Fix for saving big replay buffer, use pickle protocol>=4 (#239)

docs/misc/changelog.rst

@@ -32,6 +32,7 @@ Bug Fixes:
 - Fixed ``DQN`` predict method when using single ``gym.Env`` with ``deterministic=False``
 - Fixed bug that the arguments order of ``explained_variance()`` in ``ppo.py`` and ``a2c.py`` is not correct (@thisray)
 - Fixed bug where full ``HerReplayBuffer`` leads to an index error. (@megan-klaiber)
+- Fixed bug where replay buffer could not be saved if it was too big (> 4 Gb) for python<3.8 (thanks @hn2)
 
 Deprecations:
 ^^^^^^^^^^^^^
@@ -523,4 +524,4 @@ And all the contributors:
 @flodorner @KuKuXia @NeoExtended @PartiallyTyped @mmcenta @richardwu @kinalmehta @rolandgvc @tkelestemur @mloo3
 @tirafesi @blurLake @koulakis @joeljosephjin @shwang @rk37 @andyshih12 @RaphaelWag @xicocaio
 @diditforlulz273 @liorcohen5 @ManifoldFR @mloo3 @SwamyDev @wmmc88 @megan-klaiber @thisray
-@tfederico
+@tfederico @hn2

stable_baselines3/common/save_util.py

@@ -337,7 +337,9 @@ def save_to_pkl(path: Union[str, pathlib.Path, io.BufferedIOBase], obj: Any, ver
     :param verbose: Verbosity level, 0 means only warnings, 2 means debug information.
     """
     with open_path(path, "w", verbose=verbose, suffix="pkl") as file_handler:
-        pickle.dump(obj, file_handler)
+        # Use protocol>=4 to support saving replay buffers >= 4Gb
+        # See https://docs.python.org/3/library/pickle.html
+        pickle.dump(obj, file_handler, protocol=pickle.HIGHEST_PROTOCOL)
 
 
 def load_from_pkl(path: Union[str, pathlib.Path, io.BufferedIOBase], verbose: int = 0) -> Any: