sources: tweak ContainersStorageSources.exists to return False

When an images does not exist just return `False` instead of
raising a RuntimeError. If anything else goes wrong (unknown
output or hash mismatch) keep the RuntimeError as this is an
unexpected exception.

sources/org.osbuild.containers-storage

@@ -81,10 +81,16 @@ class ContainersStorageSource(sources.SourceService):
         # fail early if the user hasn't imported the container into
         # containers-storage
         if res.returncode != 0:
-            raise RuntimeError(f"Container does not exist in local containers storage: {res.stderr}")
+            # string not matching not ideal - this is ErrNotAnImage
+            # which is unchanged since 2016 (added in ee99172905 in
+            # containers/storage)
+            if "identifier is not an image" in res.stderr:
+                return False
+            raise RuntimeError(f"unknown skopeo error: {res.stderr}")
 
         # NOTE: this is a bit redundant because we're checking the content digest of the thing we retrieved via its
         # id (which is the content digest), but let's keep it in case we change to retrieving local containers by name
+        # See also https://github.com/containers/skopeo/pull/2236
         local_id = "sha256:" + hashlib.sha256(res.stdout.encode()).hexdigest()
         if local_id != checksum:
             raise RuntimeError(

sources/test/test_container_storage_source.py

@@ -34,6 +34,17 @@ def test_containers_storage_integration_missing(sources_module):
     checksum = "sha256:1234567890123456789012345678901234567890909b14ffb032aa20fa23d9ad6"
     sock = socket.socket(socket.AF_UNIX, socket.SOCK_STREAM)
     cnt_storage = sources_module.ContainersStorageSource.from_args(["--service-fd", str(sock.fileno())])
-    # this is not ideal, consider to just return "False" here
-    with pytest.raises(RuntimeError):
+    assert not cnt_storage.exists(checksum, None)
+
+
+@pytest.mark.skipif(not has_executable("podman"), reason="no podman executable")
+@pytest.mark.skipif(os.getuid() != 0, reason="root only")
+def test_containers_storage_integration_invalid(sources_module):
+    # put an invalid reference into the source to ensure skopeo errors with
+    # a different error than image not found
+    checksum = "sha256:["
+    sock = socket.socket(socket.AF_UNIX, socket.SOCK_STREAM)
+    cnt_storage = sources_module.ContainersStorageSource.from_args(["--service-fd", str(sock.fileno())])
+    with pytest.raises(RuntimeError) as exc:
         cnt_storage.exists(checksum, None)
+    assert "unknown skopeo error:" in str(exc)