Add empty sentinel item in WorkerInteractor queue

When pytest_runtest_protocol is called with item==nextitem multiple times, with a test that accepts some fixtures -> the second call forward raises an error.

This is causes in WorkerInteractor when the same test appears twice in a row in its items queue.

To deal with this, we define an "empty" item that shall allow us to call pytest_runtest_protocol with nextitem=None, effectively solving this problem so long sequential executions of the same items are delimited by these empty items in the queue.

src/xdist/remote.py

@@ -67,6 +67,7 @@ def worker_title(title: str) -> None:
 class Marker(enum.Enum):
     SHUTDOWN = 0
     QUEUE_REPLACED = 1
+    EMPTY = 2
 
 
 class WorkerInteractor:
@@ -144,6 +145,8 @@ def handle_command(
             self.torun.put(Marker.SHUTDOWN)
         elif name == "steal":
             self.steal(kwargs["indices"])
+        elif name == "empty":
+            self.torun.put(Marker.EMPTY)
 
     def steal(self, indices: Sequence[int]) -> None:
         indices_set = set(indices)
@@ -171,6 +174,8 @@ def pytest_runtestloop(self, session: pytest.Session) -> bool:
         self.channel.setcallback(self.handle_command, endmarker=Marker.SHUTDOWN)
         self.nextitem_index = self._get_next_item_index()
         while self.nextitem_index is not Marker.SHUTDOWN:
+            while self.nextitem_index is Marker.EMPTY:
+                self.nextitem_index = self._get_next_item_index()
             self.run_one_test()
             if session.shouldfail or session.shouldstop:
                 break
@@ -183,7 +188,7 @@ def run_one_test(self) -> None:
 
         items = self.session.items
         item = items[self.item_index]
-        if self.nextitem_index is Marker.SHUTDOWN:
+        if self.nextitem_index in [Marker.SHUTDOWN, Marker.EMPTY]:
             nextitem = None
         else:
             assert self.nextitem_index is not None