mapreduce| Fixes -jN for map/reduce Checkers (e.g. SimilarChecker)

This integrate the map/reduce functionality into lint.check_process().

We previously had `map` being invoked, here we add `reduce` support.

We do this by collecting the map-data by worker and then passing it to a
reducer function on the Checker object, if available - determined by
whether they confirm to the `mapreduce_checker.MapReduceMixin` mixin
interface or nor.

This allows Checker objects to function across file-streams when using
multiprocessing/-j2+. For example SimilarChecker needs to be able to
compare data across all files.

The tests, that we also add here, check that a Checker instance returns
and reports expected data and errors, such as error-messages and stats -
at least in a exit-ok (0) situation.

On a personal note, as we are copying more data across process
boundaries, I suspect that the memory implications of this might cause
issues for large projects already running with -jN and duplicate code
detection on. That said, given that it takes a long time to perform
lints of large code bases that is an issue for the [near?] future and
likely to be part of the performance work. Either way but let's get it
working first and deal with memory and perforamnce considerations later
- I say this as there are many quick wins we can make here, e.g.
file-batching, hashing lines, data compression and so on.

6 files changed, 105 insertions, 21 deletions

diff --git a/pylint/checkers/__init__.py b/pylint/checkers/__init__.py
index bfde22b3d..31d2df522 100644
--- a/pylint/checkers/__init__.py
+++ b/pylint/checkers/__init__.py
@@ -10,6 +10,7 @@
 # Copyright (c) 2018-2019 Pierre Sassoulas <pierre.sassoulas@gmail.com>
 # Copyright (c) 2018 ssolanki <sushobhitsolanki@gmail.com>
 # Copyright (c) 2019 Bruno P. Kinoshita <kinow@users.noreply.github.com>
+# Copyright (c) 2020 Frank Harrison <doublethefish@gmail.com>
 
 # Licensed under the GPL: https://www.gnu.org/licenses/old-licenses/gpl-2.0.html
 # For details: https://github.com/PyCQA/pylint/blob/master/COPYING
@@ -43,6 +44,7 @@ messages nor reports. XXX not true, emit a 07 report !
 """
 
 from pylint.checkers.base_checker import BaseChecker, BaseTokenChecker
+from pylint.checkers.mapreduce_checker import MapReduceMixin
 from pylint.utils import register_plugins
 
 
@@ -64,4 +66,10 @@ def initialize(linter):
     register_plugins(linter, __path__[0])
 
 
-__all__ = ["BaseChecker", "BaseTokenChecker", "initialize", "register_plugins"]
+__all__ = [
+    "BaseChecker",
+    "BaseTokenChecker",
+    "initialize",
+    "MapReduceMixin",
+    "register_plugins",
+]
diff --git a/pylint/checkers/mapreduce_checker.py b/pylint/checkers/mapreduce_checker.py
new file mode 100644
index 000000000..17a1090bf
--- /dev/null
+++ b/pylint/checkers/mapreduce_checker.py
@@ -0,0 +1,18 @@
+# Copyright (c) 2020 Frank Harrison <doublethefish@gmail.com>
+
+# Licensed under the GPL: https://www.gnu.org/licenses/old-licenses/gpl-2.0.html
+# For details: https://github.com/PyCQA/pylint/blob/master/COPYING
+import abc
+
+
+class MapReduceMixin(metaclass=abc.ABCMeta):
+    """ A mixin design to allow multiprocess/threaded runs of a Checker """
+
+    @abc.abstractmethod
+    def get_map_data(self):
+        """ Returns mergable/reducible data that will be examined """
+
+    @classmethod
+    @abc.abstractmethod
+    def reduce_map_data(cls, linter, data):
+        """ For a given Checker, receives data for all mapped runs """
diff --git a/pylint/checkers/similar.py b/pylint/checkers/similar.py
index 3ac071bb3..1f817ada2 100644
--- a/pylint/checkers/similar.py
+++ b/pylint/checkers/similar.py
@@ -31,7 +31,7 @@ from itertools import groupby
 
 import astroid
 
-from pylint.checkers import BaseChecker, table_lines_from_stats
+from pylint.checkers import BaseChecker, MapReduceMixin, table_lines_from_stats
 from pylint.interfaces import IRawChecker
 from pylint.reporters.ureports.nodes import Table
 from pylint.utils import decoding_stream
@@ -302,7 +302,7 @@ def report_similarities(sect, stats, old_stats):
 
