add support for creating empty commits to enable pull requests with n…

test/ci_provider/github/merge_request_test.py

@@ -320,3 +320,45 @@ def test_enable_auto_merge_exception(self, mock_warn, mock_info, mock_resolve, m
 
             # then
             mock_warn.assert_any_call("Could not enable auto-merge for pull request because: Network error")
+
+
+class GitHubMergeRequestEmptyCommitTest(unittest.TestCase):
+
+    @patch("valhalla.ci_provider.github.merge_request.get_valhalla_token", return_value="token")
+    @patch("valhalla.ci_provider.github.merge_request.GitRepository")
+    @patch("valhalla.ci_provider.github.merge_request.GitHubClient")
+    @patch("valhalla.ci_provider.github.merge_request.resolve")
+    @patch("valhalla.ci_provider.github.merge_request.info")
+    @patch("valhalla.ci_provider.github.merge_request.warn")
+    def test_create_empty_commit_and_retry_when_no_commits_between(self, mock_warn, mock_info, mock_resolve,
+                                                                   mock_client_cls, mock_git_cls, mock_token):
+        # given
+        with patch.dict('os.environ', {'GITHUB_REF_NAME': 'release-1.0.0'}):
+            mock_resolve.side_effect = lambda x: x
+            mock_client = MagicMock()
+            mock_client.api_url = "https://api.github.com"
+            mock_client.repo = "owner/repo"
+
+            no_commits_response = MagicMock()
+            no_commits_response.status_code = 422
+            no_commits_response.text = '{"errors":[{"message":"No commits between main and release-1.0.0"}]}'
+            pr_response = MagicMock()
+            pr_response.status_code = 201
+            pr_response.json.return_value = {"html_url": "https://github.com/owner/repo/pull/3", "number": 3}
+
+            mock_client.post.side_effect = [no_commits_response, pr_response]
+            mock_client_cls.return_value = mock_client
+            mock_git = mock_git_cls.return_value
+
+            pr = GitHubValhallaPullRequest()
+            config = MergeRequestConfig(enabled=True, target_branch="main", title="t", description="d", reviewers=[])
+
+            # when
+            pr.create(config)
+
+            # then
+            mock_git.commit_empty.assert_called_once()
+            mock_git.push.assert_called_once_with("token")
+            self.assertEqual(2, mock_client.post.call_count)
+            mock_info.assert_any_call("Created pull request: https://github.com/owner/repo/pull/3")
+            mock_warn.assert_not_called()

valhalla/ci_provider/github/merge_request.py

@@ -1,13 +1,17 @@
 import os
 
+from valhalla.ci_provider.get_token import get_valhalla_token
 from valhalla.ci_provider.github.common import get_default_branch_fallback, GitHubClient
+from valhalla.commit.commit import GitRepository
 from valhalla.common.get_config import MergeRequestConfig
 from valhalla.common.logger import info, warn
 from valhalla.common.resolver import resolve
 
 
 from valhalla.ci_provider.merge_request_hook import MergeRequestHook
 
+EMPTY_COMMIT_MSG = "Empty commit to create PR to start release"
+
 
 class GitHubValhallaPullRequest:
     def __init__(self):
@@ -39,6 +43,15 @@ def create(self, merge_request_config: MergeRequestConfig):
         }
 
         resp = self.client.post(url, json=payload)
+        # GitHub does not allow empty pull requests, see:
+        # https://stackoverflow.com/questions/46577500/why-cant-i-create-an-empty-pull-request-for-discussion-prior-to-developing-chan
+        if resp.status_code == 422 and "No commits between" in resp.text:
+            info("GitHub does not allow creating pull request without commits, creating empty commit")
+            git = GitRepository(None, None)
+            git.commit_empty(EMPTY_COMMIT_MSG)
+            git.push(get_valhalla_token())
+            resp = self.client.post(url, json=payload)
+
         if resp.status_code >= 300:
             warn(f"Failed to create pull request: {resp.status_code} {resp.text}")
             return MergeRequestHook.Skip()

valhalla/commit/commit.py

@@ -74,6 +74,11 @@ def commit(self, msg: str, add=True) -> bool:
         self.status()
         return True
 
+    def commit_empty(self, msg: str):
+        msg += " [VALHALLA SKIP]"
+        self.repository.git.commit("--allow-empty", "-m", resolve(msg))
+        info(f"Created empty commit: {self.repository.head.commit}")
+
     def push(self, token):
         info("Preparing to push")
         branch = self.repository.active_branch