Try alternate filenames for system_executable (#2442)

docs/changelog/2442.bugfix.rst

@@ -0,0 +1 @@
+In POSIX virtual environments, try alternate binary names if ``sys._base_executable`` does not exist - by :user:`vfazio`.

src/virtualenv/discovery/py_info.py

@@ -138,7 +138,24 @@ def _fast_get_system_executable(self):
                 base_executable = getattr(sys, "_base_executable", None)  # some platforms may set this to help us
                 if base_executable is not None:  # use the saved system executable if present
                     if sys.executable != base_executable:  # we know we're in a virtual environment, cannot be us
-                        return base_executable
+                        if os.path.exists(base_executable):
+                            return base_executable
+                        # Python may return "python" because it was invoked from the POSIX virtual environment
+                        # however some installs/distributions do not provide a version-less "python" binary in
+                        # the system install location (see PEP 394) so try to fallback to a versioned binary.
+                        #
+                        # Gate this to Python 3.11 as `sys._base_executable` path resolution is now relative to
+                        # the 'home' key from pyvenv.cfg which often points to the system install location.
+                        major, minor = self.version_info.major, self.version_info.minor
+                        if self.os == "posix" and (major, minor) >= (3, 11):
+                            # search relative to the directory of sys._base_executable
+                            base_dir = os.path.dirname(base_executable)
+                            for base_executable in [
+                                os.path.join(base_dir, exe)
+                                for exe in ("python{}".format(major), "python{}.{}".format(major, minor))
+                            ]:
+                                if os.path.exists(base_executable):
+                                    return base_executable
             return None  # in this case we just can't tell easily without poking around FS and calling them, bail
         # if we're not in a virtual environment, this is already a system python, so return the original executable
         # note we must choose the original and not the pure executable as shim scripts might throw us off

tests/unit/discovery/py_info/test_py_info.py

@@ -378,6 +378,29 @@ def test_custom_venv_install_scheme_is_prefered(mocker):
     assert pyinfo.install_path("purelib").replace(os.sep, "/") == f"lib/python{pyver}/site-packages"
 
 
+@pytest.mark.skipif(not (os.name == "posix" and sys.version_info[:2] >= (3, 11)), reason="POSIX 3.11+ specific")
+def test_fallback_existent_system_executable(mocker):
+    current = PythonInfo()
+    # Posix may execute a "python" out of a venv but try to set the base_executable
+    # to "python" out of the system installation path. PEP 394 informs distributions
+    # that "python" is not required and the standard `make install` does not provide one
+
+    # Falsify some data to look like we're in a venv
+    current.prefix = current.exec_prefix = "/tmp/tmp.izZNCyINRj/venv"
+    current.executable = current.original_executable = os.path.join(current.prefix, "bin/python")
+
+    # Since we don't know if the distribution we're on provides python, use a binary that should not exist
+    mocker.patch.object(sys, "_base_executable", os.path.join(os.path.dirname(current.system_executable), "idontexist"))
+    mocker.patch.object(sys, "executable", current.executable)
+
+    # ensure it falls back to an alternate binary name that exists
+    current._fast_get_system_executable()
+    assert os.path.basename(current.system_executable) in [
+        f"python{v}" for v in (current.version_info.major, f"{current.version_info.major}.{current.version_info.minor}")
+    ]
+    assert os.path.exists(current.system_executable)
+
+
 @pytest.mark.skipif(sys.version_info[:2] != (3, 10), reason="3.10 specific")
 def test_uses_posix_prefix_on_debian_3_10_without_venv(mocker):
     # this is taken from ubuntu 22.04 /usr/lib/python3.10/sysconfig.py