merge_tree: proper check for subdirectory

conda_build/utils.py

@@ -715,6 +715,17 @@ def copytree(src, dst, symlinks=False, ignore=None, dry_run=False):
     return dst_lst
 
 
+def is_subdir(child, parent, strict = True):
+    """
+    Check whether child is a (strict) subdirectory if parent.
+    """
+    parent = Path(parent).resolve()
+    child = Path(child).resolve()
+    if strict:
+        return parent in child.parents
+    return child == parent or parent in child.parents
+
+
 def merge_tree(
     src, dst, symlinks=False, timeout=900, lock=None, locking=True, clobber=False
 ):
@@ -725,9 +736,7 @@ def merge_tree(
     Like copytree(src, dst), but raises an error if merging the two trees
     would overwrite any files.
     """
-    dst = os.path.normpath(os.path.normcase(dst))
-    src = os.path.normpath(os.path.normcase(src))
-    assert not dst.startswith(src), (
+    assert not is_subdir(dst, src, strict=False), (
         "Can't merge/copy source into subdirectory of itself.  "
         "Please create separate spaces for these things.\n"
         "  src: {}\n"

tests/test_utils.py

@@ -72,11 +72,32 @@ def test_disallow_merge_conflicts(namespace_setup: os.PathLike):
 
 
 @pytest.mark.sanity
-def test_disallow_in_tree_merge(testing_workdir):
-    with open("testfile", "w") as f:
+def test_is_subdir(testing_workdir):
+    assert not utils.is_subdir(testing_workdir, testing_workdir)
+    assert utils.is_subdir(testing_workdir, testing_workdir, strict=False)
+    subdir = os.path.join(testing_workdir, "subdir")
+    assert utils.is_subdir(subdir, testing_workdir)
+    assert utils.is_subdir(subdir, testing_workdir, strict=False)
+
+
+@pytest.mark.sanity
+def test_disallow_down_tree_merge(testing_workdir):
+    src = testing_workdir
+    with open(os.path.join(src,"testfile"), "w") as f:
         f.write("test")
     with pytest.raises(AssertionError):
-        utils.merge_tree(testing_workdir, os.path.join(testing_workdir, "subdir"))
+        utils.merge_tree(src, testing_workdir)
+    with pytest.raises(AssertionError):
+        utils.merge_tree(src, os.path.join(testing_workdir, "subdir"))
+
+
+@pytest.mark.sanity
+def test_allow_up_tree_merge(testing_workdir):
+    src = os.path.join(testing_workdir, "subdir")
+    os.makedirs(src)
+    with open(os.path.join(src,"testfile"), "w") as f:
+        f.write("test")
+    utils.merge_tree(src, testing_workdir)
 
 
 def test_relative_default():