with the message passed
+    /// Track calls to `describe()` with the message passed
     describe_calls: Mutex<Vec<String>>,
+    /// Track response to return from `new_revision()`
+    new_revision_response: Result<(), Error>,
+    /// Track calls to `new_revision()`
+    new_revision_calls: Mutex<Vec<String>>,
 }
 
 impl Default for MockJjExecutor {
@@ -34,6 +38,8 @@ impl Default for MockJjExecutor {
             get_description_response: Ok(String::new()),
             is_repo_called: AtomicBool::new(false),
             describe_calls: Mutex::new(Vec::new()),
+            new_revision_response: Ok(()),
+            new_revision_calls: Mutex::new(Vec::new()),
         }
     }
 }
@@ -66,6 +72,15 @@ impl MockJjExecutor {
     pub fn describe_messages(&self) -> Vec<String> {
         self.describe_calls.lock().unwrap().clone()
     }
+
+    pub fn with_new_revision_response(mut self, response: Result<(), Error>) -> Self {
+        self.new_revision_response = response;
+        self
+    }
+
+    pub fn new_revision_calls(&self) -> Vec<String> {
+        self.new_revision_calls.lock().unwrap().clone()
+    }
 }
 
 #[async_trait(?Send)]
@@ -97,6 +112,17 @@ impl JjExecutor for MockJjExecutor {
     async fn get_description(&self, _revset: &str) -> Result<String, Error> {
         self.get_description_response.clone()
     }
+
+    async fn new_revision(&self, revset: &str) -> Result<(), Error> {
+        self.new_revision_calls
+            .lock()
+            .unwrap()
+            .push(revset.to_string());
+        match &self.new_revision_response {
+            Ok(()) => Ok(()),
+            Err(e) => Err(e.clone()),
+        }
+    }
 }
 
 #[cfg(test)]
@@ -245,4 +271,61 @@ mod tests {
         mock.is_repository().await.unwrap();
         assert!(mock.was_is_repo_called());
     }
+
+    /// Test mock new_revision() records the revset
+    #[tokio::test]
+    async fn mock_new_revision_records_revset() {
+        let mock = MockJjExecutor::new();
+        let result = mock.new_revision("@").await;
+        assert!(result.is_ok());
+        let calls = mock.new_revision_calls();
+        assert_eq!(calls.len(), 1);
+        assert_eq!(calls[0], "@");
+    }
+
+    /// Test mock new_revision() records multiple calls
+    #[tokio::test]
+    async fn mock_new_revision_records_multiple_calls() {
+        let mock = MockJjExecutor::new();
+        mock.new_revision("@").await.unwrap();
+        mock.new_revision("abc").await.unwrap();
+        mock.new_revision("xyz").await.unwrap();
+        let calls = mock.new_revision_calls();
+        assert_eq!(calls.len(), 3);
+        assert_eq!(calls[0], "@");
+        assert_eq!(calls[1], "abc");
+        assert_eq!(calls[2], "xyz");
+    }
+
+    /// Test mock new_revision() returns configured error
+    #[tokio::test]
+    async fn mock_new_revision_returns_error() {
+        let mock = MockJjExecutor::new().with_new_revision_response(Err(Error::RepositoryLocked));
+        let result = mock.new_revision("@").await;
+        assert!(result.is_err());
+        assert!(matches!(result.unwrap_err(), Error::RepositoryLocked));
+    }
+
+    /// Test mock new_revision() records revset even on error
+    #[tokio::test]
+    async fn mock_new_revision_records_revset_on_error() {
+        let mock = MockJjExecutor::new().with_new_revision_response(Err(Error::JjOperation {
+            context: "failed".to_string(),
+        }));
+        let result = mock.new_revision("abc").await;
+        assert!(result.is_err());
+        let calls = mock.new_revision_calls();
+        assert_eq!(calls.len(), 1);
+        assert_eq!(calls[0], "abc");
+    }
+
+    /// Test mock new_revision() can be inspected after success
+    #[tokio::test]
+    async fn mock_new_revision_returns_ok_and_tracks_revset() {
+        let mock = MockJjExecutor::new();
+        let result = mock.new_revision("my-feature").await;
+        assert!(result.is_ok());
+        let calls = mock.new_revision_calls();
+        assert_eq!(calls, vec!["my-feature"]);
+    }
 }

