Set store to writeable after creating a branch (earth-mover#455)

* Set store to writeable after creating a branch

Closes earth-mover#439

* Add tests

icechunk-python/python/icechunk/__init__.py

@@ -242,19 +242,20 @@ def checkout(
                     "only one of snapshot_id, branch, or tag may be specified"
                 )
             self._store.checkout_snapshot(snapshot_id)
-            self._read_only = True
+            self.set_read_only()
             return
         if branch is not None:
             if tag is not None:
                 raise ValueError(
                     "only one of snapshot_id, branch, or tag may be specified"
                 )
             self._store.checkout_branch(branch)
-            self._read_only = True
+            # We preserve the read-only status here so you can checkout a branch
+            # on a read-only store, and be guaranteed that you won't modify the store.
             return
         if tag is not None:
             self._store.checkout_tag(tag)
-            self._read_only = True
+            self.set_read_only()
             return
 
         raise ValueError("a snapshot_id, branch, or tag must be specified")
@@ -385,7 +386,9 @@ def new_branch(self, branch_name: str) -> str:
 
         This requires having no uncommitted changes.
         """
-        return self._store.new_branch(branch_name)
+        ret = self._store.new_branch(branch_name)
+        self.set_writeable()
+        return ret
 
     async def async_reset_branch(self, to_snapshot: str) -> None:
         """Reset the currently checked out branch to point to a different snapshot.

icechunk-python/tests/test_timetravel.py

@@ -20,6 +20,7 @@ def test_timetravel():
     assert air_temp[200, 6] == 42
 
     snapshot_id = store.commit("commit 1")
+    assert not store._read_only
 
     air_temp[:, :] = 54
     assert air_temp[200, 6] == 54
@@ -49,12 +50,14 @@ def test_timetravel():
     assert air_temp[200, 6] == 54
 
     store.new_branch("feature")
+    assert not store._read_only
     assert store.branch == "feature"
     air_temp[:, :] = 90
     feature_snapshot_id = store.commit("commit 3")
     store.tag("v1.0", feature_snapshot_id)
 
     store.checkout(tag="v1.0")
+    assert store._read_only
     assert store.branch is None
     assert air_temp[200, 6] == 90