Gracefully handle "diff too large" errors

coverage_comment/github.py

@@ -28,6 +28,12 @@ class NoArtifact(Exception):
     pass
 
 
+class CannotGetDiff(Exception):
+    """Raised when the diff cannot be fetched from GitHub."""
+
+    pass
+
+
 @dataclasses.dataclass
 class RepositoryInfo:
     default_branch: str
@@ -289,11 +295,19 @@ def get_pr_diff(github: github_client.GitHub, repository: str, pr_number: int) -
     """
     Get the diff of a pull request.
     """
-    return (
-        github.repos(repository)
-        .pulls(pr_number)
-        .get(headers={"Accept": "application/vnd.github.v3.diff"}, text=True)
-    )
+    try:
+        return (
+            github.repos(repository)
+            .pulls(pr_number)
+            .get(headers={"Accept": "application/vnd.github.v3.diff"}, text=True)
+        )
+    except github_client.ApiError as exc:
+        if _is_too_large_error(exc):
+            raise CannotGetDiff(
+                "The diff for this PR is too large to be retrieved from GitHub's API "
+                "(maximum 300 files). Diff coverage is not available for this PR."
+            ) from exc
+        raise
 
 
 def get_branch_diff(
@@ -302,8 +316,31 @@ def get_branch_diff(
     """
     Get the diff of branch.
     """
-    return (
-        github.repos(repository)
-        .compare(f"{base_branch}...{head_branch}")
-        .get(headers={"Accept": "application/vnd.github.v3.diff"}, text=True)
-    )
+    try:
+        return (
+            github.repos(repository)
+            .compare(f"{base_branch}...{head_branch}")
+            .get(headers={"Accept": "application/vnd.github.v3.diff"}, text=True)
+        )
+    except github_client.ApiError as exc:
+        if _is_too_large_error(exc):
+            raise CannotGetDiff(
+                "The diff for this branch is too large to be retrieved from GitHub's API "
+                "(maximum 300 files). Diff coverage is not available for this branch."
+            ) from exc
+        raise
+
+
+def _is_too_large_error(exc: github_client.ApiError) -> bool:
+    """
+    Check if the error is a "too_large" error from GitHub API.
+
+    GitHub returns this error when the diff exceeds the maximum number of files (300).
+    The error response body is JSON from GitHub's API.
+    """
+    try:
+        error_data: dict[str, Any] = json.loads(str(exc))
+        errors: list[dict[str, Any]] = error_data.get("errors", [])
+        return any(error.get("code") == "too_large" for error in errors)
+    except (json.JSONDecodeError, TypeError):
+        return False

coverage_comment/main.py

@@ -133,21 +133,27 @@ def process_pr(
     )
     base_ref = config.GITHUB_BASE_REF or repo_info.default_branch
 
-    if config.GITHUB_BRANCH_NAME:
-        diff = github.get_branch_diff(
-            github=gh,
-            repository=config.GITHUB_REPOSITORY,
-            base_branch=base_ref,
-            head_branch=config.GITHUB_BRANCH_NAME,
-        )
-    elif config.GITHUB_PR_NUMBER:
-        diff = github.get_pr_diff(
-            github=gh,
-            repository=config.GITHUB_REPOSITORY,
-            pr_number=config.GITHUB_PR_NUMBER,
-        )
-    else:  # pragma: no cover
-        raise Exception("Unreachable code")
+    failure_msg: str | None = None
+    try:
+        if config.GITHUB_BRANCH_NAME:
+            diff = github.get_branch_diff(
+                github=gh,
+                repository=config.GITHUB_REPOSITORY,
+                base_branch=base_ref,
+                head_branch=config.GITHUB_BRANCH_NAME,
+            )
+        elif config.GITHUB_PR_NUMBER:
+            diff = github.get_pr_diff(
+                github=gh,
+                repository=config.GITHUB_REPOSITORY,
+                pr_number=config.GITHUB_PR_NUMBER,
+            )
+        else:  # pragma: no cover
+            raise Exception("Unreachable code")
+    except github.CannotGetDiff as exc:
+        failure_msg = str(exc)
+        log.warning(failure_msg, exc_info=True)
+        diff = ""
 
     added_lines = coverage_module.get_added_lines(diff=diff)
     diff_coverage = coverage_module.get_diff_coverage_info(
@@ -219,6 +225,7 @@ def process_pr(
             pr_targets_default_branch=pr_targets_default_branch,
             marker=marker,
             subproject_id=config.SUBPROJECT_ID,
+            failure_msg=failure_msg,
         )
         # Same as above except `max_files` is None
         summary_comment = template.get_comment_markdown(
@@ -240,6 +247,7 @@ def process_pr(
             pr_targets_default_branch=pr_targets_default_branch,
             marker=marker,
             subproject_id=config.SUBPROJECT_ID,
+            failure_msg=failure_msg,
         )
     except template.MissingMarker:
         log.error(

coverage_comment/template.py

@@ -141,6 +141,7 @@ def get_comment_markdown(
     subproject_id: str | None = None,
     custom_template: str | None = None,
     pr_targets_default_branch: bool = True,
+    failure_msg: str | None = None,
 ):
     loader = CommentLoader(base_template=base_template, custom_template=custom_template)
     env = SandboxedEnvironment(loader=loader)
@@ -188,6 +189,7 @@ def get_comment_markdown(
             subproject_id=subproject_id,
             marker=marker,
             pr_targets_default_branch=pr_targets_default_branch,
+            failure_msg=failure_msg,
         )
     except jinja2.exceptions.TemplateError as exc:
         raise TemplateError from exc

coverage_comment/template_files/comment.md.j2

@@ -19,12 +19,25 @@
 {#- Coverage diff badge -#}
 {#- space #} {# space -#}
 {%- block diff_coverage_badge -%}
+{%- if failure_msg -%}
+<img title="Diff coverage unavailable" src="{{ 'PR Coverage' | generate_badge(message='N/A', color='grey') }}">
+
+{%- else -%}
 {%- set text = (diff_coverage.total_percent_covered | pct) ~ " of the statement lines added by this PR are covered" -%}
 <img title="{{ text }}" src="{{ 'PR Coverage' | generate_badge(message=diff_coverage.total_percent_covered | pct(precision=0), color=diff_coverage.total_percent_covered | x100 | get_badge_color) }}">
 
+{%- endif -%}
 {%- endblock diff_coverage_badge -%}
 {%- endblock coverage_badges -%}
 
+{%- block diff_coverage_failure_message -%}
+{%- if failure_msg %}
+
+> [!WARNING]
+> {{ failure_msg }}
+
+{% endif -%}
+{%- endblock diff_coverage_failure_message -%}
 
 {%- macro statements_badge(path, statements_count, previous_statements_count) -%}
 {% if previous_statements_count is not none -%}

tests/integration/test_github.py

@@ -4,7 +4,7 @@
 
 import pytest
 
-from coverage_comment import github
+from coverage_comment import github, github_client
 
 
 @pytest.mark.parametrize(
@@ -491,3 +491,99 @@ def test_get_branch_diff(gh, session):
     )
 
     assert result == "diff --git a/foo.py b/foo.py..."
+
+
+def test_get_pr_diff__too_large(gh, session):
+    error_response = {
+        "message": "Sorry, the diff exceeded the maximum number of files (300).",
+        "errors": [{"resource": "PullRequest", "field": "diff", "code": "too_large"}],
+        "documentation_url": "https://docs.github.com/rest/pulls/pulls#list-pull-requests-files",
+        "status": "406",
+    }
+    session.register(
+        "GET",
+        "/repos/foo/bar/pulls/123",
+        headers={"Accept": "application/vnd.github.v3.diff"},
+    )(json=error_response, status_code=406)
+
+    with pytest.raises(github.CannotGetDiff) as exc_info:
+        github.get_pr_diff(github=gh, repository="foo/bar", pr_number=123)
+
+    assert "too large" in str(exc_info.value)
+    assert "maximum 300 files" in str(exc_info.value)
+
+
+def test_get_branch_diff__too_large(gh, session):
+    error_response = {
+        "message": "Sorry, the diff exceeded the maximum number of files (300).",
+        "errors": [{"resource": "PullRequest", "field": "diff", "code": "too_large"}],
+        "documentation_url": "https://docs.github.com/rest/pulls/pulls#list-pull-requests-files",
+        "status": "406",
+    }
+    session.register(
+        "GET",
+        "/repos/foo/bar/compare/main...feature",
+        headers={"Accept": "application/vnd.github.v3.diff"},
+    )(json=error_response, status_code=406)
+
+    with pytest.raises(github.CannotGetDiff) as exc_info:
+        github.get_branch_diff(
+            github=gh, repository="foo/bar", base_branch="main", head_branch="feature"
+        )
+
+    assert "too large" in str(exc_info.value)
+    assert "maximum 300 files" in str(exc_info.value)
+
+
+def test_get_pr_diff__other_error(gh, session):
+    error_response = {"message": "Some other error", "errors": []}
+    session.register(
+        "GET",
+        "/repos/foo/bar/pulls/123",
+        headers={"Accept": "application/vnd.github.v3.diff"},
+    )(json=error_response, status_code=500)
+
+    with pytest.raises(github_client.ApiError):
+        github.get_pr_diff(github=gh, repository="foo/bar", pr_number=123)
+
+
+def test_get_branch_diff__other_error(gh, session):
+    error_response = {"message": "Some other error", "errors": []}
+    session.register(
+        "GET",
+        "/repos/foo/bar/compare/main...feature",
+        headers={"Accept": "application/vnd.github.v3.diff"},
+    )(json=error_response, status_code=500)
+
+    with pytest.raises(github_client.ApiError):
+        github.get_branch_diff(
+            github=gh, repository="foo/bar", base_branch="main", head_branch="feature"
+        )
+
+
+@pytest.mark.parametrize(
+    "error_str,expected",
+    [
+        # Valid JSON with too_large error
+        ('{"errors": [{"code": "too_large"}]}', True),
+        # Valid JSON with too_large error and extra fields
+        (
+            '{"message": "Diff too large", "errors": [{"resource": "PR", "code": "too_large"}]}',
+            True,
+        ),
+        # Valid JSON without too_large error
+        ('{"errors": [{"code": "other"}]}', False),
+        # Valid JSON with empty errors
+        ('{"errors": []}', False),
+        # Valid JSON with no errors key
+        ('{"message": "error"}', False),
+        # Non-JSON string (returns False, not a fallback match)
+        ("not valid json", False),
+        # Empty string
+        ("", False),
+    ],
+)
+def test__is_too_large_error(error_str, expected):
+    """Test the _is_too_large_error helper function with various inputs."""
+    exc = github_client.ApiError(error_str)
+    assert github._is_too_large_error(exc) is expected

