fix(stack): Rebase branch in dry-run mode for accurate plan output

mergify_cli/stack/changes.py

@@ -203,6 +203,11 @@ class Changes:
     locals: list[LocalChange] = dataclasses.field(default_factory=list)
     orphans: list[OrphanChange] = dataclasses.field(default_factory=list)
 
+    def replace_local_action(self, old: ActionT, new: ActionT) -> None:
+        for change in self.locals:
+            if change.action == old:
+                change.action = new
+
 
 def display_plan(
     changes: Changes,

mergify_cli/stack/push.py

@@ -215,6 +215,20 @@ async def stack_push(
                 await utils.git("pull", "--rebase", remote, base_branch)
             console.log(f"branch `{dest_branch}` rebased on `{remote}/{base_branch}`")
 
+    rebase_required = False
+    if dry_run and not skip_rebase:
+        commits_behind = int(
+            await utils.git("rev-list", "--count", f"HEAD..{remote}/{base_branch}"),
+        )
+        rebase_required = commits_behind > 0
+
+        if rebase_required:
+            console.log(
+                f"[orange]branch `{dest_branch}` is behind `{remote}/{base_branch}` "
+                f"by {commits_behind} {'commit' if commits_behind == 1 else 'commits'}, "
+                f"commit SHAs will differ after rebase[/]",
+            )
+
     base_commit_sha = await utils.git(
         "merge-base",
         "--fork-point",
@@ -249,6 +263,12 @@ async def stack_push(
                 next_only=next_only,
             )
 
+        if rebase_required:
+            # If the branch is behind, we know for sure that all the existing
+            # pull requests will need to be updated, so we can directly plan
+            # them as "update" instead of "skip-up-to-date".
+            planned_changes.replace_local_action(old="skip-up-to-date", new="update")
+
         changes.display_plan(
             planned_changes,
             create_as_draft=create_as_draft,

mergify_cli/tests/stack/test_push.py

@@ -19,6 +19,7 @@
 
 import pytest
 
+from mergify_cli.stack import changes
 from mergify_cli.stack import push
 from mergify_cli.tests import utils as test_utils
 
