cli: add "--limit N" option to log-like commands

Copied from Mercurial. This isn't a revset predicate since our revset is
conceptually unordered.

CHANGELOG.md

@@ -50,6 +50,9 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
   external program. For configuration, see [the documentation](docs/config.md).
   [#1886](https://github.com/martinvonz/jj/issues/1886)
 
+* `jj log`/`obslog`/`op log` now supports `--limit N` option to show the first
+  `N` entries.
+
 ### Fixed bugs
 
 * SSH authentication could hang when ssh-agent couldn't be reached

cli/src/commands/mod.rs

@@ -368,6 +368,11 @@ struct LogArgs {
     /// Show revisions in the opposite order (older revisions first)
     #[arg(long)]
     reversed: bool,
+    /// Limit number of revisions to show
+    ///
+    /// Applied after revisions are filtered and reordered.
+    #[arg(long, short)]
+    limit: Option<usize>,
     /// Don't show the graph, show a flat list of revisions
     #[arg(long)]
     no_graph: bool,
@@ -390,6 +395,9 @@ struct LogArgs {
 struct ObslogArgs {
     #[arg(long, short, default_value = "@")]
     revision: RevisionArg,
+    /// Limit number of revisions to show
+    #[arg(long, short)]
+    limit: Option<usize>,
     /// Don't show the graph, show a flat list of revisions
     #[arg(long)]
     no_graph: bool,
@@ -1664,7 +1672,7 @@ fn cmd_log(ui: &mut Ui, command: &CommandHelper, args: &LogArgs) -> Result<(), C
             } else {
                 Box::new(forward_iter)
             };
-            for (commit_id, edges) in iter {
+            for (commit_id, edges) in iter.take(args.limit.unwrap_or(usize::MAX)) {
                 let mut graphlog_edges = vec![];
                 // TODO: Should we update RevsetGraphIterator to yield this flag instead of all
                 // the missing edges since we don't care about where they point here
@@ -1727,7 +1735,7 @@ fn cmd_log(ui: &mut Ui, command: &CommandHelper, args: &LogArgs) -> Result<(), C
             } else {
                 Box::new(revset.iter())
             };
-            for commit_or_error in iter.commits(store) {
+            for commit_or_error in iter.commits(store).take(args.limit.unwrap_or(usize::MAX)) {
                 let commit = commit_or_error?;
                 with_content_format
                     .write(formatter, |formatter| template.format(&commit, formatter))?;
@@ -1791,11 +1799,14 @@ fn cmd_obslog(ui: &mut Ui, command: &CommandHelper, args: &ObslogArgs) -> Result
     let formatter = formatter.as_mut();
     formatter.push_label("log")?;
 
-    let commits = topo_order_reverse(
+    let mut commits = topo_order_reverse(
         vec![start_commit],
         |commit: &Commit| commit.id().clone(),
         |commit: &Commit| commit.predecessors(),
     );
+    if let Some(n) = args.limit {
+        commits.truncate(n);
+    }
     if !args.no_graph {
         let mut graph = get_graphlog(command.settings(), formatter.raw());
         let default_node_symbol = graph.default_node_symbol().to_owned();

cli/src/commands/operation.rs

@@ -26,6 +26,9 @@ pub enum OperationCommands {
 /// Show the operation log
 #[derive(clap::Args, Clone, Debug)]
 pub struct OperationLogArgs {
+    /// Limit number of operations to show
+    #[arg(long, short)]
+    limit: Option<usize>,
     /// Render each operation using the given template
     ///
     /// For the syntax, see https://github.com/martinvonz/jj/blob/main/docs/templates.md
@@ -119,7 +122,7 @@ fn cmd_op_log(
     let formatter = formatter.as_mut();
     let mut graph = get_graphlog(command.settings(), formatter.raw());
     let default_node_symbol = graph.default_node_symbol().to_owned();
-    for op in operation::walk_ancestors(&head_op) {
+    for op in operation::walk_ancestors(&head_op).take(args.limit.unwrap_or(usize::MAX)) {
         let mut edges = vec![];
         for parent in op.parents() {
             edges.push(Edge::direct(parent.id().clone()));