src/jj/mod.rs

@@ -21,6 +21,11 @@ pub trait JjExecutor: Send + Sync {
 
     /// Get the current description of a specific revision
     async fn get_description(&self, revset: &str) -> Result<String, Error>;
+
+    /// Create a new empty child revision parented on `revset`.
+    ///
+    /// Equivalent to `jj new <revset>`
+    async fn new_revision(&self, revset: &str) -> Result<(), Error>;
 }
 
 #[cfg(test)]

src/lib.rs

@@ -1,5 +1,5 @@
 mod commit;
-mod error;
+pub mod error;
 mod jj;
 mod prompts;
 

src/main.rs

@@ -10,7 +10,7 @@ const EXIT_CANCELLED: i32 = 130; // Same as SIGINT (Ctrl+C)
 const EXIT_ERROR: i32 = 1;
 
 /// Map application errors to appropriate exit codes
-fn error_to_exit_code(error: &Error) -> i32 {
+fn error_to_exit_code(error: Error) -> i32 {
     match error {
         Error::Cancelled => EXIT_CANCELLED,
         Error::NotARepository => EXIT_ERROR,
@@ -24,6 +24,7 @@ fn error_to_exit_code(error: &Error) -> i32 {
         Error::FailedReadingConfig { .. } => EXIT_ERROR,
         Error::RevsetResolutionError { .. } => EXIT_ERROR,
         Error::MultipleRevisions { .. } => EXIT_ERROR,
+        Error::NewFlagWithMultipleRevisions => EXIT_ERROR,
     }
 }
 
@@ -34,25 +35,26 @@ fn is_interactive_terminal() -> bool {
 }
 
 #[tokio::main]
-async fn main() {
+async fn main() -> Result<(), ()> {
     let cli = cli::Cli::parse();
+
+    cli.validate().map_err(exit_on_error)?;
+
     if !is_interactive_terminal() {
         eprintln!("❌ Error: jj-cz requires an interactive terminal (TTY)");
         eprintln!("   This tool cannot be used in non-interactive mode or when piping input.");
         eprintln!("   Use --help for usage information.");
         process::exit(EXIT_ERROR);
     }
-    let executor = match JjLib::new().await {
+    let executor = JjLib::new().await.map_err(exit_on_error)?;
-        Ok(e) => e,
-        Err(e) => {
-            eprintln!("❌ Error: {}", e);
-            process::exit(EXIT_ERROR);
-        }
-    };
     let workflow = CommitWorkflow::new(executor);
     for revset in cli.revsets() {
         let result = workflow.run_for_revset(revset).await;
         handle_result(result);
+        if cli.create_new() {
+            println!("Creating a new revision after {revset}");
+            workflow.new_revision(revset).await.map_err(exit_on_error)?;
+        }
     }
 
     fn handle_result(result: Result<(), Error>) {
@@ -62,11 +64,13 @@ async fn main() {
                 println!("🟡 Operation cancelled by user.");
                 process::exit(EXIT_CANCELLED);
             }
-            Err(e) => {
+            Err(e) => exit_on_error(e),
-                eprintln!("❌ Error: {}", e);
-                process::exit(error_to_exit_code(&e));
-            }
         }
     }
     process::exit(EXIT_SUCCESS);
 }
+
+fn exit_on_error(e: Error) {
+    eprintln!("❌ Error: {}", e);
+    process::exit(error_to_exit_code(e));
+}

src/prompts/workflow.rs

@@ -184,6 +184,10 @@ impl<J: JjExecutor, P: Prompter> CommitWorkflow<J, P> {
             Err(Error::Cancelled)
         }
     }
+
+    pub async fn new_revision(&self, revset: &str) -> Result<(), Error> {
+        self.executor.new_revision(revset).await
+    }
 }
 
