Add --validate-rules, --dry-run, --comment-on-failure to land command

Extends ghstack land with merge rules validation:

CLI options:
- --validate-rules: Validate PRs against .github/merge_rules.yaml
- --dry-run: Validate without actually merging
- --comment-on-failure: Post validation errors as PR comments

The validation happens before any git operations:
1. Load merge rules from repository
2. For each PR in the stack, validate approvers and CI checks
3. If any PR fails validation, abort with detailed error
4. Optionally post error details as a PR comment

Example usage:
  ghstack land --validate-rules --comment-on-failure <PR_URL>


ghstack-source-id: a063004
ghstack-comment-id: 3807838942
Pull-Request: #318

Author: seemethere

src/ghstack/cli.py

@@ -164,8 +164,29 @@ def cherry_pick(stack: bool, pull_request: str) -> None:
 
 @main.command("land")
 @click.option("--force", is_flag=True, help="force land even if the PR is closed")
+@click.option(
+    "--validate-rules",
+    is_flag=True,
+    help="validate against merge rules defined in .github/merge_rules.yaml",
+)
+@click.option(
+    "--dry-run",
+    is_flag=True,
+    help="validate merge rules but don't actually merge",
+)
+@click.option(
+    "--comment-on-failure",
+    is_flag=True,
+    help="post validation errors as a PR comment",
+)
 @click.argument("pull_request", metavar="PR")
-def land(force: bool, pull_request: str) -> None:
+def land(
+    force: bool,
+    validate_rules: bool,
+    dry_run: bool,
+    comment_on_failure: bool,
+    pull_request: str,
+) -> None:
     """
     Land a PR stack
     """
@@ -177,6 +198,9 @@ def land(force: bool, pull_request: str) -> None:
             github_url=config.github_url,
             remote_name=config.remote_name,
             force=force,
+            validate_rules=validate_rules,
+            dry_run=dry_run,
+            comment_on_failure=comment_on_failure,
         )
 
 

src/ghstack/land.py

@@ -2,11 +2,12 @@
 
 import logging
 import re
-from typing import List, Tuple
+from typing import List, Optional, Tuple
 
 import ghstack.git
 import ghstack.github
 import ghstack.github_utils
+import ghstack.merge_rules
 import ghstack.shell
 from ghstack.diff import PullRequestResolved
 from ghstack.types import GitCommitHash
@@ -50,6 +51,10 @@ def main(
     github_url: str,
     *,
     force: bool = False,
+    validate_rules: bool = False,
+    dry_run: bool = False,
+    comment_on_failure: bool = False,
+    rules_file: Optional[str] = None,
 ) -> None:
 
     # We land the entire stack pointed to by a URL.
@@ -60,19 +65,22 @@ def main(
     params = ghstack.github_utils.parse_pull_request(
         pull_request, sh=sh, remote_name=remote_name
     )
+    owner = params["owner"]
+    name = params["name"]
+
     default_branch = ghstack.github_utils.get_github_repo_info(
         github=github,
         sh=sh,
-        repo_owner=params["owner"],
-        repo_name=params["name"],
+        repo_owner=owner,
+        repo_name=name,
         github_url=github_url,
         remote_name=remote_name,
     )["default_branch"]
 
     needs_force = False
     try:
         protection = github.get(
-            f"repos/{params['owner']}/{params['name']}/branches/{default_branch}/protection"
+            f"repos/{owner}/{name}/branches/{default_branch}/protection"
         )
         if not protection["allow_force_pushes"]["enabled"]:
             raise RuntimeError(
@@ -91,12 +99,12 @@ def main(
 
     orig_ref, closed = lookup_pr_to_orig_ref_and_closed(
         github,
-        owner=params["owner"],
-        name=params["name"],
+        owner=owner,
+        name=name,
         number=params["number"],
     )
 
-    if closed:
+    if closed and not force:
         raise RuntimeError("PR is already closed, cannot land it!")
 
     if sh is None:
@@ -117,6 +125,64 @@ def main(
         github_url=github_url,
     )
 
+    # Compute the metadata for each commit
+    stack_orig_refs: List[Tuple[str, PullRequestResolved]] = []
+    for s in stack:
+        pr_resolved = s.pull_request_resolved
+        # We got this from GitHub, this better not be corrupted
+        assert pr_resolved is not None
+
+        ref, stack_closed = lookup_pr_to_orig_ref_and_closed(
+            github,
+            owner=pr_resolved.owner,
+            name=pr_resolved.repo,
+            number=pr_resolved.number,
+        )
+        if stack_closed and not force:
+            continue
+        stack_orig_refs.append((ref, pr_resolved))
+
+    # Validate merge rules if requested
+    if validate_rules:
+        logging.info("Validating merge rules for PR stack...")
+
+        # Load merge rules
+        loader = ghstack.merge_rules.MergeRulesLoader(github, owner, name)
+        if rules_file:
+            rules = loader.load_from_file(rules_file)
+        else:
+            rules = loader.load_from_repo()
+
+        if not rules:
+            logging.warning("No merge rules found, skipping validation")
+        else:
+            # Validate each PR in the stack
+            validator = ghstack.merge_rules.MergeValidator(github, owner, name)
+            for _, pr_resolved in stack_orig_refs:
+                result = validator.validate_pr(pr_resolved.number, rules)
+                if not result.valid:
+                    logging.error(
+                        f"Validation failed for PR #{pr_resolved.number}: "
+                        f"{', '.join(result.errors)}"
+                    )
+                    if comment_on_failure:
+                        comment_body = (
+                            ghstack.merge_rules.format_validation_error_comment(result)
+                        )
+                        github.post_issue_comment(
+                            owner, name, pr_resolved.number, comment_body
+                        )
+                    raise ghstack.merge_rules.MergeValidationError(result)
+                logging.info(
+                    f"PR #{pr_resolved.number} passed validation (rule: {result.rule_name})"
+                )
+
+            logging.info("All PRs in stack passed merge rules validation")
+
+        if dry_run:
+            logging.info("Dry run complete - no changes made")
+            return
+
     # Switch working copy
     try:
         prev_ref = sh.git("symbolic-ref", "--short", "HEAD")
@@ -127,23 +193,6 @@ def main(
     sh.git("checkout", f"{remote_name}/{default_branch}")
 
     try:
-        # Compute the metadata for each commit
-        stack_orig_refs: List[Tuple[str, PullRequestResolved]] = []
-        for s in stack:
-            pr_resolved = s.pull_request_resolved
-            # We got this from GitHub, this better not be corrupted
-            assert pr_resolved is not None
-
-            ref, closed = lookup_pr_to_orig_ref_and_closed(
-                github,
-                owner=pr_resolved.owner,
-                name=pr_resolved.repo,
-                number=pr_resolved.number,
-            )
-            if closed and not force:
-                continue
-            stack_orig_refs.append((ref, pr_resolved))
-
         # OK, actually do the land now
         for orig_ref, pr_resolved in stack_orig_refs:
             try: