fix: fix Python venv in core24 classic snaps (#4946)

snapcraft/parts/plugins/python_plugin.py

@@ -17,9 +17,10 @@
 """The Snapcraft Python plugin."""
 
 import logging
+from pathlib import Path
 from typing import Optional
 
-from craft_parts import errors
+from craft_parts import StepInfo, errors
 from craft_parts.plugins import python_plugin
 from overrides import override
 
@@ -64,3 +65,35 @@ def _get_system_python_interpreter(self) -> Optional[str]:
             confinement,
         )
         return interpreter
+
+    @classmethod
+    def post_prime(cls, step_info: StepInfo) -> None:
+        """Perform Python-specific actions right before packing."""
+        base = step_info.project_base
+
+        if base in ("core20", "core22"):
+            # Only fix pyvenv.cfg on core24+ snaps
+            return
+
+        root_path: Path = step_info.prime_dir
+
+        pyvenv = root_path / "pyvenv.cfg"
+        if not pyvenv.is_file():
+            return
+
+        snap_path = Path(f"/snap/{step_info.project_name}/current")
+        new_home = f"home = {snap_path}"
+
+        candidates = (
+            step_info.part_install_dir,
+            step_info.stage_dir,
+        )
+
+        old_contents = contents = pyvenv.read_text()
+        for candidate in candidates:
+            old_home = f"home = {candidate}"
+            contents = contents.replace(old_home, new_home)
+
+        if old_contents != contents:
+            logger.debug("Updating pyvenv.cfg to:\n%s", contents)
+            pyvenv.write_text(contents)

snapcraft/services/lifecycle.py

@@ -77,6 +77,7 @@ def setup(self) -> None:
             extra_build_snaps=project.get_extra_build_snaps(),
             confinement=project.confinement,
             project_base=project.base or "",
+            project_name=project.name,
         )
         callbacks.register_prologue(parts.set_global_environment)
         callbacks.register_pre_step(parts.set_step_environment)
@@ -85,8 +86,19 @@ def setup(self) -> None:
     @overrides
     def post_prime(self, step_info: StepInfo) -> bool:
         """Run post-prime parts steps for Snapcraft."""
+        from snapcraft.parts import plugins
+
         project = cast(models.Project, self._project)
 
+        part_name = step_info.part_name
+        plugin_name = project.parts[part_name]["plugin"]
+
+        # Handle plugin-specific prime fixes
+        if plugin_name == "python":
+            plugins.PythonPlugin.post_prime(step_info)
+
+        # Handle patch-elf
+
         # do not use system libraries in classic confinement
         use_system_libs = not bool(project.confinement == "classic")
 

tests/spread/core24-suites/environment/test-variables/task.yaml

@@ -33,7 +33,7 @@ execute: |
     cat "$file"
     for exp in \
       "^SNAPCRAFT_PROJECT_GRADE=devel$" \
-      "^SNAPCRAFT_PROJECT_NAME=None$" \
+      "^SNAPCRAFT_PROJECT_NAME=variables$" \
       "^SNAPCRAFT_PROJECT_VERSION=1$" \
       "^SNAPCRAFT_PARALLEL_BUILD_COUNT=[0-9]\+$" \
       "^SNAPCRAFT_PROJECT_DIR=${root}$" \

tests/spread/core24/python-hello/classic/snap/snapcraft.yaml

@@ -16,10 +16,13 @@ parts:
   hello:
     plugin: python
     source: src
+    python-packages:
+      - black
     build-attributes:
       - enable-patchelf
     stage-packages:
       - libpython3.12-minimal
       - libpython3.12-stdlib
       - python3.12-minimal
       - python3.12-venv
+      - python3-minimal # (for the "python3" symlink)

tests/spread/core24/python-hello/src/hello/__init__.py

@@ -1,2 +1,5 @@
+import black
+
+
 def main():
-    print("hello world")
+    print(f"hello world! black version: {black.__version__}")

tests/spread/core24/python-hello/strict/snap/snapcraft.yaml

@@ -12,3 +12,5 @@ parts:
   hello:
     plugin: python
     source: src
+    python-packages:
+      - black

tests/spread/core24/python-hello/task.yaml

@@ -1,5 +1,10 @@
 summary: Build and run Python-based snaps in core24
 
+systems:
+  # Must *not* run this on 24.04, which can give false-positives due to the
+  # presence of the system Python 3.12.
+  - ubuntu-22.04*
+
 environment:
   PARAM/strict: ""
   PARAM/classic: "--classic"
@@ -17,4 +22,4 @@ execute: |
   # shellcheck disable=SC2086
   snap install python-hello-"${SPREAD_VARIANT}"_1.0_*.snap --dangerous ${PARAM}
 
