feat: add smart_rebase algorithm for merged commit handling

AGENTS.md

@@ -0,0 +1,20 @@
+# Agent Instructions
+
+## CLI Output Style
+
+Use compact Unicode symbols for terminal output, not emoji. This keeps new and
+updated terminal output consistent and avoids rendering issues across terminals.
+
+**Preferred symbols:**
+- `✓` success, approved, merged
+- `✗` failure, conflict, changes requested
+- `●` active, pending, in progress
+- `○` inactive, skipped, none
+- `—` unknown, not applicable
+
+**Avoid in terminal output:** `✅`, `❌`, `🟢`, `🔴`, `📝`, `⚠️`, `🔄`, `📦`, `🤖`, `🔗`
+and other emoji. They can render at inconsistent widths across terminals and
+break column alignment.
+
+**Exception:** Emoji are acceptable in markdown output destined for GitHub
+(PR comments, CI summaries) where they render consistently.

mergify_cli/stack/hooks/scripts/pre-push.sh

@@ -40,7 +40,7 @@ if test "$has_change_id" -eq 0; then
 fi
 
 echo ""
-echo "⚠  This branch is managed by Mergify stacks."
+echo "This branch is managed by Mergify stacks."
 echo "   Use 'mergify stack push' instead of 'git push'."
 echo ""
 echo "   'mergify stack push' will:"

mergify_cli/stack/push.py

@@ -212,11 +212,28 @@ async def stack_push(
         if skip_rebase:
             console.log(f"branch `{dest_branch}` rebase skipped (--skip-rebase)")
         else:
+            from mergify_cli.stack import sync as stack_sync_mod
+
             with console.status(
                 f"Rebasing branch `{dest_branch}` on `{remote}/{base_branch}`...",
             ):
-                await utils.git("pull", "--rebase", remote, base_branch)
-            console.log(f"branch `{dest_branch}` rebased on `{remote}/{base_branch}`")
+                await utils.git("fetch", remote, base_branch)
+                sync_status = await stack_sync_mod.smart_rebase(
+                    github_server,
+                    token,
+                    trunk=trunk,
+                    branch_prefix=branch_prefix,
+                    author=author,
+                )
+            if sync_status.merged:
+                console.log(
+                    f"branch `{dest_branch}` rebased on `{remote}/{base_branch}` "
+                    f"(dropped {len(sync_status.merged)} merged commit(s))",
+                )
+            else:
+                console.log(
+                    f"branch `{dest_branch}` rebased on `{remote}/{base_branch}`",
+                )
 
     rebase_required = False
     if dry_run and not skip_rebase:

mergify_cli/stack/sync.py

@@ -0,0 +1,266 @@
+#
+#  Copyright © 2021-2026 Mergify SAS
+#
+# Licensed under the Apache License, Version 2.0 (the "License"); you may
+# not use this file except in compliance with the License. You may obtain
+# a copy of the License at
+#
+#      http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
+# WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
+# License for the specific language governing permissions and limitations
+# under the License.
+
+from __future__ import annotations
+
+import dataclasses
+import os
+import pathlib
+import sys
+import tempfile
+import typing
+
+from mergify_cli import console
+from mergify_cli import utils
+from mergify_cli.stack import changes
+from mergify_cli.stack.push import LocalBranchInvalidError
+from mergify_cli.stack.push import check_local_branch
+
+
+@dataclasses.dataclass
+class MergedCommit:
+    """A commit whose PR has been merged."""
+
+    commit_sha: str
+    title: str
+    pull_number: int
+    pull_url: str
+
+
+@dataclasses.dataclass
+class RemainingCommit:
+    """A commit that still has an open PR or no PR yet."""
+
+    commit_sha: str
+    title: str
+
+
+@dataclasses.dataclass
+class SyncStatus:
+    """Result of a sync status check for the current stack."""
+
+    branch: str
+    trunk: str
+    merged: list[MergedCommit]
+    remaining: list[RemainingCommit]
+
+    @property
+    def all_merged(self) -> bool:
+        """True when every commit in the stack has been merged."""
+        return bool(self.merged) and not self.remaining
+
+    @property
+    def up_to_date(self) -> bool:
+        """True when there are no merged commits (nothing to rebase away)."""
+        return not self.merged
+
+
+async def get_sync_status(
+    github_server: str,
+    token: str,
+    *,
+    trunk: tuple[str, str],
+    branch_prefix: str | None = None,
+    author: str | None = None,
+) -> SyncStatus:
+    """Compute the sync status for the current stack.
+
+    Args:
+        github_server: GitHub API server URL
+        token: GitHub personal access token
+        trunk: Tuple of (remote, branch) for the trunk
+        branch_prefix: Optional branch prefix for stack branches
+        author: Optional author filter (defaults to token owner)
+
+    Returns:
+        SyncStatus classifying each commit as merged or remaining
+    """
+    dest_branch = await utils.git_get_branch_name()
+
+    if author is None:
+        async with utils.get_github_http_client(github_server, token) as client:
+            r_author = await client.get("/user")
+            author = typing.cast("str", r_author.json()["login"])
+
+    if branch_prefix is None:
+        branch_prefix = await utils.get_default_branch_prefix(author)
+
+    try:
+        check_local_branch(branch_name=dest_branch, branch_prefix=branch_prefix)
+    except LocalBranchInvalidError as e:
+        console.print(f"[red] {e.message} [/]")
+        console.print(
+            "You should run `mergify stack sync` on the branch you created in the first place",
+        )
+        sys.exit(1)
+
+    remote, base_branch = trunk
+
+    user, repo = utils.get_slug(
+        await utils.git("config", "--get", f"remote.{remote}.url"),
+    )
+
+    if base_branch == dest_branch:
+        remote_url = await utils.git("remote", "get-url", remote)
+        console.print(
+            f"Your local branch `{dest_branch}` targets itself: "
+            f"`{remote}/{base_branch}` (at {remote_url}@{base_branch}).\n"
+            "You should either fix the target branch or rename your local branch.\n\n"
+            f"* To fix the target branch: "
+            f"`git branch {dest_branch} --set-upstream-to={remote}/{base_branch}`\n"
+            f"* To rename your local branch: "
+            f"`git branch -M {dest_branch} new-branch-name`",
+            style="red",
+        )
+        sys.exit(1)
+
+    stack_prefix = f"{branch_prefix}/{dest_branch}" if branch_prefix else dest_branch
+
+    base_commit_sha = await utils.git(
+        "merge-base",
+        "--fork-point",
+        f"{remote}/{base_branch}",
+    )
+    if not base_commit_sha:
+        console.print(
+            f"Common commit between `{remote}/{base_branch}` and `{dest_branch}` branches not found",
+            style="red",
+        )
+        sys.exit(1)
+
+    async with utils.get_github_http_client(github_server, token) as client:
+        remote_changes = await changes.get_remote_changes(
+            client,
+            user,
+            repo,
+            stack_prefix,
+            author,
+        )
+
+        stack_changes = await changes.get_changes(