cli/tests/test_log_command.rs

@@ -796,6 +796,88 @@ fn test_log_filtered_by_path() {
     "###);
 }
 
+#[test]
+fn test_log_limit() {
+    let test_env = TestEnvironment::default();
+    test_env.jj_cmd_success(test_env.env_root(), &["init", "repo", "--git"]);
+    let repo_path = test_env.env_root().join("repo");
+
+    test_env.jj_cmd_success(&repo_path, &["describe", "-m", "a"]);
+    std::fs::write(repo_path.join("a"), "").unwrap();
+    test_env.jj_cmd_success(&repo_path, &["new", "-m", "b"]);
+    std::fs::write(repo_path.join("b"), "").unwrap();
+    test_env.jj_cmd_success(&repo_path, &["new", "-m", "c", "description(a)"]);
+    std::fs::write(repo_path.join("c"), "").unwrap();
+    test_env.jj_cmd_success(
+        &repo_path,
+        &["new", "-m", "d", "description(c)", "description(b)"],
+    );
+
+    let stdout = test_env.jj_cmd_success(&repo_path, &["log", "-T", "description", "--limit=3"]);
+    insta::assert_snapshot!(stdout, @r###"
+    @    d
+    ├─╮
+    │ ◉  b
+    ◉ │  c
+    ├─╯
+    "###);
+
+    // Applied on sorted DAG
+    let stdout = test_env.jj_cmd_success(&repo_path, &["log", "-T", "description", "--limit=2"]);
+    insta::assert_snapshot!(stdout, @r###"
+    @    d
+    ├─╮
+    │ ◉  b
+    "###);
+
+    let stdout = test_env.jj_cmd_success(
+        &repo_path,
+        &["log", "-T", "description", "--limit=2", "--no-graph"],
+    );
+    insta::assert_snapshot!(stdout, @r###"
+    d
+    c
+    "###);
+
+    // Applied on reversed DAG
+    let stdout = test_env.jj_cmd_success(
+        &repo_path,
+        &["log", "-T", "description", "--limit=3", "--reversed"],
+    );
+    insta::assert_snapshot!(stdout, @r###"
+    ◉
+    ◉    a
+    ├─╮
+    │ ◉  c
+    "###);
+    let stdout = test_env.jj_cmd_success(
+        &repo_path,
+        &[
+            "log",
+            "-T",
+            "description",
+            "--limit=3",
+            "--reversed",
+            "--no-graph",
+        ],
+    );
+    insta::assert_snapshot!(stdout, @r###"
+    a
+    b
+    "###);
+
+    // Applied on filtered commits
+    let stdout = test_env.jj_cmd_success(
+        &repo_path,
+        &["log", "-T", "description", "--limit=1", "b", "c"],
+    );
+    insta::assert_snapshot!(stdout, @r###"
+    ◉  c
+    │
+    ~
+    "###);
+}
+
 #[test]
 fn test_log_warn_path_might_be_revset() {
     let test_env = TestEnvironment::default();

cli/tests/test_obslog_command.rs

@@ -80,6 +80,15 @@ fn test_obslog_with_or_without_diff() {
        (empty) my description
     "###);
 
+    // Test `--limit`
+    let stdout = test_env.jj_cmd_success(&repo_path, &["obslog", "--limit=2"]);
+    insta::assert_snapshot!(stdout, @r###"
+    @  rlvkpnrz test.user@example.com 2001-02-03 04:05:10.000 +07:00 66b42ad3
+    │  my description
+    ◉  rlvkpnrz hidden test.user@example.com 2001-02-03 04:05:09.000 +07:00 af536e5a conflict
+    │  my description
+    "###);
+
     // Test `--no-graph`
     let stdout = test_env.jj_cmd_success(&repo_path, &["obslog", "--no-graph"]);
     insta::assert_snapshot!(stdout, @r###"

cli/tests/test_operations.rs

@@ -118,6 +118,18 @@ fn test_op_log() {
     "###);
 }
 
+#[test]
+fn test_op_log_limit() {
+    let test_env = TestEnvironment::default();
+    test_env.jj_cmd_success(test_env.env_root(), &["init", "repo", "--git"]);
+    let repo_path = test_env.env_root().join("repo");
+
+    let stdout = test_env.jj_cmd_success(&repo_path, &["op", "log", "-Tdescription", "--limit=1"]);
+    insta::assert_snapshot!(stdout, @r###"
+    @  add workspace 'default'
+    "###);
+}
+
 #[test]
 fn test_op_log_template() {
     let test_env = TestEnvironment::default();