@@ -491,6 +492,142 @@ async def test_stack_update_keep_title_and_body(
     }
 
 
+@pytest.mark.respx(base_url="https://api.github.com/")
+async def test_stack_dry_run_does_not_rebase(
+    git_mock: test_utils.GitMock,
+    respx_mock: respx.MockRouter,
+) -> None:
+    git_mock.commit(
+        test_utils.Commit(
+            sha="commit1_sha",
+            title="Title commit 1",
+            message="Message commit 1",
+            change_id="I29617d37762fd69809c255d7e7073cb11f8fbf50",
+        ),
+    )
+    git_mock.finalize()
+    git_mock.mock("rev-list", "--count", "HEAD..origin/main", output="0")
+
+    respx_mock.get("/user").respond(200, json={"login": "author"})
+    respx_mock.get("/search/issues").respond(200, json={"items": []})
+
+    with pytest.raises(SystemExit, match="0"):
+        await push.stack_push(
+            github_server="https://api.github.com/",
+            token="",
+            skip_rebase=False,
+            next_only=False,
+            branch_prefix="",
+            dry_run=True,
+            trunk=("origin", "main"),
+        )
+
+    # Dry-run never rebases.
+    assert not git_mock.has_been_called_with("pull", "--rebase", "origin", "main")
+
+    # No branches are pushed.
+    assert not git_mock.has_been_called_with(
+        "push",
+        "-f",
+        "origin",
+        "commit1_sha:refs/heads/current-branch/I29617d37762fd69809c255d7e7073cb11f8fbf50",
+    )
+
+
+@pytest.mark.respx(base_url="https://api.github.com/")
+async def test_stack_dry_run_behind_flips_up_to_date_to_update(
+    git_mock: test_utils.GitMock,
+    respx_mock: respx.MockRouter,
+) -> None:
+    # PR exists with matching SHA — normally "skip-up-to-date".
+    # But branch is behind base, so rebase would change SHAs → "update".
+    git_mock.commit(
+        test_utils.Commit(
+            sha="commit1_sha",
+            title="Title commit 1",
+            message="Message commit 1",
+            change_id="I29617d37762fd69809c255d7e7073cb11f8fbf50",
+        ),
+    )
+    git_mock.finalize()
+    git_mock.mock("rev-list", "--count", "HEAD..origin/main", output="3")
+
+    respx_mock.get("/user").respond(200, json={"login": "author"})
+    respx_mock.get("/search/issues").respond(
+        200,
+        json={
+            "items": [
+                {
+                    "pull_request": {
+                        "url": "https://api.github.com/repos/user/repo/pulls/42",
+                    },
+                },
+            ],
+        },
+    )
+    respx_mock.get("/repos/user/repo/pulls/42").respond(
+        200,
+        json={
+            "html_url": "",
+            "head": {
+                "sha": "commit1_sha",
+                "ref": "current-branch/I29617d37762fd69809c255d7e7073cb11f8fbf50",
+            },
+            "state": "open",
+            "merged_at": None,
+            "draft": False,
+        },
+    )
+
+    with pytest.raises(SystemExit, match="0"):
+        await push.stack_push(
+            github_server="https://api.github.com/",
+            token="",
+            skip_rebase=False,
+            next_only=False,
+            branch_prefix="",
+            dry_run=True,
+            trunk=("origin", "main"),
+        )
+
+    # Dry-run never rebases.
+    assert not git_mock.has_been_called_with("pull", "--rebase", "origin", "main")
+
+
+@pytest.mark.respx(base_url="https://api.github.com/")
+async def test_stack_dry_run_skip_rebase(
+    git_mock: test_utils.GitMock,
+    respx_mock: respx.MockRouter,
+) -> None:
+    git_mock.commit(
+        test_utils.Commit(
+            sha="commit1_sha",
+            title="Title commit 1",
+            message="Message commit 1",
+            change_id="I29617d37762fd69809c255d7e7073cb11f8fbf50",
+        ),
+    )
+    git_mock.finalize()
+
+    respx_mock.get("/user").respond(200, json={"login": "author"})
+    respx_mock.get("/search/issues").respond(200, json={"items": []})
+
+    with pytest.raises(SystemExit, match="0"):
+        await push.stack_push(
+            github_server="https://api.github.com/",
+            token="",
+            skip_rebase=True,
+            next_only=False,
+            branch_prefix="",
+            dry_run=True,
+            trunk=("origin", "main"),
+        )
+
+    # Rebase check is skipped when --skip-rebase is passed.
+    assert not git_mock.has_been_called_with("rev-list", "--count", "HEAD..origin/main")
+    assert not git_mock.has_been_called_with("pull", "--rebase", "origin", "main")
+
+
 @pytest.mark.respx(base_url="https://api.github.com/")
 async def test_stack_on_destination_branch_raises_an_error(
     git_mock: test_utils.GitMock,
@@ -535,3 +672,26 @@ async def test_stack_without_common_commit_raises_an_error(
             dry_run=False,
             trunk=("origin", "main"),
         )
+
+
+def test_replace_local_action_flips_up_to_date() -> None:
+    def _make_local_change(action: changes.ActionT) -> changes.LocalChange:
+        return changes.LocalChange(
+            id=changes.ChangeId(""),
+            pull=None,
+            commit_sha="",
+            title="",
+            message="",
+            base_branch="",
+            dest_branch="",
+            action=action,
+        )
+
+    planned = changes.Changes(stack_prefix="")
+    planned.locals.append(_make_local_change("skip-up-to-date"))
+    planned.locals.append(_make_local_change("create"))
+
+    planned.replace_local_action(old="skip-up-to-date", new="update")
+
+    assert planned.locals[0].action == "update"
+    assert planned.locals[1].action == "create"