Require a path separator when detecting the active virtualenv

[tarann26]

Pull Request Check List

Resolves: #10357

• Added tests for changed code.
• Updated documentation for changed code.

Problem

When Poetry is itself hosted in a virtualenv whose name is a textual prefix-superset of the project venv — for example Poetry runs from .venv-poetry while the project venv is .venv — create_venv() mistakes the two for the same environment and installs the project's dependencies into the base/system Python instead of the project venv.

After building the target venv, create_venv() walks sys.executable's symlink chain to decide whether Poetry is already running inside it:

p_venv = os.path.normcase(str(venv))
if any(p.startswith(p_venv) for p in paths):
    # Running properly in the virtualenv, don't need to do anything
    return self.get_system_env()

str.startswith has no path boundary, so ".../.venv-poetry/bin/python".startswith(".../.venv") is True. Poetry wrongly concludes it is inside .venv, returns get_system_env(), and the install lands in the base Python.

Fix

Require a path-separator boundary:

if any(p == p_venv or p.startswith(p_venv + os.sep) for p in paths):

A real in-venv interpreter (<venv>/bin/python) still matches via p_venv + os.sep, while a sibling such as .venv-poetry no longer does. Both p and p_venv are already passed through os.path.normcase, so os.sep is the correct native boundary on POSIX and Windows.

A regression test covering the sibling-venv scenario is included.

[sourcery-ai]

Hey - I've left some high level feedback:

• In the new regression test, consider using os.environ.pop("VIRTUAL_ENV", None) instead of del os.environ["VIRTUAL_ENV"] to avoid a potential KeyError when the variable is absent.
• The test hardcodes a bin/python3.9 path which is POSIX-specific; if the test suite is expected to run on Windows, you may want to construct the sibling executable path in a platform-aware way.

Prompt for AI Agents

Please address the comments from this code review:

## Overall Comments
- In the new regression test, consider using `os.environ.pop("VIRTUAL_ENV", None)` instead of `del os.environ["VIRTUAL_ENV"]` to avoid a potential `KeyError` when the variable is absent.
- The test hardcodes a `bin/python3.9` path which is POSIX-specific; if the test suite is expected to run on Windows, you may want to construct the sibling executable path in a platform-aware way.

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