stages/org.osbuild.tar: implement disk-full test

this should be an example environment
for more stages to test if they return a proper error
in a "disk full scenario"

stages/test/conftest.py

@@ -1,5 +1,6 @@
 import os
 import pathlib
+import subprocess
 from types import ModuleType
 
 import pytest
@@ -43,3 +44,17 @@ def stage_schema(request: pytest.FixtureRequest) -> osbuild.meta.Schema:
     root = caller_dir.parent.parent
     mod_info = osbuild.meta.ModuleInfo.load(root, "Stage", stage_name)
     return osbuild.meta.Schema(mod_info.get_schema(version=schema_version), stage_name)
+
+
+@pytest.fixture
+def tmp_path_disk_full(tmp_path, tmp_path_disk_full_size):
+    small_dir = tmp_path / "small-dir"
+    small_dir.mkdir()
+    subprocess.run(["mount",
+                    "-t", "tmpfs",
+                    "-o", f"size={tmp_path_disk_full_size}",
+                    "tmpfs",
+                    small_dir
+                    ], check=True)
+    yield small_dir
+    subprocess.run(["umount", small_dir], check=True)

stages/test/test_tar.py

@@ -1,6 +1,7 @@
 #!/usr/bin/python3
 
 import os.path
+import re
 import subprocess
 
 import pytest
@@ -145,3 +146,17 @@ def test_tar_transform_no_sh(tmp_path, stage_module, fake_inputs):
     with pytest.raises(subprocess.CalledProcessError) as ex:
         stage_module.main(fake_inputs, tmp_path, options)
     assert "exit status 2" in str(ex.value)
+
+
+@pytest.mark.parametrize('tmp_path_disk_full_size', [5 * 1024])
+@pytest.mark.skipif(os.getuid() != 0, reason="needs root")
+def test_tar_disk_full(stage_module, fake_inputs, tmp_path_disk_full, capfd):
+    options = {
+        "filename": "out.tar",
+    }
+
+    with pytest.raises(subprocess.CalledProcessError) as ex:
+        stage_module.main(fake_inputs, tmp_path_disk_full, options)
+
+    assert ex.value.returncode == 2
+    assert re.match(r".*Wrote only \d+ of \d+ bytes.*", str(capfd.readouterr().err))