test_abandon_command: create another test for bug 2600

See comments inline for details. Cc jj-vcs#2600.

In particular, I wanted to make sure these behaviors are not affected by jj-vcs#2646.
They don't seem to be.

The tests ended up weirder than expected because of
jj-vcs#2600 (comment). Even
though, for now, the behavior of tests is unaffected by that issue, the
*expected* behavior is different.

cli/tests/test_abandon_command.rs

@@ -176,6 +176,193 @@ fn test_basics() {
     "###);
 }
 
+// TODO(#2600): Make sure the results here become saner as #2600 is fixed. There
+// is an simpler demo of #2600 at https://github.com/martinvonz/jj/pull/2655.
+// However, fixing #2600 will likely change how `abandon` works. This test
+// exists to track how that happens. See also the corresponding test in
+// `test_rebase_command`
+#[test]
+fn test_bug_2600() {
+    let test_env = TestEnvironment::default();
+    test_env.jj_cmd_ok(test_env.env_root(), &["init", "repo", "--git"]);
+    let repo_path = test_env.env_root().join("repo");
+
+    // We will not touch "nottherootcommit". See the
+    // `test_bug_2600_rootcommit_special_case` for the one case where base being the
+    // child of the root commit changes the expected behavior.
+    create_commit(&test_env, &repo_path, "nottherootcommit", &[]);
+    create_commit(&test_env, &repo_path, "base", &["nottherootcommit"]);
+    create_commit(&test_env, &repo_path, "a", &["base"]);
+    create_commit(&test_env, &repo_path, "b", &["base", "a"]);
+    create_commit(&test_env, &repo_path, "c", &["b"]);
+
+    // Test the setup
+    insta::assert_snapshot!(get_log_output(&test_env, &repo_path), @r###"
+    @  [znk] c
+    ◉    [vru] b
+    ├─╮
+    │ ◉  [roy] a
+    ├─╯
+    ◉  [zsu] base
+    ◉  [rlv] nottherootcommit
+    ◉  [zzz]
+    "###);
+    let setup_opid = test_env.current_operation_id(&repo_path);
+
+    test_env.jj_cmd_ok(&repo_path, &["op", "restore", &setup_opid]);
+    let (stdout, stderr) = test_env.jj_cmd_ok(&repo_path, &["abandon", "base"]);
+    insta::assert_snapshot!(stdout, @"");
+    insta::assert_snapshot!(stderr, @r###"
+    Abandoned commit zsuskuln 73c929fc base | base
+    Rebased 3 descendant commits onto parents of abandoned commits
+    Working copy now at: znkkpsqq 510f8756 c | c
+    Parent commit      : vruxwmqv 7301d9ab b | b
+    Added 0 files, modified 0 files, removed 1 files
+    "###);
+    // BUG. The user would expect
+    // @  c
+    // ├─╮
+    // │ ◉  a
+    // ├─╯
+    // ◉  base nottherootcommit
+    // ◉
+    // This is likely caused by DescendantRebaser
+    insta::assert_snapshot!(get_log_output(&test_env, &repo_path), @r###"
+    @  [znk] c
+    ◉  [vru] b
+    ◉  [roy] a
+    ◉  [rlv] base nottherootcommit
+    ◉  [zzz]
+    "###);
+
+    test_env.jj_cmd_ok(&repo_path, &["op", "restore", &setup_opid]);
+    let (stdout, stderr) = test_env.jj_cmd_ok(&repo_path, &["abandon", "a"]);
+    insta::assert_snapshot!(stdout, @"");
+    insta::assert_snapshot!(stderr, @r###"
+    Abandoned commit royxmykx 98f3b9ba a | a
+    Rebased 2 descendant commits onto parents of abandoned commits
+    Working copy now at: znkkpsqq 683b9435 c | c
+    Parent commit      : vruxwmqv c10cb7b4 b | b
+    Added 0 files, modified 0 files, removed 1 files
+    "###);
+    // This is likely what the user will expect.
+    insta::assert_snapshot!(get_log_output(&test_env, &repo_path), @r###"
+    @  [znk] c
+    ◉  [vru] b
+    ◉  [zsu] a base
+    ◉  [rlv] nottherootcommit
+    ◉  [zzz]
+    "###);
+
+    test_env.jj_cmd_ok(&repo_path, &["op", "restore", &setup_opid]);
+    let (stdout, stderr) = test_env.jj_cmd_ok(&repo_path, &["abandon", "b"]);
+    insta::assert_snapshot!(stdout, @"");
+    insta::assert_snapshot!(stderr, @r###"
