stages(container-deploy): podman mount failure test/tweaks

This commit adds a test that ensures that the output of
podman mount is part of the error message. While writing
the test I also tweaked the code slightly so that we only
try to `podman umount` if we managed to successfully mount.

stages/org.osbuild.container-deploy

@@ -44,18 +44,18 @@ SCHEMA_2 = r"""
 
 @contextlib.contextmanager
 def mount_container(image_tag):
+    result = subprocess.run(
+        ["podman", "image", "mount", image_tag],
+        stdout=subprocess.PIPE,
+        stderr=subprocess.PIPE,
+        encoding="utf-8",
+        check=False,
+    )
+    if result.returncode != 0:
+        code = result.returncode
+        msg = result.stderr.strip()
+        raise RuntimeError(f"Failed to mount image ({code}): {msg}")
     try:
-        result = subprocess.run(
-            ["podman", "image", "mount", image_tag],
-            stdout=subprocess.PIPE,
-            stderr=subprocess.PIPE,
-            encoding="utf-8",
-            check=False,
-        )
-        if result.returncode != 0:
-            code = result.returncode
-            msg = result.stderr.strip()
-            raise RuntimeError(f"Failed to mount image ({code}): {msg}")
         yield result.stdout.strip()
     finally:
         subprocess.run(

stages/test/test_container_deploy.py

@@ -5,10 +5,12 @@ import os.path
 import random
 import string
 import subprocess
+import textwrap
 from unittest.mock import call, patch
 
 import pytest
 
+import osbuild.testutil
 from osbuild.testutil import has_executable, make_fake_tree
 
 STAGE_NAME = "org.osbuild.container-deploy"
@@ -127,3 +129,18 @@ def test_container_deploy_exclude(tmp_path, stage_module):
     assert (output_dir / "dir1/file3").read_bytes() == b"dir1/file3 content"
     assert not (output_dir / "dir2/file4").exists()
     assert not (output_dir / "dir2").exists()
+
+
+def test_container_deploy_error(stage_module):
+    fake_podman = textwrap.dedent("""\
+    #!/bin/sh
+    echo "some msg on stdout"
+    echo "other error on stderr" >&2
+    exit 1
+    """)
+    with osbuild.testutil.mock_command("podman", fake_podman):
+        with pytest.raises(RuntimeError) as exp:
+            with stage_module.mount_container("some-image-tag"):
+                pass
+    assert "some msg on stdout" not in str(exp.value)
+    assert "other error on stderr" in str(exp.value)