fix --forever interaction with pytest-xdist

Author: Liam-DeVoe

README.md

@@ -84,7 +84,8 @@ those fixtures are shared between threads.
       the user explicitly ends the process with Ctrl-C. This is especially
       helpful when trying to reproduce thread safety bugs that might only
       occur rarely. Note that pytest's progress indicator will keep showing
-      100% forever after the first pass of the test suite.
+      100% forever after the first pass of the test suite. `forever` is not
+      compatible with `-n` from [pytest-xdist](https://github.com/pytest-dev/pytest-xdist).
     - `--ignore-gil-enabled` to ignore the RuntimeWarning generated
       when the GIL is enabled at runtime on the free-threaded build
       and run the tests despite the fact that the GIL is enabled.

pyproject.toml

@@ -54,6 +54,7 @@ psutil = [
 test = [
     "coverage[toml]>=7.5",
     "pytest-order",
+    "pytest-xdist",
     "hypothesis>=6.135.33",
     "check-manifest",
 ]

src/pytest_run_parallel/plugin.py

@@ -190,6 +190,10 @@ def pytest_runtestloop(self, session: pytest.Session):
         Based on the default implementation in pytest, but also adds support
         for running the tests in an endless loop.
         """
+        if not self.forever:
+            # let the default pytest_runtestloop run if we don't need any
+            # customization for --forever.
+            return None
 
         if (
             session.testsfailed
@@ -205,12 +209,10 @@ def pytest_runtestloop(self, session: pytest.Session):
         number_of_items = len(session.items)
         iter_number = 0
         idx = 0
-        next_idx = idx + 1
-        if self.forever:
-            next_idx = next_idx % number_of_items
+        next_idx = (idx + 1) % number_of_items
 
         while idx < number_of_items:
-            if idx == 0 and self.forever:
+            if idx == 0:
                 print("\n\n", end="")
                 print("==========================================================")
                 print("You ran the test suite with 'forever' mode enabled.")
@@ -228,9 +230,7 @@ def pytest_runtestloop(self, session: pytest.Session):
                 raise session.Interrupted(session.shouldstop)
 
             idx = next_idx
-            next_idx = idx + 1
-            if self.forever:
-                next_idx = next_idx % number_of_items
+            next_idx = (idx + 1) % number_of_items
 
         return True
 
@@ -407,6 +407,14 @@ def thread_comp(num_parallel_threads):
 
 
 def pytest_configure(config):
+    if (
+        config.option.forever
+        and (n := getattr(config.option, "numprocesses", None)) is not None
+    ):
+        raise pytest.UsageError(
+            f"--forever from pytest-run-parallel is incompatible with `-n {n}` from pytest-xdist."
+        )
+
     config.addinivalue_line(
         "markers",
         "parallel_threads(n): run the given test function in parallel "
@@ -491,7 +499,8 @@ def pytest_addoption(parser):
         default=False,
         help="Run the test loop forever (starting from the top when all the tests have been run), "
         "until one crashes or the user explicitly stops the process with Ctrl-C. This is especially "
-        "helpful for hitting thread safety bugs that only occur rarely.",
+        "helpful for hitting thread safety bugs that only occur rarely. --forever is not compatible "
+        "with -n from pytest-xdist.",
     )
     parser.addini(
         "thread_unsafe_fixtures",

tests/test_run_parallel.py

@@ -698,3 +698,32 @@ def test_warning_gil_enabled_ignore_option():
     """)
     result = pytester.runpytest("-v", "--ignore-gil-enabled", "-W", "default")
     assert result.ret == 0
+
+
+def test_runs_with_xdist(pytester):
+    pytester.makepyfile("""
+    def test_parallel1(num_parallel_threads):
+        assert num_parallel_threads == 10
+
+    def test_parallel2(num_parallel_threads):
+        assert num_parallel_threads == 10
+    """)
+    result = pytester.runpytest("--parallel-threads=10", "-n", "2", "-v")
+    result.stdout.fnmatch_lines(["*test_parallel1*"])
+    result.stdout.fnmatch_lines(["*test_parallel2*"])
+    result.stdout.fnmatch_lines(["*All tests were run in parallel!*"])
+    assert result.ret == 0
+
+
+def test_forever_with_xdist_errors(pytester):
+    pytester.makepyfile("""
+    def test_example():
+        assert True
+    """)
+    result = pytester.runpytest("--forever", "-n", "2")
+    assert result.ret != 0
+    result.stderr.fnmatch_lines(
+        [
+            "*--forever from pytest-run-parallel is incompatible with `-n 2` from pytest-xdist*"
+        ]
+    )