_artifactcache.py: Don't push artifact blobs when no files are present

Previously, if an artifact proto had no files at all in it, we would
fail at pushing it, making BuildStream crash.

When no files are part of an artifact proto, we can short-circuit
the call and avoid pushing them unecessarily.

- Add a test to ensure this doesn't come back.

3 files changed, 47 insertions, 2 deletions

diff --git a/src/buildstream/_artifactcache.py b/src/buildstream/_artifactcache.py
index f8d856be7..b4d4efe00 100644
--- a/src/buildstream/_artifactcache.py
+++ b/src/buildstream/_artifactcache.py
@@ -560,7 +560,8 @@ class ArtifactCache(BaseCache):
         artifact_proto = artifact._get_proto()
 
         try:
-            self.cas._send_directory(remote, artifact_proto.files)
+            if str(artifact_proto.files):
+                self.cas._send_directory(remote, artifact_proto.files)
 
             if str(artifact_proto.buildtree):
                 try:
diff --git a/src/buildstream/_cas/casserver.py b/src/buildstream/_cas/casserver.py
index 716178bb4..d4241435a 100644
--- a/src/buildstream/_cas/casserver.py
+++ b/src/buildstream/_cas/casserver.py
@@ -502,7 +502,8 @@ class _ArtifactServicer(artifact_pb2_grpc.ArtifactServiceServicer):
 
         if self.update_cas:
             # Check that the files specified are in the CAS
-            self._check_directory("files", artifact.files, context)
+            if str(artifact.files):
+                self._check_directory("files", artifact.files, context)
 
             # Unset protocol buffers don't evaluated to False but do return empty
             # strings, hence str()
diff --git a/tests/integration/cachedfail.py b/tests/integration/cachedfail.py
index 08733ab71..e3c05e691 100644
--- a/tests/integration/cachedfail.py
+++ b/tests/integration/cachedfail.py
@@ -183,6 +183,49 @@ def test_push_cached_fail(cli, tmpdir, datafiles, on_error):
         assert share.get_artifact(cli.get_artifact_name(project, 'test', 'element.bst'))
 
 
+@pytest.mark.skipif(not HAVE_SANDBOX, reason='Only available with a functioning sandbox')
+@pytest.mark.datafiles(DATA_DIR)
+@pytest.mark.parametrize("on_error", ("continue", "quit"))
+def test_push_failed_missing_shell(cli, tmpdir, datafiles, on_error):
+    """Test that we can upload a built artifact that didn't have a valid shell inside.
+
+    When we don't have a valid shell, the artifact will be empty, not even the root directory.
+    This ensures we handle the case of an entirely empty artifact correctly.
+    """
+    if on_error == 'quit':
+        pytest.xfail('https://gitlab.com/BuildStream/buildstream/issues/534')
+
+    project = str(datafiles)
+    element_path = os.path.join(project, 'elements', 'element.bst')
+
+    # Write out our test target
+    element = {
+        'kind': 'script',
+        'config': {
+            'commands': [
+                'false',
+                # Ensure unique cache key for different test variants
+                'TEST="{}"'.format(os.environ.get('PYTEST_CURRENT_TEST')),
+            ],
+        },
+    }
+    _yaml.roundtrip_dump(element, element_path)
+
+    with create_artifact_share(os.path.join(str(tmpdir), 'remote')) as share:
+        cli.configure({
+            'artifacts': {'url': share.repo, 'push': True},
+        })
+
+        # Build the element, continuing to finish active jobs on error.
+        result = cli.run(project=project, args=['--on-error={}'.format(on_error), 'build', 'element.bst'])
+        result.assert_main_error(ErrorDomain.STREAM, None)
+
+        # This element should have failed
+        assert cli.get_element_state(project, 'element.bst') == 'failed'
+        # This element should have been pushed to the remote
+        assert share.get_artifact(cli.get_artifact_name(project, 'test', 'element.bst'))
+
+
 @pytest.mark.skipif(HAVE_SANDBOX != 'bwrap', reason='Only available with bubblewrap on Linux')
 @pytest.mark.datafiles(DATA_DIR)
 def test_host_tools_errors_are_not_cached(cli, datafiles, tmp_path):