Prevent infinite recursion with pip wheel with $TMPDIR in $PWD

During a build of extension module within `pip wheel` the source directory is
recursively copied in a temporary directory.

See #7555

When the temporary directory is inside the source directory
(for example by setting `TMPDIR=$PWD/tmp`) this caused an infinite recursion
that ended in:

    [Errno 36] File name too long

We prevent that buy never copying the target to the target in _copy_source_tree.

Fixes #7872

news/7872.bugfix

@@ -0,0 +1 @@
+Prevent an infinite recursion with ``pip wheel`` when ``$TMPDIR`` is within the source directory.

src/pip/_internal/operations/prepare.py

@@ -156,13 +156,25 @@ def _copy2_ignoring_special_files(src, dest):
 
 def _copy_source_tree(source, target):
     # type: (str, str) -> None
+    target_abspath = os.path.abspath(target)
+    target_basename = os.path.basename(target_abspath)
+    target_dirname = os.path.dirname(target_abspath)
+
     def ignore(d, names):
         # type: (str, List[str]) -> List[str]
-        # Pulling in those directories can potentially be very slow,
-        # exclude the following directories if they appear in the top
-        # level dir (and only it).
-        # See discussion at https://github.com/pypa/pip/pull/6770
-        return ['.tox', '.nox'] if d == source else []
+        skipped = []  # type: List[str]
+        if d == source:
+            # Pulling in those directories can potentially be very slow,
+            # exclude the following directories if they appear in the top
+            # level dir (and only it).
+            # See discussion at https://github.com/pypa/pip/pull/6770
+            skipped += ['.tox', '.nox']
+        if os.path.abspath(d) == target_dirname:
+            # Prevent an infinite recursion if the target is in source.
+            # This can happen when TMPDIR is set to ${PWD}/...
+            # and we copy PWD to TMPDIR.
+            skipped += [target_basename]
+        return skipped
 
     kwargs = dict(ignore=ignore, symlinks=True)  # type: CopytreeKwargs
 

tests/data/src/extension/setup.py

@@ -0,0 +1,4 @@
+from setuptools import Extension, setup
+
+module = Extension('extension', sources=['extension.c'])
+setup(name='extension', version='0.0.1', ext_modules = [module])

tests/functional/test_wheel.py

@@ -289,6 +289,17 @@ def test_pip_wheel_with_user_set_in_config(script, data, common_wheels):
     assert "Successfully built withpyproject" in result.stdout, result.stdout
 
 
+def test_pip_wheel_ext_module_with_tmpdir_inside(script, data, common_wheels):
+    tmpdir = data.src / 'extension/tmp'
+    tmpdir.mkdir()
+    script.environ['TMPDIR'] = str(tmpdir)
+    result = script.pip(
+        'wheel', data.src / 'extension',
+        '--no-index', '-f', common_wheels
+    )
+    assert "Successfully built extension" in result.stdout, result.stdout
+
+
 @pytest.mark.network
 def test_pep517_wheels_are_not_confused_with_other_files(script, tmpdir, data):
     """Check correct wheels are copied. (#6196)