 #[cfg(test)]
@@ -762,4 +766,59 @@ mod tests {
         assert_eq!(messages.len(), 1);
         assert_eq!(messages[0], "fix: fix crash");
     }
+
+    /// Test workflow new_revision() records the revset
+    #[tokio::test]
+    async fn workflow_new_revision_records_revset() {
+        let mock_executor = MockJjExecutor::new();
+        let workflow = CommitWorkflow::new(mock_executor);
+
+        let result = workflow.new_revision("@").await;
+        assert!(result.is_ok());
+
+        let calls = workflow.executor.new_revision_calls();
+        assert_eq!(calls, vec!["@"]);
+    }
+
+    /// Test workflow new_revision() propagates executor errors
+    #[tokio::test]
+    async fn workflow_new_revision_propagates_error() {
+        let mock_executor =
+            MockJjExecutor::new().with_new_revision_response(Err(Error::RepositoryLocked));
+        let workflow = CommitWorkflow::new(mock_executor);
+
+        let result = workflow.new_revision("@").await;
+        assert!(result.is_err());
+        assert!(matches!(result.unwrap_err(), Error::RepositoryLocked));
+    }
+
+    /// Test workflow run_for_revset() followed by new_revision() records both
+    ///
+    /// This mirrors the actual usage pattern in main.rs.
+    #[tokio::test]
+    async fn workflow_describe_then_new_revision() {
+        let mock_executor = MockJjExecutor::new().with_is_repo_response(Ok(true));
+        let mock_prompts = MockPrompts::new()
+            .with_commit_type(CommitType::Feat)
+            .with_scope(Scope::empty())
+            .with_description(Description::parse("add feature").unwrap())
+            .with_breaking_change(BreakingChange::No)
+            .with_body(Body::default())
+            .with_confirm(true);
+
+        let workflow = CommitWorkflow::with_prompts(mock_executor, mock_prompts);
+
+        workflow.run_for_revset("@").await.expect("describe failed");
+        workflow
+            .new_revision("@")
+            .await
+            .expect("new_revision failed");
+
+        let messages = workflow.executor.describe_messages();
+        assert_eq!(messages.len(), 1);
+        assert!(messages[0].contains("feat:"));
+
+        let calls = workflow.executor.new_revision_calls();
+        assert_eq!(calls, vec!["@"]);
+    }
 }