+            base_commit_sha=base_commit_sha,
+            stack_prefix=stack_prefix,
+            base_branch=base_branch,
+            dest_branch=dest_branch,
+            remote_changes=remote_changes,
+            only_update_existing_pulls=False,
+            next_only=False,
+        )
+
+    merged: list[MergedCommit] = []
+    remaining: list[RemainingCommit] = []
+
+    for local_change in stack_changes.locals:
+        if local_change.action == "skip-merged":
+            pull = local_change.pull
+            merged.append(
+                MergedCommit(
+                    commit_sha=local_change.commit_sha,
+                    title=local_change.title,
+                    pull_number=int(pull["number"]) if pull else 0,
+                    pull_url=pull["html_url"] if pull else "",
+                ),
+            )
+        else:
+            remaining.append(
+                RemainingCommit(
+                    commit_sha=local_change.commit_sha,
+                    title=local_change.title,
+                ),
+            )
+
+    return SyncStatus(
+        branch=dest_branch,
+        trunk=f"{remote}/{base_branch}",
+        merged=merged,
+        remaining=remaining,
+    )
+
+
+def _write_drop_script(merged_shas: set[str]) -> pathlib.Path:
+    """Write a temporary bash script that acts as GIT_SEQUENCE_EDITOR to drop merged commits.
+
+    The script reads the rebase todo file, replaces "pick <sha>" with
+    "drop <sha>" for each merged commit, and writes it back.
+
+    Returns the path to the script. Caller is responsible for cleanup.
+    """
+    short_shas = sorted(sha[:7] for sha in merged_shas)
+
+    # Build sed expressions — one per merged SHA
+    sed_expressions = " ".join(f"-e 's/^pick {sha}/drop {sha}/'" for sha in short_shas)
+
+    script = f'#!/bin/sh\nsed {sed_expressions} "$1" > "$1.tmp" && mv "$1.tmp" "$1"\n'
+
+    fd, path = tempfile.mkstemp(suffix=".sh", prefix="mergify_drop_")
+    script_path = pathlib.Path(path)
+    os.close(fd)
+    script_path.write_text(script, encoding="utf-8")
+    script_path.chmod(0o755)
+    return script_path
+
+
+async def smart_rebase(
+    github_server: str,
+    token: str,
+    *,
+    trunk: tuple[str, str],
+    branch_prefix: str | None = None,
+    author: str | None = None,
+) -> SyncStatus:
+    """Rebase the stack onto trunk, dropping any merged commits.
+
+    If merged commits are found, does a single `git rebase -i` that both
+    drops them and rebases onto the latest trunk. Otherwise, falls back
+    to a simple `git pull --rebase`.
+
+    Callers are responsible for fetching the remote before calling this.
+
+    Returns the SyncStatus so callers can inspect what happened.
+    """
+    remote, base_branch = trunk
+
+    status = await get_sync_status(
+        github_server,
+        token,
+        trunk=trunk,
+        branch_prefix=branch_prefix,
+        author=author,
+    )
+
+    if status.all_merged or status.up_to_date:
+        # Simple rebase — no merged commits to drop
+        await utils.git("pull", "--rebase", remote, base_branch)
+        return status
+
+    # Merged commits found — drop them and rebase in one operation.
+    # Using git rebase -i onto trunk with a script that changes "pick" to "drop"
+    # for merged commits. This avoids conflicts from trying to reapply commits
+    # whose content was modified on GitHub before merge.
+    merged_shas = {m.commit_sha for m in status.merged}
+    script_path = _write_drop_script(merged_shas)
+
+    env_backup = os.environ.get("GIT_SEQUENCE_EDITOR")
+    os.environ["GIT_SEQUENCE_EDITOR"] = str(script_path)
+    try:
+        await utils.git("rebase", "-i", f"{remote}/{base_branch}")
+    finally:
+        if env_backup is None:
+            os.environ.pop("GIT_SEQUENCE_EDITOR", None)
+        else:
+            os.environ["GIT_SEQUENCE_EDITOR"] = env_backup
+        script_path.unlink(missing_ok=True)
+
+    return status

mergify_cli/tests/conftest.py

@@ -80,8 +80,16 @@ def git_mock(
         "remote.origin.url",
         output="https://github.com/user/repo",
     )
-    # Mock pull command
+    # Mock fetch and pull commands (used by smart_rebase)
+    git_mock_object.mock("fetch", "origin", "main", output="")
     git_mock_object.mock("pull", "--rebase", "origin", "main", output="")
+    # Mock branch prefix config (used by smart_rebase via get_sync_status)
+    git_mock_object.mock(
+        "config",
+        "--get",
+        "mergify-cli.stack-branch-prefix",
+        output="",
+    )
 
     with mock.patch("mergify_cli.utils.git", git_mock_object):
         yield git_mock_object