feat: implement fallback behavior for compare API 404 errors in DataM…

docs/features/compare_mode.md

@@ -98,6 +98,43 @@ Issues are always filtered by timestamp regardless of mode.
 
 ---
 
+## Fallback Behavior — When the Target Tag Doesn't Exist Yet
+
+In CI/CD pipelines, it's common to call the release notes generator *before* the target tag is created.
+When the compare API receives a request for a non-existent tag, it returns a 404 error.
+
+**Fallback strategy:**
+
+If the compare API fails with a 404 or any other error:
+1. The action falls back to fetching the latest commit SHA of the target tag/ref
+2. A warning is logged to indicate the fallback occurred
+3. Processing continues with just that single commit as the comparison baseline
+
+This ensures the action completes gracefully even when tags are created in a sequence:
+
+```yaml
+- name: Build & Tag Release
+  run: |
+    ./scripts/build.sh
+    git tag v2.6.4
+    git push origin v2.6.4
+
+- name: Generate Release Notes  # Runs immediately after tagging
+  uses: AbsaOSS/generate-release-notes@v1
+  with:
+    tag-name: v2.6.4
+    from-tag-name: v2.6.3
+```
+
+**Log output when fallback is triggered:**
+
+```
+2026-06-25 12:22:27 - INFO - Compare mode: using repo.compare('v2.6.3', 'v2.6.4').
+2026-06-25 12:22:27 - WARNING - Compare API failed for 'v2.6.3'...'v2.6.4' (target tag may not exist yet). Falling back to the latest commit SHA of 'v2.6.4'.
+```
+
+---
+
 ## Data Flow
 
 ```
@@ -109,8 +146,14 @@ from-tag-name provided?
   GitHub Compare API:          get_commits(since=data.since)
   commits unique to to-tag     get_pulls(state=closed)
      │                              │
-  extract PR numbers           FilterByRelease drops
-  from commit messages         PRs/commits before since
+    API fails (404)?           FilterByRelease drops
+     │                         PRs/commits before since
+     ├─ YES: Fallback to
+     │       target ref SHA
+     │       (log warning)
+     │
+  extract PR numbers
+  from commit messages
      │
   fetch each PR by number
      │

release_notes_generator/data/miner.py

@@ -99,6 +99,7 @@ def _handle_compare_mode(self, repo: Repository, data: MinedData) -> None:
 
         Logic:
           - Fetch commits between from_tag and to_tag using repo.compare().
+          - If compare fails (404), fall back to the latest commit SHA of to_tag.
           - Extract PR numbers from commit messages and fetch those PRs.
           - Filter out commits that already have a PR reference to avoid duplication.
         """
@@ -109,12 +110,26 @@ def _handle_compare_mode(self, repo: Repository, data: MinedData) -> None:
         )
         comparison = self._safe_call(repo.compare)(ActionInputs.get_from_tag_name(), ActionInputs.get_tag_name())
         if comparison is None:
-            logger.error(
-                "Compare API returned no result for '%s'...'%s'. Ending!",
+            logger.warning(
+                "Compare API failed for '%s'...'%s' (target tag may not exist yet). "
+                "Falling back to the latest commit SHA of '%s'.",
                 ActionInputs.get_from_tag_name(),
                 ActionInputs.get_tag_name(),
+                ActionInputs.get_tag_name(),
             )
-            sys.exit(1)
+            # Fall back: fetch the latest commit of the target tag/ref
+            target_commit = self._safe_call(repo.get_commit)(ActionInputs.get_tag_name())
+            if target_commit is None:
+                logger.error(
+                    "Could not retrieve commit for '%s'. Ending!",
+                    ActionInputs.get_tag_name(),
+                )
+                sys.exit(1)
+            # Create a minimal comparison object with just the target commit
+            class MinimalComparison:
+                def __init__(self, commit):
+                    self.commits = [commit]
+            comparison = MinimalComparison(target_commit)
         compare_commits: list[GithubCommit] = list(comparison.commits)
         total_commits = getattr(comparison, "total_commits", None)
         if isinstance(total_commits, int) and total_commits > len(compare_commits):

tests/unit/release_notes_generator/data/test_miner.py

@@ -746,6 +746,82 @@ def test_mine_data_compare_mode_warns_on_retrieval_cap_without_total(mocker, moc
     )
 
 
+def test_mine_data_compare_mode_fallback_to_target_sha_on_404(mocker, mock_repo):
+    """Test that when compare API returns None (404), fallback to target tag's latest commit SHA."""
+    target_commit = mocker.Mock()
+    target_commit.sha = "targetsha123"
+    target_commit.commit.message = "Latest commit on target tag (#99)"
+
+    mocker.patch("release_notes_generator.action_inputs.ActionInputs.is_from_tag_name_defined", return_value=True)
+    mocker.patch("release_notes_generator.action_inputs.ActionInputs.get_from_tag_name", return_value="v2.6.3")
+    mocker.patch("release_notes_generator.action_inputs.ActionInputs.get_tag_name", return_value="v2.6.4")
+    mocker.patch("release_notes_generator.action_inputs.ActionInputs.get_github_repository", return_value="org/repo")
+    mocker.patch("release_notes_generator.action_inputs.ActionInputs.get_published_at", return_value=False)
+
+    release_mock = mocker.Mock(spec=GitRelease)
+    release_mock.created_at = datetime(2026, 5, 7)
+    release_mock.published_at = None
+    release_mock.tag_name = "v2.6.3"
+    mock_repo.get_release.return_value = release_mock
+
+    # Compare API returns None (404)
+    mock_repo.compare.return_value = None
+    # Fall back to fetching the target commit
+    mock_repo.get_commit.return_value = target_commit
+    mock_repo.get_issues.return_value = []
+    mock_repo.get_pull.return_value = mocker.Mock(spec=PullRequest)
+
+    warning_mock = mocker.patch("release_notes_generator.data.miner.logger.warning")
+
+    github_mock = mocker.Mock(spec=Github)
+    github_mock.get_repo.return_value = mock_repo
+
+    decorator_mock = lambda f: f
+    miner = DataMiner(github_mock, mocker.Mock())
+    miner._safe_call = decorator_mock
+    data = miner.mine_data()
+
+    # Verify warning was logged
+    warning_mock.assert_called_once()
+    call_args = warning_mock.call_args[0]
+    assert "Compare API failed" in call_args[0]
+    assert "Falling back to the latest commit SHA" in call_args[0]
+
+    # Verify the target commit was used
+    assert "targetsha123" in data.compare_commit_shas
+    assert len(data.commits) <= 1  # May be 0 if PR number found and filtered
+
+
+def test_mine_data_compare_mode_exits_when_fallback_fails(mocker, mock_repo):
+    """Test that action exits when both compare API and fallback commit fetch fail."""
+    mocker.patch("release_notes_generator.action_inputs.ActionInputs.is_from_tag_name_defined", return_value=True)
+    mocker.patch("release_notes_generator.action_inputs.ActionInputs.get_from_tag_name", return_value="v2.6.3")
+    mocker.patch("release_notes_generator.action_inputs.ActionInputs.get_tag_name", return_value="v2.6.4")
+    mocker.patch("release_notes_generator.action_inputs.ActionInputs.get_github_repository", return_value="org/repo")
+    mocker.patch("release_notes_generator.action_inputs.ActionInputs.get_published_at", return_value=False)
+
+    release_mock = mocker.Mock(spec=GitRelease)
+    release_mock.created_at = datetime(2026, 5, 7)
+    release_mock.published_at = None
+    release_mock.tag_name = "v2.6.3"
+    mock_repo.get_release.return_value = release_mock
+
+    # Both compare and get_commit fail
+    mock_repo.compare.return_value = None
+    mock_repo.get_commit.return_value = None
+    mock_repo.get_issues.return_value = []
+
+    github_mock = mocker.Mock(spec=Github)
+    github_mock.get_repo.return_value = mock_repo
+
+    decorator_mock = lambda f: f
+    miner = DataMiner(github_mock, mocker.Mock())
+    miner._safe_call = decorator_mock
+
+    with pytest.raises(SystemExit):
+        miner.mine_data()
+
+
 # --- mine_data timestamp mode (regression) ---