Tolerate absence of rpm at import time

CHANGELOG.md

@@ -10,6 +10,8 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 ### Changed
 
 - Internally created executors are now named for improved metrics and debuggability.
+- Library no longer has a hard dependency on Python bindings to `rpm`, if RPM-related
+  functionality is not used.
 
 ## [2.9.0] - 2021-07-05
 

src/pushsource/_impl/backend/broken_rpmlib.py

@@ -0,0 +1,17 @@
+import six
+
+# A replacement for kobo.rpmlib to give a decent error message
+# when rpm bindings are not available.
+
+# importer is expected to put a more specific cause here.
+CAUSE = RuntimeError("unknown error")
+
+MESSAGE = "kobo.rpmlib is not available (consider 'pip install rpm-py-installer')"
+
+
+def not_available(*_args, **_kwargs):
+    six.raise_from(RuntimeError(MESSAGE), CAUSE)
+
+
+get_rpm_header = not_available
+get_keys_from_header = not_available

src/pushsource/_impl/backend/staged/staged_rpm.py

@@ -1,6 +1,14 @@
 import logging
 
-import kobo.rpmlib
+try:
+    from kobo import rpmlib
+except Exception as ex:  # pragma: no cover, pylint: disable=broad-except
+    # If kobo.rpmlib is unavailable, let's not immediately crash.
+    # We will hold this exception and re-raise it only if there's an
+    # attempt to use the related functionality.
+    from .. import broken_rpmlib as rpmlib
+
+    rpmlib.CAUSE = ex
 
 from ...model import RpmPushItem
 from .staged_base import StagedBaseMixin, handles_type
@@ -19,8 +27,8 @@ def __push_item(self, leafdir, _, entry):
             LOG.warning("Unexpected non-RPM %s (ignored)", entry.path)
             return None
 
-        header = kobo.rpmlib.get_rpm_header(entry.path)
-        key_id = kobo.rpmlib.get_keys_from_header(header)
+        header = rpmlib.get_rpm_header(entry.path)
+        key_id = rpmlib.get_keys_from_header(header)
 
         return RpmPushItem(
             name=entry.name,

tests/staged/test_broken_rpmlib.py

@@ -0,0 +1,15 @@
+import pytest
+
+from pushsource._impl.backend import broken_rpmlib
+
+
+@pytest.mark.parametrize(
+    "fn", [broken_rpmlib.get_keys_from_header, broken_rpmlib.get_rpm_header]
+)
+def test_functions_raise(fn):
+    """Expected functions are present in broken_rpmlib and raise on use."""
+
+    with pytest.raises(RuntimeError) as excinfo:
+        fn("a", "b", "c")
+
+    assert "kobo.rpmlib is not available" in str(excinfo)