Handle HTTP errors when fetching revisions (#5234)

This PR adds exception handling to `_get_url_content` in `revisions.py`
to prevent the testcase detail page from crashing with a 500 error when
external requests to Gitiles (`chromium.googlesource.com`) fail with
HTTP errors (such as 429, 403, or 503). This was caused by Gitiles
throttling anonymous traffic due to an outage (see b/499989694 for
context). This fix allows the UI to degrade gracefully, displaying 'No
component revisions found' instead of crashing.

Author: hunsche

src/clusterfuzz/_internal/build_management/revisions.py

@@ -182,7 +182,11 @@ def _get_url_content(url):
     url_content = url_data.decode('utf-8')
   else:
     # Fetch a regular url without authentication.
-    url_content = utils.fetch_url(url)
+    try:
+      url_content = utils.fetch_url(url)
+    except Exception:
+      logs.error(f'Failed to fetch URL {url}')
+      return None
 
     # Urls on googlesource.com return file data as base64 encoded to avoid
     # cross-site scripting attacks. If the requested url contains |format=text|,

src/clusterfuzz/_internal/tests/core/build_management/revisions_test.py

@@ -177,6 +177,13 @@ def test_convert_revision_to_integer_version_string(self, _):
     with self.assertRaises(ValueError):
       revisions.convert_revision_to_integer('...')
 
+  @mock.patch('clusterfuzz._internal.base.utils.fetch_url')
+  def test_get_url_content_exception(self, mock_fetch_url):
+    """Test that _get_url_content returns None when fetch_url raises an exception."""
+    mock_fetch_url.side_effect = Exception('Network error')
+    result = revisions._get_url_content('https://example.com/api')  # pylint: disable=protected-access
+    self.assertIsNone(result)
+
   @mock.patch('clusterfuzz._internal.config.db_config.get')
   @mock.patch(
       'clusterfuzz._internal.build_management.revisions._get_url_content')