feat: ensure hash consistency between lockfile and venv (#207)

This change fixes an issue observable in this test run in which riot generated a requirements lockfile for a Venv instance other than the one it was running tests for. This behavior was happening due to riot logic that skipped Venv instances with pkgs == None while preparing the environment, but not while running tests. Thus the fix is to stop riot from ignoring pkgs-less Venvs in all cases.

Note these two lines of output from the example test run:

```
Compiling requirements file .riot/requirements/118238b.in
RIOT_VENV_HASH=32bd6c2
```

This illustrates the mismatch. RIOT_VENV_HASH is the hash of the environment in which the command will be run, and for which the lockfile should be generated. The prepare() function had been ignoring that environment and instead preparing an environment for one of its ancestors.

releasenotes/notes/emptypkgs-21c00f01a644170d.yaml

@@ -0,0 +1,6 @@
+---
+fixes:
+  - |
+    Fixes an issue in which riot generated a requirements lockfile for a Venv instance other than the one it was running tests for.
+    This behavior was happening due to logic that skipped Venv instances with pkgs == None while preparing the environment,
+    but not while running tests. The fix is to stop riot from ignoring pkgs-less Venvs in all cases.

riot/riot.py

@@ -383,7 +383,7 @@ def prefix(self) -> t.Optional[str]:
         This will return a python version + package specific path name.
         If no packages are defined it will return ``None``.
         """
-        if self.py is None or not self.pkgs:
+        if self.py is None:
             return None
 
         venv_path = self.py.venv_path
@@ -416,8 +416,6 @@ def venv_path(self) -> t.Optional[str]:
     @property
     def ident(self) -> t.Optional[str]:
         """Return prefix identifier string based on packages."""
-        if not self.pkgs:
-            return None
         return "_".join(
             (
                 f"{rmchars('<=>.,:+@/', n)}"
@@ -587,7 +585,6 @@ def prepare(
         installed = False
         if (
             py is not None
-            and self.pkgs
             and self.prefix is not None
             # We only install dependencies if the prefix directory does not
             # exist already. If it does exist, we assume it is in a good state.

tests/test_integration.py

@@ -935,6 +935,39 @@ def test_requirements_lockfile_created(
         assert "#" in lines[0]
 
 
+def test_consistent_hashes_between_prepare_and_run_when_child_has_no_pkgs(
+    tmp_path: pathlib.Path, tmp_run: _T_TmpRun
+) -> None:
+    version = "{}.{}".format(sys.version_info[0], sys.version_info[1])
+    rf_path = tmp_path / "riotfile.py"
+    rf_path.write_text(
+        """
+from riot import Venv
+venv = Venv(
+    pkgs={{"requests": ""}},
+    name="parent",
+    command="env",
+    venvs=[Venv(pys=["{}"], name="child")]
+)
+""".format(
+            version
+        ),
+    )
+    _hash = tmp_run("riot list --hash-only child").stdout.strip("\n")
+    result = tmp_run("riot run -p {} {}".format(version, _hash))
+    assert result.returncode == 0, result.stderr
+
+    lockfile_path = str(
+        tmp_path
+        / os.path.join(
+            ".riot",
+            "requirements",
+            "{}.txt".format(_hash),
+        )
+    )
+    assert os.path.exists(lockfile_path)
+
+
 def test_recompile_flag(tmp_path: pathlib.Path, tmp_run: _T_TmpRun) -> None:
     rf_path = tmp_path / "riotfile.py"
     rf_path.write_text(