prevent PermissionError when using venv creator on some systems (#2543)

docs/changelog/2543.bugfix.rst

@@ -0,0 +1,2 @@
+Prevent ``PermissionError`` when using venv creator on systems that deliver files without user write
+permission - by :user:`kulikjak`.

src/virtualenv/activation/via_template.py

@@ -43,6 +43,10 @@ def _generate(self, replacements, templates, to_folder, creator):
         for template in templates:
             text = self.instantiate_template(replacements, template, creator)
             dest = to_folder / self.as_name(template)
+            # remove the file if it already exists - this prevents permission
+            # errors when the dest is not writable
+            if dest.exists():
+                dest.unlink()
             # use write_bytes to avoid platform specific line normalization (\n -> \r\n)
             dest.write_bytes(text.encode("utf-8"))
             generated.append(dest)

tests/unit/create/test_creator.py

@@ -611,3 +611,25 @@ def _get_sys_path(flag=None):
         assert non_python_path == [i for i in base if i not in extra_as_python_path]
     else:
         assert base == extra_all
+
+
+# Make sure that the venv creator works on systems where vendor-delivered files
+# (specifically venv scripts delivered with Python itself) are not writable.
+#
+# https://github.com/pypa/virtualenv/issues/2419
+@pytest.mark.skipif("venv" not in CURRENT_CREATORS, reason="test needs venv creator")
+def test_venv_creator_without_write_perms(tmp_path, mocker):
+    from virtualenv.run.session import Session
+
+    prev = Session._create
+
+    def func(self):
+        prev(self)
+        scripts_dir = self.creator.dest / "bin"
+        for script in scripts_dir.glob("*ctivate*"):
+            script.chmod(stat.S_IREAD | stat.S_IRGRP | stat.S_IROTH)
+
+    mocker.patch("virtualenv.run.session.Session._create", side_effect=func, autospec=True)
+
+    cmd = [str(tmp_path), "--seeder", "app-data", "--without-pip", "--creator", "venv"]
+    cli_run(cmd)