tests/error_tests.rs

@@ -135,3 +135,155 @@ fn test_jj_operation_context() {
         panic!("Expected JjOperation variant");
     }
 }
+
+/// Test conversion from std::io::Error
+#[test]
+fn test_from_io_error() {
+    let io_err = std::io::Error::new(std::io::ErrorKind::NotFound, "file not found");
+    let error: Error = io_err.into();
+    assert!(matches!(error, Error::FailedGettingCurrentDir));
+}
+
+/// Test conversion from std::sync::PoisonError
+#[test]
+fn test_from_poison_error() {
+    let mutex = std::sync::Mutex::new(());
+    // Poison the mutex by panicking while holding the lock
+    let poison_err = std::panic::catch_unwind(|| {
+        let _guard = mutex.lock().unwrap();
+        panic!("deliberate panic");
+    });
+    assert!(poison_err.is_err());
+
+    // Now lock should fail with PoisonError
+    let result = mutex.lock();
+    assert!(result.is_err());
+
+    let error: Error = result.unwrap_err().into();
+    assert!(matches!(error, Error::JjOperation { .. }));
+    assert_eq!(
+        format!("{}", error),
+        "Repository operation failed: internal lock poisoned"
+    );
+}
+
+/// Test from_revset_evaluation_error constructs RevsetResolutionError
+#[test]
+fn test_from_revset_evaluation_error() {
+    let underlying = std::io::Error::new(std::io::ErrorKind::Other, "store failure");
+    let eval_err = jj_lib::revset::RevsetEvaluationError::Other(Box::new(underlying));
+    let error = Error::from_revset_evaluation_error("@", eval_err);
+    assert!(matches!(error, Error::RevsetResolutionError { .. }));
+    let description = format!("{}", error);
+    assert!(description.contains("@"));
+    assert!(description.contains("store failure"));
+}
+
+/// Test from_revset_resolution_error constructs RevsetResolutionError
+#[test]
+fn test_from_revset_resolution_error() {
+    let resolution_err = jj_lib::revset::RevsetResolutionError::NoSuchRevision {
+        name: "nonexistent".to_string(),
+        candidates: Vec::new(),
+    };
+    let error = Error::from_revset_resolution_error("@", resolution_err);
+    assert!(matches!(error, Error::RevsetResolutionError { .. }));
+    let description = format!("{}", error);
+    assert!(description.contains("@"));
+    assert!(description.contains("nonexistent"));
+}
+
+/// Test NewFlagWithMultipleRevisions error display
+#[test]
+fn test_new_flag_with_multiple_revisions() {
+    let error = Error::NewFlagWithMultipleRevisions;
+    assert_eq!(
+        format!("{}", error),
+        "--new cannot be used with multiple revisions"
+    );
+}
+
+/// Test NonInteractive error display
+#[test]
+fn test_non_interactive() {
+    let error = Error::NonInteractive;
+    assert_eq!(format!("{}", error), "Non-interactive terminal detected");
+}
+
+/// Test FailedReadingConfig error display
+#[test]
+fn test_failed_reading_config() {
+    let error = Error::FailedReadingConfig {
+        context: "config parse error".to_string(),
+    };
+    let description = format!("{}", error);
+    assert!(description.contains("config parse error"));
+}
+
+/// Test MultipleRevisions error display
+#[test]
+fn test_multiple_revisions() {
+    let error = Error::MultipleRevisions {
+        revset: "abc | def".to_string(),
+    };
+    let description = format!("{}", error);
+    assert!(description.contains("abc | def"));
+    assert!(description.contains("multiple commits"));
+}
+
+/// Test RepositoryLocked error display
+#[test]
+fn test_repository_locked() {
+    let error = Error::RepositoryLocked;
+    assert_eq!(
+        format!("{}", error),
+        "Repository is locked by another process"
+    );
+}
+
+/// Test FailedGettingCurrentDir error display
+#[test]
+fn test_failed_getting_current_dir() {
+    let error = Error::FailedGettingCurrentDir;
+    assert_eq!(format!("{}", error), "Could not get current directory");
+}
+
+/// Test error matching on all variants
+#[test]
+fn test_error_matching_all_variants() {
+    let variants: Vec<Error> = vec![
+        Error::InvalidScope("s".into()),
+        Error::InvalidDescription("d".into()),
+        Error::InvalidCommitMessage("m".into()),
+        Error::NotARepository,
+        Error::JjOperation {
+            context: "c".into(),
+        },
+        Error::RepositoryLocked,
+        Error::FailedGettingCurrentDir,
+        Error::FailedReadingConfig {
+            context: "c".into(),
+        },
+        Error::Cancelled,
+        Error::NonInteractive,
+        Error::RevsetResolutionError {
+            revset: "@".into(),
+            context: "c".into(),
+        },
+        Error::MultipleRevisions { revset: "@".into() },
+        Error::NewFlagWithMultipleRevisions,
+    ];
+
+    // All variants should be displayable without panicking
+    for variant in &variants {
+        let _ = format!("{}", variant);
+        let _ = format!("{:?}", variant);
+    }
+
+    // Verify all variants can be cloned
+    let cloned: Vec<Error> = variants.iter().map(|e| e.clone()).collect();
+    assert_eq!(variants.len(), cloned.len());
+    for (original, clone) in variants.iter().zip(cloned.iter()) {
+        assert_eq!(original, clone);
+    }
+}