Add upper bound for prefix_int

streaming/base/constant.py

@@ -34,3 +34,6 @@
 
 # Default download timeout
 DEFAULT_TIMEOUT = 60.0
+
+# Maximum prefix integers
+MAX_PREFIX_INT = 1000

streaming/base/shared/prefix.py

@@ -16,7 +16,7 @@
 import numpy as np
 from torch import distributed as dist
 
-from streaming.base.constant import BARRIER_FILELOCK, CACHE_FILELOCK, LOCALS, SHM_TO_CLEAN, TICK
+from streaming.base.constant import BARRIER_FILELOCK, CACHE_FILELOCK, LOCALS, SHM_TO_CLEAN, TICK, MAX_PREFIX_INT
 from streaming.base.shared import SharedMemory
 from streaming.base.world import World
 
@@ -113,6 +113,12 @@ def _check_and_find(streams_local: list[str], streams_remote: list[Union[str, No
 
     for prefix_int in _each_prefix_int():
 
+        if prefix_int >= MAX_PREFIX_INT:
+            raise ValueError(f"prefix_int exceeds {MAX_PREFIX_INT}. This may happen " +
+                             f"when you mock os.path.exists or os.stat so the filelock " +
+                             f"checks always returns ``True`` " +
+                             f"you need to clean up TMPDIR.")
+
         name = _get_path(prefix_int, shm_name)
 
         # Check if any shared memory filelocks exist for the current prefix

tests/test_shared.py

@@ -190,3 +190,16 @@ def test_shared_memory_permission_error(mock_shared_memory_class: MagicMock):
     with patch('os.path.exists', return_value=False):
         next_prefix = _check_and_find(['local'], [None], LOCALS)
         assert next_prefix == 1
+
+
+@pytest.mark.usefixtures('local_remote_dir')
+def test_shared_memory_infinity_exception(local_remote_dir: tuple[str, str]):
+    local, remote = local_remote_dir
+    with patch('os.path.exists', return_value=True):
+        with pytest.raises(ValueError, match='prefix_int exceeds .*clean up TMPDIR.'):
+            _, _ = get_shm_prefix(streams_local=[local],
+                                  streams_remote=[remote],
+                                  world=World.detect())
+
+
+