tests/integration/test_main.py

@@ -927,3 +927,76 @@ def test_action__workflow_run__post_comment(
     assert get_logs("INFO", "Comment file found in artifact, posting to PR")
     assert get_logs("INFO", "Comment posted in PR")
     assert summary_file.read_text() == ""
+
+
+def test_action__pull_request__diff_too_large(
+    pull_request_config,
+    session,
+    in_integration_env,
+    output_file,
+    summary_file,
+    git,
+    get_logs,
+):
+    """Test that when the diff is too large, a warning is shown in the comment."""
+    session.register("GET", "/repos/py-cov-action/foobar")(
+        json={"default_branch": "main", "visibility": "public"}
+    )
+    # No existing badge in this test
+    session.register("GET", "/repos/py-cov-action/foobar/contents/data.json")(
+        status_code=404
+    )
+
+    # Who am I
+    session.register("GET", "/user")(json={"login": "foo"})
+    # Are there already comments
+    session.register("GET", "/repos/py-cov-action/foobar/issues/2/comments")(json=[])
+
+    comment = None
+
+    def checker(payload):
+        body = payload["body"]
+        assert "## Coverage report" in body
+        nonlocal comment
+        comment = body
+        return True
+
+    # Post a new comment
+    session.register(
+        "POST", "/repos/py-cov-action/foobar/issues/2/comments", json=checker
+    )(status_code=200)
+
+    # The diff is too large - returns 406 with error
+    error_response = {
+        "message": "Sorry, the diff exceeded the maximum number of files (300).",
+        "errors": [{"resource": "PullRequest", "field": "diff", "code": "too_large"}],
+        "documentation_url": "https://docs.github.com/rest/pulls/pulls#list-pull-requests-files",
+        "status": "406",
+    }
+    session.register("GET", "/repos/py-cov-action/foobar/pulls/2")(
+        json=error_response, status_code=406
+    )
+
+    result = main.action(
+        config=pull_request_config(
+            GITHUB_OUTPUT=output_file, GITHUB_STEP_SUMMARY=summary_file
+        ),
+        github_session=session,
+        http_session=session,
+        git=git,
+    )
+    assert result == 0
+
+    # Check that a warning was logged
+    assert get_logs("WARNING", "too large")
+
+    # Comment was posted successfully, no fallback file should exist
+    assert not pathlib.Path("python-coverage-comment-action.txt").exists()
+
+    # Check that the error message is in the comment
+    assert "too large" in comment
+    assert "maximum 300 files" in comment
+    # Check that the warning block is in the comment
+    assert "[!WARNING]" in comment
+    # Check the N/A badge is shown for PR coverage (URL-encoded in badge URL)
+    assert "PR%20Coverage-N/A-grey" in comment