fix: avoid unnecessary env updates to reduce chances of segfault (#956)

* fix: __terminal_color shouldn't modify os.environ unless it has to

See #955 for a
detailed explanation of why this can cause thread safety issues
resulting in a segfault when another thread calls `getenv`

* fix: pytest_assertrepr_compare should be a no-op for other types

If neither operand is a `SnapshotAssertion` then the rest of the
code is not going to do anything, so bail early rather than
unnecessarily setting up `__terminal_color` etc. which can cause
issues c.f. #955

* chore: run linter

---------

Co-authored-by: Noah Ulster <noah.u@roserocket.com>

Author: sparrowt

src/syrupy/__init__.py

@@ -1,4 +1,5 @@
 import argparse
+import contextlib
 import sys
 from functools import lru_cache
 from gettext import gettext
@@ -105,11 +106,14 @@ def pytest_addoption(parser: "pytest.Parser") -> None:
 
 
 def __terminal_color(config: "pytest.Config") -> "ContextManager[None]":
-    env = {}
     if config.option.no_colors:
-        env[DISABLE_COLOR_ENV_VAR] = "true"
-
-    return env_context(**env)
+        env = {
+            DISABLE_COLOR_ENV_VAR: "true",
+        }
+        return env_context(**env)
+    else:
+        # No-op to avoid unnecessary env updates
+        return contextlib.nullcontext()
 
 
 def pytest_assertrepr_compare(
@@ -119,6 +123,12 @@ def pytest_assertrepr_compare(
     Return explanation for comparisons in failing assert expressions.
     https://docs.pytest.org/en/latest/reference.html#_pytest.hookspec.pytest_assertrepr_compare
     """
+    if not isinstance(left, SnapshotAssertion) and not isinstance(
+        right, SnapshotAssertion
+    ):
+        # Shortcut to minimise overhead in the case of other unrelated assertions
+        return None
+
     with __terminal_color(config):
         received_name = received_style("[+ received]")