+    Abandoned commit vruxwmqv 8c0dced0 b | b
+    Rebased 1 descendant commits onto parents of abandoned commits
+    Working copy now at: znkkpsqq 924bdd1c c | c
+    Parent commit      : royxmykx 98f3b9ba a b?? | a
+    Added 0 files, modified 0 files, removed 1 files
+    "###);
+    // BUG. The user would expect
+    // @  c
+    // ├─╮
+    // │ ◉  a
+    // ├─╯
+    // ◉  base
+    // ◉  nottherootcommit
+    // ◉
+    // This is likely caused by logic in `cmd_abandon`, not DescendantRebaser
+    insta::assert_snapshot!(get_log_output(&test_env, &repo_path), @r###"
+    @  [znk] c
+    ◉  [roy] a b??
+    ◉  [zsu] b?? base
+    ◉  [rlv] nottherootcommit
+    ◉  [zzz]
+    "###);
+
+    test_env.jj_cmd_ok(&repo_path, &["op", "restore", &setup_opid]);
+    // ========= Reminder of the setup ===========
+    insta::assert_snapshot!(get_log_output(&test_env, &repo_path), @r###"
+    @  [znk] c
+    ◉    [vru] b
+    ├─╮
+    │ ◉  [roy] a
+    ├─╯
+    ◉  [zsu] base
+    ◉  [rlv] nottherootcommit
+    ◉  [zzz]
+    "###);
+    let (stdout, stderr) = test_env.jj_cmd_ok(&repo_path, &["abandon", "a", "b"]);
+    insta::assert_snapshot!(stdout, @"");
+    insta::assert_snapshot!(stderr, @r###"
+    Abandoned the following commits:
+      royxmykx 98f3b9ba a | a
+      vruxwmqv 8c0dced0 b | b
+    Rebased 1 descendant commits onto parents of abandoned commits
+    Working copy now at: znkkpsqq 84fac1f8 c | c
+    Parent commit      : zsuskuln 73c929fc a b base | base
+    Added 0 files, modified 0 files, removed 2 files
+    "###);
+    // This is likely what the user would expect
+    insta::assert_snapshot!(get_log_output(&test_env, &repo_path), @r###"
+    @  [znk] c
+    ◉  [zsu] a b base
+    ◉  [rlv] nottherootcommit
+    ◉  [zzz]
+    "###);
+    let (stdout, stderr) = test_env.jj_cmd_ok(&repo_path, &["branch", "list", "b"]);
+    insta::assert_snapshot!(stdout, @r###"
+    b: zsuskuln 73c929fc base
+    "###);
+    insta::assert_snapshot!(stderr, @"");
+}
+
+#[test]
+fn test_bug_2600_rootcommit_special_case() {
+    let test_env = TestEnvironment::default();
+    test_env.jj_cmd_ok(test_env.env_root(), &["init", "repo", "--git"]);
+    let repo_path = test_env.env_root().join("repo");
+
+    // Set up like `test_bug_2600`, but without the `nottherootcommit` commit.
+    create_commit(&test_env, &repo_path, "base", &[]);
+    create_commit(&test_env, &repo_path, "a", &["base"]);
+    create_commit(&test_env, &repo_path, "b", &["base", "a"]);
+    create_commit(&test_env, &repo_path, "c", &["b"]);
+
+    // Setup
+    insta::assert_snapshot!(get_log_output(&test_env, &repo_path), @r###"
+    @  [vru] c
+    ◉    [roy] b
+    ├─╮
+    │ ◉  [zsu] a
+    ├─╯
+    ◉  [rlv] base
+    ◉  [zzz]
+    "###);
+
+    // Now, the test
+    let (stdout, stderr) = test_env.jj_cmd_ok(&repo_path, &["abandon", "base"]);
+    insta::assert_snapshot!(stdout, @"");
+    insta::assert_snapshot!(stderr, @r###"
+    Abandoned commit rlvkpnrz 0c61db1b base | base
+    Rebased 3 descendant commits onto parents of abandoned commits
+    Working copy now at: vruxwmqv 73e9185c c | c
+    Parent commit      : royxmykx 80dd9cba b | b
+    Added 0 files, modified 0 files, removed 1 files
+    "###);
+    // The current behavior is either correct or should be replaced with an error
+    // message. Even though the user would expect `b` to still be a descendant of
+    // `base`, it is impossible in the Git backend.
+    // See also https://github.com/martinvonz/jj/issues/2600#issuecomment-1835418824
+    insta::assert_snapshot!(get_log_output(&test_env, &repo_path), @r###"
+    @  [vru] c
+    ◉  [roy] b
+    ◉  [zsu] a
+    ◉  [zzz] base
+    "###);
+}
+
 #[test]
 fn test_double_abandon() {
     let test_env = TestEnvironment::default();