Support cyclic-import message with --jobs

doc/user_guide/usage/run.rst

@@ -160,11 +160,6 @@ This will spawn 4 parallel Pylint sub-process, where each provided module will
 be checked in parallel. Discovered problems by checkers are not displayed
 immediately. They are shown just after checking a module is complete.
 
-There is one known limitation with running checks in parallel as currently
-implemented. Since the division of files into worker processes is indeterminate,
-checkers that depend on comparing multiple files (e.g. ``cyclic-import``
-and ``duplicate-code``) can produce indeterminate results.
-
 Exit codes
 ----------
 

doc/whatsnew/fragments/4171.bugfix

@@ -0,0 +1,3 @@
+Support ``cyclic-import`` message when parallelizing with ``--jobs``.
+
+Closes #4171

pylint/checkers/imports.py

@@ -455,6 +455,7 @@ def __init__(self, linter: PyLinter) -> None:
             ("RP0401", "External dependencies", self._report_external_dependencies),
             ("RP0402", "Modules dependencies graph", self._report_dependencies_graph),
         )
+        self._excluded_edges: defaultdict[str, set[str]] = defaultdict(set)
 
     def open(self) -> None:
         """Called before visiting project (i.e set of modules)."""
@@ -463,7 +464,6 @@ def open(self) -> None:
         self.import_graph = defaultdict(set)
         self._module_pkg = {}  # mapping of modules to the pkg they belong in
         self._current_module_package = False
-        self._excluded_edges: defaultdict[str, set[str]] = defaultdict(set)
         self._ignored_modules: Sequence[str] = self.linter.config.ignored_modules
         # Build a mapping {'module': 'preferred-module'}
         self.preferred_modules = dict(
@@ -488,6 +488,17 @@ def close(self) -> None:
             for cycle in get_cycles(graph, vertices=vertices):
                 self.add_message("cyclic-import", args=" -> ".join(cycle))
 
+    def get_map_data(self) -> defaultdict[str, set[str]]:
+        return self.import_graph
+
+    def reduce_map_data(
+        self, linter: PyLinter, data: list[defaultdict[str, set[str]]]
+    ) -> None:
+        self.import_graph = defaultdict(set)
+        for graph in data:
+            self.import_graph.update(graph)
+        self.close()
+
     def deprecated_modules(self) -> set[str]:
         """Callback returning the deprecated modules."""
         # First get the modules the user indicated

tests/test_check_parallel.py

@@ -13,6 +13,7 @@
 import sys
 from concurrent.futures import ProcessPoolExecutor
 from concurrent.futures.process import BrokenProcessPool
+from pathlib import Path
 from pickle import PickleError
 from typing import TYPE_CHECKING
 from unittest.mock import patch
@@ -23,11 +24,13 @@
 import pylint.interfaces
 import pylint.lint.parallel
 from pylint.checkers import BaseRawFileChecker
+from pylint.checkers.imports import ImportsChecker
 from pylint.lint import PyLinter, augmented_sys_path
 from pylint.lint.parallel import _worker_check_single_file as worker_check_single_file
 from pylint.lint.parallel import _worker_initialize as worker_initialize
 from pylint.lint.parallel import check_parallel
 from pylint.testutils import GenericTestReporter as Reporter
+from pylint.testutils.utils import _test_cwd
 from pylint.typing import FileItem
 from pylint.utils import LinterStats, ModuleStats
 
@@ -625,3 +628,30 @@ def test_no_deadlock_due_to_initializer_error(self) -> None:
                 # because arguments has to be an Iterable.
                 extra_packages_paths=1,  # type: ignore[arg-type]
             )
+
+    @pytest.mark.needs_two_cores
+    def test_cyclic_import_parallel(self) -> None:
+        tests_dir = Path("tests")
+        package_path = Path("input") / "func_w0401_package"
+        linter = PyLinter(reporter=Reporter())
+        linter.register_checker(ImportsChecker(linter))
+
+        with _test_cwd(tests_dir):
+            check_parallel(
+                linter,
+                jobs=2,
+                files=[
+                    FileItem(
+                        name="input.func_w0401_package.all_the_things",
+                        filepath=str(package_path / "all_the_things.py"),
+                        modpath="input.func_w0401_package",
+                    ),
+                    FileItem(
+                        name="input.func_w0401_package.thing2",
+                        filepath=str(package_path / "thing2.py"),
+                        modpath="input.func_w0401_package",
+                    ),
+                ],
+            )
+
+        assert "cyclic-import" in linter.stats.by_msg