[Bugfix]: During testing, use pytest monkeypatch for safely overriding the env...

[Bugfix]: During testing, use pytest monkeypatch for safely overriding the env var that indicates the vLLM backend (#5210)

tests/kernels/test_attention_selector.py

-import os
 from unittest.mock import patch
 
 import pytest
 import torch
 
+from tests.kernels.utils import (STR_FLASH_ATTN_VAL, STR_INVALID_VAL,
+                                 override_backend_env_variable)
 from vllm.attention.selector import which_attn_to_use
 
 
 @pytest.mark.parametrize(
     "name", ["TORCH_SDPA", "ROCM_FLASH", "XFORMERS", "FLASHINFER"])
 @pytest.mark.parametrize("device", ["cpu", "hip", "cuda"])
-def test_env(name: str, device: str):
+def test_env(name: str, device: str, monkeypatch):
     """Test that the attention selector can be set via environment variable.
     Note that we do not test FlashAttn because it is the default backend.
     """
-    name_backup = os.environ.get("VLLM_ATTENTION_BACKEND", None)
-    os.environ["VLLM_ATTENTION_BACKEND"] = name
+
+    override_backend_env_variable(monkeypatch, name)
 
     if device == "cpu":
         with patch("vllm.attention.selector.is_cpu", return_value=True):
@@ -32,14 +33,11 @@ def test_env(name: str, device: str):
                                     torch.float16, 16)
         assert backend.name == name
 
-    if name_backup is not None:
-        os.environ["VLLM_ATTENTION_BACKEND"] = name_backup
-
 
-def test_flash_attn():
+def test_flash_attn(monkeypatch):
     """Test FlashAttn validation."""
-    name_backup = os.environ.get("VLLM_ATTENTION_BACKEND", None)
-    os.environ["VLLM_ATTENTION_BACKEND"] = "FLASH_ATTN"
+
+    override_backend_env_variable(monkeypatch, STR_FLASH_ATTN_VAL)
 
     # Unsupported CUDA arch
     with patch("torch.cuda.get_device_capability", return_value=[7, 5]):
@@ -71,14 +69,9 @@ def test_flash_attn():
     backend = which_attn_to_use(8, 17, 8, None, torch.float16, None, 16)
     assert backend.name != "FLASH_ATTN"
 
-    if name_backup is not None:
-        os.environ["VLLM_ATTENTION_BACKEND"] = name_backup
-
 
-def test_invalid_env():
+def test_invalid_env(monkeypatch):
     """Throw an exception if the backend name is invalid."""
-    name_backup = os.environ.get("VLLM_ATTENTION_BACKEND", None)
-    os.environ["VLLM_ATTENTION_BACKEND"] = "INVALID"
+    override_backend_env_variable(monkeypatch, STR_INVALID_VAL)
     with pytest.raises(ValueError):
         which_attn_to_use(8, 16, 8, None, torch.float16, None, 16)
-    os.environ["VLLM_ATTENTION_BACKEND"] = name_backup

tests/kernels/utils.py

+"""Kernel test utils"""
+
+import pytest
+
+STR_BACKEND_ENV_VAR: str = "VLLM_ATTENTION_BACKEND"
+STR_FLASH_ATTN_VAL: str = "FLASH_ATTN"
+STR_INVALID_VAL: str = "INVALID"
+
+
+def override_backend_env_variable(mpatch: pytest.MonkeyPatch,
+                                  backend_name: str) -> None:
+    '''
+    Override the environment variable indicating the vLLM backend temporarily,
+    using pytest monkeypatch to ensure that the env vars get
+    reset once the test context exits.
+
+    Arguments:
+
+    * mpatch: pytest monkeypatch instance
+    * backend_name: attention backend name to force
+    '''
+    mpatch.setenv(STR_BACKEND_ENV_VAR, backend_name)