-  python-hello-"${SPREAD_VARIANT}" | MATCH "hello world"
+  python-hello-"${SPREAD_VARIANT}" | MATCH "hello world! black version"

tests/unit/parts/plugins/test_python_plugin.py

@@ -17,7 +17,7 @@
 from textwrap import dedent
 
 import pytest
-from craft_parts import Part, PartInfo, ProjectInfo, errors
+from craft_parts import Part, PartInfo, ProjectInfo, Step, StepInfo, errors
 
 from snapcraft.parts.plugins import PythonPlugin
 
@@ -190,3 +190,41 @@ def test_get_system_python_interpreter_unknown_base(confinement, new_dir):
     expected_error = "Don't know which interpreter to use for base core10"
     with pytest.raises(errors.PartsError, match=expected_error):
         plugin._get_system_python_interpreter()
+
+
+@pytest.mark.parametrize("home_attr", ["part_install_dir", "stage_dir"])
+def test_fix_pyvenv(new_dir, home_attr):
+    part_info = PartInfo(
+        project_info=ProjectInfo(
+            application_name="test",
+            project_name="test-snap",
+            base="core24",
+            confinement="classic",
+            project_base="core24",
+            cache_dir=new_dir,
+        ),
+        part=Part("my-part", {"plugin": "python"}),
+    )
+
+    prime_dir = part_info.prime_dir
+    prime_dir.mkdir()
+
+    pyvenv = prime_dir / "pyvenv.cfg"
+    pyvenv.write_text(
+        dedent(
+            f"""\
+        home = {getattr(part_info, home_attr)}/usr/bin
+        include-system-site-packages = false
+        version = 3.12.3
+        executable = /root/parts/my-part/install/usr/bin/python3.12
+        command = /root/parts/my-part/install/usr/bin/python3 -m venv /root/parts/my-part/install
+        """
+        )
+    )
+
+    step_info = StepInfo(part_info, Step.PRIME)
+
+    PythonPlugin.post_prime(step_info)
+
+    new_contents = pyvenv.read_text()
+    assert "home = /snap/test-snap/current/usr/bin" in new_contents

tests/unit/services/test_lifecycle.py

@@ -43,14 +43,18 @@ def test_lifecycle_installs_base(lifecycle_service, mocker):
     )
 
 
-def test_post_prime_no_patchelf(fp, tmp_path, lifecycle_service):
+def test_post_prime_no_patchelf(fp, tmp_path, lifecycle_service, default_project):
+    new_attrs = {"parts": {"my-part": {"plugin": "nil"}}}
+    default_project.__dict__.update(**new_attrs)
+
     mock_step_info = mock.Mock()
     mock_step_info.configure_mock(
         **{
             "base": "core24",
             "build_attributes": [],
             "state.files": ["usr/bin/ls"],
             "prime_dir": tmp_path / "prime",
+            "part_name": "my-part",
         }
     )
 
@@ -82,7 +86,7 @@ def test_post_prime_patchelf(
     use_system_libs,
 ):
     patchelf_spy = mocker.spy(snapcraft.parts, "patch_elf")
-    new_attrs = {"confinement": confinement}
+    new_attrs = {"confinement": confinement, "parts": {"my-part": {"plugin": "nil"}}}
     default_project.__dict__.update(**new_attrs)
 
     mock_step_info = mock.Mock()
@@ -92,6 +96,7 @@ def test_post_prime_patchelf(
             "build_attributes": ["enable-patchelf"],
             "state.files": ["usr/bin/ls"],
             "prime_dir": tmp_path / "prime",
+            "part_name": "my-part",
         }
     )
 
@@ -207,6 +212,7 @@ def test_lifecycle_custom_arguments(
 
     assert info.project_base == expected_base
     assert info.confinement == expected_confinement
+    assert info.project_name == default_project.name == "default"
 
 
 @pytest.mark.usefixtures("default_project")