_yaml.pyx: Handle OSError when loading file

src/buildstream/_yaml.pyx

@@ -284,6 +284,9 @@ cpdef MappingNode load(str filename, str shortname, bint copy_tree=False, object
     except IsADirectoryError as e:
         raise LoadError("{} is a directory".format(filename),
                         LoadErrorReason.LOADING_DIRECTORY) from e
+    except OSError as e:
+         raise LoadError("Failed to load file at {}: {}".format(filename, e.strerror),
+                        LoadErrorReason.LOAD_FAILED) from e
     except LoadError as e:
         raise LoadError("{}: {}".format(displayname, e), e.reason) from e
 

src/buildstream/exceptions.py

@@ -160,3 +160,6 @@ class LoadErrorReason(Enum):
     """
     Thee source provenance attribute specified was not defined in the project config
     """
+
+    LOAD_FAILED = 30
+    """Failed to load a yaml file due to some error"""

tests/internals/loader.py

@@ -110,3 +110,25 @@ def test_invalid_directory_load(datafiles):
         loader.load(["element.bst"])
 
     assert exc.value.reason == LoadErrorReason.LOADING_DIRECTORY
+
+
+@pytest.mark.datafiles(os.path.join(DATA_DIR, "onefile"))
+def test_invalid_nested_file_path(datafiles):
+
+    basedir = str(datafiles)
+    with make_loader(basedir) as loader, pytest.raises(LoadError) as exc:
+        loader.load(["elements/onefile.bst/onefile.bst"])
+
+    assert exc.value.reason == LoadErrorReason.LOAD_FAILED
+
+
+@pytest.mark.datafiles(os.path.join(DATA_DIR, "onefile"))
+def test_fail_load_open_error(datafiles):
+
+    basedir = str(datafiles)
+    # remove read prermission
+    os.chmod(f"{basedir}/elements/onefile.bst", 0o333)
+    with make_loader(basedir) as loader, pytest.raises(LoadError) as exc:
+        loader.load(["elements/onefile.bst"])
+
+    assert exc.value.reason == LoadErrorReason.LOAD_FAILED