[3.8] bpo-38688, shutil.copytree: consume iterator and create list of entries to prevent infinite recursion (GH-17397)

(cherry picked from commit 9bbcbc9f6dfe1368fe7330b117707f828e6a2c18)

Co-authored-by: Bruno P. Kinoshita <kinow@users.noreply.github.com>
diff --git a/Lib/test/test_shutil.py b/Lib/test/test_shutil.py
index b98e7dc..286e333 100644
--- a/Lib/test/test_shutil.py
+++ b/Lib/test/test_shutil.py
@@ -1605,6 +1605,18 @@
         rv = shutil.copytree(src_dir, dst_dir)
         self.assertEqual(['foo'], os.listdir(rv))
 
+    def test_copytree_subdirectory(self):
+        # copytree where dst is a subdirectory of src, see Issue 38688
+        base_dir = self.mkdtemp()
+        self.addCleanup(shutil.rmtree, base_dir, ignore_errors=True)
+        src_dir = os.path.join(base_dir, "t", "pg")
+        dst_dir = os.path.join(src_dir, "somevendor", "1.0")
+        os.makedirs(src_dir)
+        src = os.path.join(src_dir, 'pol')
+        write_file(src, 'pol')
+        rv = shutil.copytree(src_dir, dst_dir)
+        self.assertEqual(['pol'], os.listdir(rv))
+
 
 class TestWhich(unittest.TestCase):