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.

README.md

@@ -371,6 +371,14 @@ If you have decided not to use Syrupy for your project after giving us a try, we
 
 Benchmarks are automatically published to https://tophat.github.io/syrupy/dev/bench/.
 
+## Known Limitations
+
+- `pytest-xdist` support only partially exists. There is no issue when it comes to reads however when you attempt to run `pytest --snapshot-update`, if running with more than 1 process, the ability to detect unused snapshots is disabled. See [#535](https://github.com/tophat/syrupy/issues/535) for more information.
+- _Extremely_ large snapshots may fail due to a known Python core library bug. See [#577](https://github.com/tophat/syrupy/issues/577) and [cpython #65452](https://github.com/python/cpython/issues/65452).
+
+_We welcome contributions to patch these known limitations._
+
+
 ## Contributing
 
 Feel free to open a PR or GitHub issue. Contributions welcome!

src/syrupy/report.py

@@ -4,6 +4,7 @@
     dataclass,
     field,
 )
+from functools import cached_property
 from gettext import (
     gettext,
     ngettext,
@@ -53,11 +54,14 @@ class SnapshotReport:
     information used for removal of unused or orphaned snapshots and fossils.
     """
 
+    # Initial arguments to the report
     base_dir: str
     collected_items: Set["pytest.Item"]
     selected_items: Dict[str, bool]
     options: "argparse.Namespace"
     assertions: List["SnapshotAssertion"]
+
+    # All of these are derived from the initial arguments and via walking the filesystem
     discovered: "SnapshotFossils" = field(default_factory=SnapshotFossils)
     created: "SnapshotFossils" = field(default_factory=SnapshotFossils)
     failed: "SnapshotFossils" = field(default_factory=SnapshotFossils)
@@ -66,9 +70,6 @@ class SnapshotReport:
     used: "SnapshotFossils" = field(default_factory=SnapshotFossils)
     _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:
@@ -82,12 +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 @@
 from .constants import EXIT_STATUS_FAIL_UNUSED
 from .data import SnapshotFossils
 from .report import SnapshotReport
+from .utils import (
+    is_xdist_controller,
+    is_xdist_worker,
+)
 
 if TYPE_CHECKING:
     from .assertion import SnapshotAssertion
@@ -79,6 +83,18 @@ def finish(self) -> int:
             assertions=self._assertions,
             options=self.pytest_session.config.option,
         )
+
+        if is_xdist_worker():
+            # TODO: If we're in a pytest-xdist worker, we need to combine the reports
+            # of all the workers so that the controller can handle unused
+            # snapshot removal.
+            return exitstatus
+        elif is_xdist_controller():
+            # TODO: If we're in a pytest-xdist controller, merge all the reports.
+            # Until this is implemented, running syrupy with pytest-xdist is only
+            # partially functional.
+            return exitstatus
+
         if self.report.num_unused:
             if self.update_snapshots:
                 self.remove_unused_snapshots(

src/syrupy/utils.py

@@ -14,6 +14,16 @@
 from .exceptions import FailedToLoadModuleMember
 
 
+def is_xdist_worker() -> bool:
+    worker_name = os.getenv("PYTEST_XDIST_WORKER")
+    return bool(worker_name and worker_name != "master")
+
+
+def is_xdist_controller() -> bool:
+    worker_count = os.getenv("PYTEST_XDIST_WORKER_COUNT")
+    return bool(worker_count and int(worker_count) > 0 and not is_xdist_worker())
+
+
 def in_snapshot_dir(path: Path) -> bool:
     return SNAPSHOT_DIRNAME in path.parts