Implement new with_read_only Zarr Store method (#1021)

* Implement new `with_read_only` Zarr `Store` method

This new method is not really implementable in Icechunk. If the user
created a readonly session there is no real good way to turn it into a
writable session, and viceversa.

The implementation we are writing "lies" in this sense. It's pretending
to return a new store with the requested readonly-ness, but not really.

* Add FAQ on read-only

Author: paraseba

docs/docs/icechunk-python/faq.md

@@ -16,3 +16,10 @@ This function also accepts the filter directive. If you prefer not to use enviro
 ```python
 icechunk.set_logs_filter("debug,icechunk=trace")
 ```
+
+**How to get a read-only Icechunk store?**
+
+Zarr has a few mechanisms to define read-only zarr stores. These don't always work perfectly with Icechunk,
+because Icechunk has a more advanced session model. The safest way to make sure you don't write to
+an Icechunk repo is to use `Repository.readonly_session` to create the session. It doesn't matter what
+you do to the Zarr store, a read-only session cannot do writes.

icechunk-python/python/icechunk/store.py

@@ -42,14 +42,16 @@ def __init__(
         self,
         store: PyStore,
         allow_pickling: bool,
+        read_only: bool | None = None,
         *args: Any,
         **kwargs: Any,
     ):
         """Create a new IcechunkStore.
 
         This should not be called directly, instead use the `create`, `open_existing` or `open_or_create` class methods.
         """
-        super().__init__(read_only=store.read_only)
+        read_only = read_only if read_only is not None else store.read_only
+        super().__init__(read_only=read_only)
         if store is None:
             raise ValueError(
                 "An IcechunkStore should not be created with the default constructor, instead use either the create or open_existing class methods."
@@ -80,6 +82,13 @@ def __setstate__(self, state: Any) -> None:
         state["_read_only"] = state["_store"].read_only
         self.__dict__ = state
 
+    def with_read_only(self, read_only: bool = False) -> Store:
+        new_store = IcechunkStore(
+            store=self._store, allow_pickling=self._allow_pickling, read_only=read_only
+        )
+        new_store._is_open = False
+        return new_store
+
     @property
     def session(self) -> "Session":
         from icechunk import Session

icechunk-python/tests/test_store.py

@@ -4,7 +4,7 @@
 
 import zarr
 from tests.conftest import parse_repo
-from zarr.core.buffer import default_buffer_prototype
+from zarr.core.buffer import cpu, default_buffer_prototype
 
 rng = np.random.default_rng(seed=12345)
 
@@ -74,3 +74,24 @@ def test_doesnt_support_consolidated_metadata() -> None:
     session = repo.writable_session("main")
     store = session.store
     assert not store.supports_consolidated_metadata
+
+
+async def test_with_readonly() -> None:
+    repo = parse_repo("memory", "test")
+    session = repo.readonly_session("main")
+    store = session.store
+    assert store.read_only
+
+    session = repo.writable_session("main")
+    store = session.store
+    writer = store.with_read_only(read_only=False)
+    assert not writer._is_open
+    assert not writer.read_only
+
+    root = zarr.group(store=store)
+    root.create_array(name="foo", shape=(1,), chunks=(1,), dtype=np.int64)
+    await writer.set("foo/c/0", cpu.Buffer.from_bytes(b"bar"))
+    await writer.delete("foo/c/0")
+
+    reader = store.with_read_only(read_only=True)
+    assert reader.read_only