From e78207c936c43478aa5d5531d7c0b90aa240c9e0 Mon Sep 17 00:00:00 2001
From: Prashant Anand <p-anand@mercari.com>
Date: Tue, 9 Jun 2020 09:54:22 +0900
Subject: [PATCH] 7119: data loss with mistyped --basetemp (#7170)

Co-authored-by: Bruno Oliveira <nicoddemus@gmail.com>
Co-authored-by: Ran Benita <ran@unusedvar.com>
---
 AUTHORS                        |  1 +
 changelog/7119.improvement.rst |  2 ++
 src/_pytest/main.py            | 31 +++++++++++++++++++++++++++++++
 testing/test_main.py           | 23 +++++++++++++++++++++++
 4 files changed, 57 insertions(+)
 create mode 100644 changelog/7119.improvement.rst

diff --git a/AUTHORS b/AUTHORS
index e1b195b9a..4c5ca41af 100644
--- a/AUTHORS
+++ b/AUTHORS
@@ -227,6 +227,7 @@ Pedro Algarvio
 Philipp Loose
 Pieter Mulder
 Piotr Banaszkiewicz
+Prashant Anand
 Pulkit Goyal
 Punyashloka Biswal
 Quentin Pradet
diff --git a/changelog/7119.improvement.rst b/changelog/7119.improvement.rst
new file mode 100644
index 000000000..6cef98836
--- /dev/null
+++ b/changelog/7119.improvement.rst
@@ -0,0 +1,2 @@
+Exit with an error if the ``--basetemp`` argument is empty, the current working directory or parent directory of it.
+This is done to protect against accidental data loss, as any directory passed to this argument is cleared.
diff --git a/src/_pytest/main.py b/src/_pytest/main.py
index 84ee00881..a95f2f2e7 100644
--- a/src/_pytest/main.py
+++ b/src/_pytest/main.py
@@ -1,4 +1,5 @@
 """ core implementation of testing process: init, session, runtest loop. """
+import argparse
 import fnmatch
 import functools
 import importlib
@@ -30,6 +31,7 @@ from _pytest.config import UsageError
 from _pytest.config.argparsing import Parser
 from _pytest.fixtures import FixtureManager
 from _pytest.outcomes import exit
+from _pytest.pathlib import Path
 from _pytest.reports import CollectReport
 from _pytest.reports import TestReport
 from _pytest.runner import collect_one_node
@@ -177,6 +179,7 @@ def pytest_addoption(parser: Parser) -> None:
         "--basetemp",
         dest="basetemp",
         default=None,
+        type=validate_basetemp,
         metavar="dir",
         help=(
             "base temporary directory for this test run."
@@ -185,6 +188,34 @@ def pytest_addoption(parser: Parser) -> None:
     )
 
 
+def validate_basetemp(path: str) -> str:
+    # GH 7119
+    msg = "basetemp must not be empty, the current working directory or any parent directory of it"
+
+    # empty path
+    if not path:
+        raise argparse.ArgumentTypeError(msg)
+
+    def is_ancestor(base: Path, query: Path) -> bool:
+        """ return True if query is an ancestor of base, else False."""
+        if base == query:
+            return True
+        for parent in base.parents:
+            if parent == query:
+                return True
+        return False
+
+    # check if path is an ancestor of cwd
+    if is_ancestor(Path.cwd(), Path(path).absolute()):
+        raise argparse.ArgumentTypeError(msg)
+
+    # check symlinks for ancestors
+    if is_ancestor(Path.cwd().resolve(), Path(path).resolve()):
+        raise argparse.ArgumentTypeError(msg)
+
+    return path
+
+
 def wrap_session(
     config: Config, doit: Callable[[Config, "Session"], Optional[Union[int, ExitCode]]]
 ) -> Union[int, ExitCode]:
diff --git a/testing/test_main.py b/testing/test_main.py
index 07aca3a1e..ee8349a9f 100644
--- a/testing/test_main.py
+++ b/testing/test_main.py
@@ -1,7 +1,9 @@
+import argparse
 from typing import Optional
 
 import pytest
 from _pytest.config import ExitCode
+from _pytest.main import validate_basetemp
 from _pytest.pytester import Testdir
 
 
@@ -75,3 +77,24 @@ def test_wrap_session_exit_sessionfinish(
         assert result.ret == ExitCode.NO_TESTS_COLLECTED
     assert result.stdout.lines[-1] == "collected 0 items"
     assert result.stderr.lines == ["Exit: exit_pytest_sessionfinish"]
+
+
+@pytest.mark.parametrize("basetemp", ["foo", "foo/bar"])
+def test_validate_basetemp_ok(tmp_path, basetemp, monkeypatch):
+    monkeypatch.chdir(str(tmp_path))
+    validate_basetemp(tmp_path / basetemp)
+
+
+@pytest.mark.parametrize("basetemp", ["", ".", ".."])
+def test_validate_basetemp_fails(tmp_path, basetemp, monkeypatch):
+    monkeypatch.chdir(str(tmp_path))
+    msg = "basetemp must not be empty, the current working directory or any parent directory of it"
+    with pytest.raises(argparse.ArgumentTypeError, match=msg):
+        if basetemp:
+            basetemp = tmp_path / basetemp
+        validate_basetemp(basetemp)
+
+
+def test_validate_basetemp_integration(testdir):
+    result = testdir.runpytest("--basetemp=.")
+    result.stderr.fnmatch_lines("*basetemp must not be*")