Fall back to the rosdistro cache for snapshots without dependency metadata

vinca/distro.py

@@ -148,6 +148,7 @@ def __init__(
         self._last_archive = None
         self._depends_cache = {}
         self._direct_depends_cache = {}
+        self._warned_snapshot_without_dependencies = False
 
         os.environ["ROS_VERSION"] = "1" if self.check_ros1() else "2"
 
@@ -205,14 +206,21 @@ def _get_direct_depends(self, pkg: str) -> set[str]:
         additional_packages_snapshot = self.additional_packages_snapshot or {}
         is_additional_package = pkg in additional_packages_snapshot
         if snapshot_info is not None and not is_additional_package:
-            if "dependencies" not in snapshot_info:
-                raise RuntimeError(
-                    f"Snapshot metadata for '{pkg}' has no dependencies; "
-                    "regenerate the rosdistro snapshot"
+            if "dependencies" in snapshot_info:
+                direct = set(snapshot_info["dependencies"] or [])
+                self._direct_depends_cache[pkg] = set(direct)
+                return direct
+            # Snapshots written before dependencies were recorded: resolve them
+            # from the rosdistro cache, as vinca-snapshot itself does, so such a
+            # snapshot keeps working until it is regenerated.
+            if not getattr(self, "_warned_snapshot_without_dependencies", False):
+                print(
+                    "Warning: the rosdistro snapshot has no dependency metadata "
+                    f"(e.g. for '{pkg}'); resolving dependencies from the rosdistro "
+                    "cache instead. Regenerate the snapshot with vinca-snapshot to "
+                    "pin them."
                 )
-            direct = set(snapshot_info["dependencies"] or [])
-            self._direct_depends_cache[pkg] = set(direct)
-            return direct
+                self._warned_snapshot_without_dependencies = True
 
         # if pkg comes from additional_packages_snapshot, extract from its package.xml
         if is_additional_package:

vinca/test_snapshot_metadata.py

@@ -1,8 +1,6 @@
 from typing import Any
 from unittest.mock import Mock, patch
 
-import pytest
-
 import vinca.main as main
 import vinca.recipes as recipes
 from vinca.distro import Distro
@@ -154,12 +152,20 @@ def test_snapshot_package_xml_does_not_use_live_cache_after_snapshot_change(
     distro._distro.get_release_package_xml.assert_not_called()
 
 
-def test_snapshot_without_dependencies_requires_regeneration(monkeypatch):
+def test_snapshot_without_dependencies_falls_back_to_rosdistro_cache(
+    monkeypatch, capsys
+):
     distro = make_snapshot_distro(monkeypatch)
     del distro.snapshot["snapshot_package"]["dependencies"]
+    distro._walker = Mock()
+    distro._walker.get_depends.side_effect = (
+        lambda pkg, dependency_type, ros_packages_only: (
+            {"cache_dependency"} if dependency_type == "build" else set()
+        )
+    )
 
-    with pytest.raises(RuntimeError, match="regenerate the rosdistro snapshot"):
-        distro.get_depends("snapshot_package")
+    assert distro.get_direct_depends("snapshot_package") == {"cache_dependency"}
+    assert "Regenerate the snapshot" in capsys.readouterr().out
 
 
 def test_snapshot_metadata_generates_dependency_required_by_pinned_source(