fix: fix Python venv in core24 classic snaps

Changes to virtual environment handling made our default approach - creating a
venv in the build system and then just moving it into the snap - not work for
Python 3.12.

My understanding of the issue is that the "home" key in the "pyvenv.cfg" file
points to a path in the build system, which obviously does not exist at
snap-run-time. This apparently works fine *somehow* in core22 - likely there's
some fallback logic that makes the interpreter correctly find the bundled
Python libraries, but not in core24. So to address this we update this "home"
key to point to its final destination in the snap.

Fixes #4942

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 errors, StepInfo
 from craft_parts.plugins import python_plugin
 from overrides import override
 
@@ -64,3 +65,32 @@ def _get_system_python_interpreter(self) -> Optional[str]:
             confinement,
         )
         return interpreter
+
+    @classmethod
+    def post_prime(cls, step_info: StepInfo) -> None:
+        base = step_info.project_base
+
+        if base != "core24":
+            # 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,
+        )
+
+        contents = pyvenv.read_text()
+        for candidate in candidates:
+            old_home = f"home = {candidate}"
+            contents = contents.replace(old_home, new_home)
+
+        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/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, errors, StepInfo, Step
 
 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")