fix: improve pytest-xdist compatibility

NOTE: When pytest-xdist is detected, we do not remove unused snapshots, since that requires coordination between the workers and the controller. A disclaimer has been added to the README and TODO comments added to the source code to help solve this problem at some point.

src/syrupy/extensions/base.py

@@ -199,10 +199,7 @@ def write_snapshot(
                 warnings.warn(warning_msg)
 
         # Ensures the folder path for the snapshot file exists.
-        try:
-            Path(snapshot_location).parent.mkdir(parents=True)
-        except FileExistsError:
-            pass
+        Path(snapshot_location).parent.mkdir(parents=True, exist_ok=True)
 
         cls._write_snapshot_collection(snapshot_collection=snapshot_collection)
 

src/syrupy/report.py

@@ -4,6 +4,7 @@
     dataclass,
     field,
 )
+from functools import cached_property
 from gettext import (
     gettext,
     ngettext,
@@ -69,9 +70,6 @@ class SnapshotReport:
     used: "SnapshotCollections" = field(default_factory=SnapshotCollections)
     _provided_test_paths: Dict[str, List[str]] = field(default_factory=dict)
     _keyword_expressions: Set["Expression"] = field(default_factory=set)
-    _collected_items_by_nodeid: Dict[str, "pytest.Item"] = field(
-        default_factory=dict, init=False
-    )
 
     @property
     def update_snapshots(self) -> bool:
@@ -85,12 +83,15 @@ def warn_unused_snapshots(self) -> bool:
     def include_snapshot_details(self) -> bool:
         return bool(self.options.include_snapshot_details)
 
-    def __post_init__(self) -> None:
-        self.__parse_invocation_args()
-        self._collected_items_by_nodeid = {
+    @cached_property
+    def _collected_items_by_nodeid(self) -> Dict[str, "pytest.Item"]:
+        return {
             getattr(item, "nodeid"): item for item in self.collected_items  # noqa: B009
         }
 
+    def __post_init__(self) -> None:
+        self.__parse_invocation_args()
+
         # We only need to discover snapshots once per test file, not once per assertion.
         locations_discovered: DefaultDict[str, Set[Any]] = defaultdict(set)
         for assertion in self.assertions:

src/syrupy/session.py

@@ -20,6 +20,10 @@
 import pytest
 
 from syrupy.location import PyTestLocation
+from syrupy.utils import (
+    is_xdist_controller,
+    is_xdist_worker,
+)
 
 from .constants import EXIT_STATUS_FAIL_UNUSED
 from .data import SnapshotCollections