Fallback to direct write for readonly dirs and use temp path for checkpoints (#1516)

Author: Darshan808

jupyter_server/services/contents/filecheckpoints.py

@@ -4,6 +4,7 @@
 
 import os
 import shutil
+import tempfile
 
 from anyio.to_thread import run_sync
 from jupyter_core.utils import ensure_dir_exists
@@ -111,6 +112,10 @@ def checkpoint_path(self, checkpoint_id, path):
         filename = f"{basename}-{checkpoint_id}{ext}"
         os_path = self._get_os_path(path=parent)
         cp_dir = os.path.join(os_path, self.checkpoint_dir)
+        # If parent directory isn't writable, use system temp
+        if not os.access(os.path.dirname(cp_dir), os.W_OK):
+            rel = os.path.relpath(os_path, start=self.root_dir)
+            cp_dir = os.path.join(tempfile.gettempdir(), "jupyter_checkpoints", rel)
         with self.perm_to_403():
             ensure_dir_exists(cp_dir)
         cp_path = os.path.join(cp_dir, filename)

jupyter_server/services/contents/fileio.py

@@ -112,6 +112,21 @@ def atomic_writing(path, text=True, encoding="utf-8", log=None, **kwargs):
     if os.path.islink(path):
         path = os.path.join(os.path.dirname(path), os.readlink(path))
 
+    # Fall back to direct write for existing file in a non-writable dir
+    dirpath = os.path.dirname(path) or os.getcwd()
+    if os.path.isfile(path) and not os.access(dirpath, os.W_OK) and os.access(path, os.W_OK):
+        mode = "w" if text else "wb"
+        # direct open on the target file
+        if text:
+            fileobj = open(path, mode, encoding=encoding, **kwargs)  # noqa: SIM115
+        else:
+            fileobj = open(path, mode, **kwargs)  # noqa: SIM115
+        try:
+            yield fileobj
+        finally:
+            fileobj.close()
+        return
+
     tmp_path = path_to_intermediate(path)
 
     if os.path.isfile(path):

tests/services/contents/test_fileio.py

@@ -136,6 +136,26 @@ def test_path_to_invalid(tmpdir):
     assert path_to_invalid(tmpdir) == str(tmpdir) + ".invalid"
 
 
+@pytest.mark.skipif(sys.platform.startswith("win"), reason="requires POSIX directory perms")
+def test_atomic_writing_in_readonly_dir(tmp_path):
+    # Setup: non-writable dir but a writable file inside
+    nonw = tmp_path / "nonwritable"
+    nonw.mkdir()
+    f = nonw / "file.txt"
+    f.write_text("original content")
+    os.chmod(str(nonw), 0o500)
+    os.chmod(str(f), 0o700)
+
+    # direct write fallback succeeds
+    with atomic_writing(str(f)) as ff:
+        ff.write("new content")
+    assert f.read_text() == "new content"
+
+    # dir perms unchanged
+    mode = stat.S_IMODE(os.stat(str(nonw)).st_mode)
+    assert mode == 0o500
+
+
 @pytest.mark.skipif(os.name == "nt", reason="test fails on Windows")
 def test_file_manager_mixin(tmp_path):
     mixin = FileManagerMixin()