 
 # wrapper to get a pylint checker from the similar class
-class SimilarChecker(BaseChecker, Similar):
+class SimilarChecker(BaseChecker, Similar, MapReduceMixin):
     """checks for similarities and duplicated code. This computation may be
     memory / CPU intensive, so you should disable it if you experiment some
     problems.
diff --git a/pylint/lint/parallel.py b/pylint/lint/parallel.py
index fa1e65d8f..fe9ce0605 100644
--- a/pylint/lint/parallel.py
+++ b/pylint/lint/parallel.py
@@ -67,28 +67,59 @@ def _worker_check_single_file(file_item):
 
     _worker_linter.open()
     _worker_linter.check_single_file(name, filepath, modname)
-
+    mapreduce_data = collections.defaultdict(list)
+    for checker in _worker_linter.get_checkers():
+        try:
+            data = checker.get_map_data()
+        except AttributeError:
+            continue
+        mapreduce_data[checker.name].append(data)
     msgs = [_get_new_args(m) for m in _worker_linter.reporter.messages]
     _worker_linter.reporter.reset()
     return (
+        id(multiprocessing.current_process()),
         _worker_linter.current_name,
         filepath,
         _worker_linter.file_state.base_name,
         msgs,
         _worker_linter.stats,
         _worker_linter.msg_status,
+        mapreduce_data,
     )
 
 
+def _merge_mapreduce_data(linter, all_mapreduce_data):
+    """ Merges map/reduce data across workers, invoking relevant APIs on checkers """
+    # First collate the data, preparing it so we can send it to the checkers for
+    # validation. The intent here is to collect all the mapreduce data for all checker-
+    # runs across processes - that will then be passed to a static method on the
+    # checkers to be reduced and further processed.
+    collated_map_reduce_data = collections.defaultdict(list)
+    for linter_data in all_mapreduce_data.values():
+        for run_data in linter_data:
+            for checker_name, data in run_data.items():
+                collated_map_reduce_data[checker_name].extend(data)
+
+    # Send the data to checkers that support/require consolidated data
+    original_checkers = linter.get_checkers()
+    for checker in original_checkers:
+        if checker.name in collated_map_reduce_data:
+            # Assume that if the check has returned map/reduce data that it has the
+            # reducer function
+            checker.reduce_map_data(linter, collated_map_reduce_data[checker.name])
+
+
 def check_parallel(linter, jobs, files, arguments=None):
-    """Use the given linter to lint the files with given amount of workers (jobs)"""
-    # The reporter does not need to be passed to worker processess, i.e. the reporter does
-    # not need to be pickleable
+    """Use the given linter to lint the files with given amount of workers (jobs)
+    This splits the work filestream-by-filestream. If you need to do work across
+    multiple files, as in the similarity-checker, then inherit from MapReduceMixin and
+    implement the map/reduce mixin functionality"""
+    # The reporter does not need to be passed to worker processes, i.e. the reporter does
     original_reporter = linter.reporter
     linter.reporter = None
 
     # The linter is inherited by all the pool's workers, i.e. the linter
-    # is identical to the linter object here. This is requred so that
+    # is identical to the linter object here. This is required so that
     # a custom PyLinter object can be used.
     initializer = functools.partial(_worker_initialize, arguments=arguments)
     pool = multiprocessing.Pool(jobs, initializer=initializer, initargs=[linter])
@@ -97,30 +128,36 @@ def check_parallel(linter, jobs, files, arguments=None):
     # correct reporter
     linter.set_reporter(original_reporter)
     linter.open()
-
-    all_stats = []
-
     try:
+        all_stats = []
+        all_mapreduce_data = collections.defaultdict(list)
+
+        # Maps each file to be worked on by a single _worker_check_single_file() call,
+        # collecting any map/reduce data by checker module so that we can 'reduce' it
+        # later.
         for (
+            worker_idx,  # used to merge map/reduce data across workers
             module,
             file_path,
             base_name,
             messages,
             stats,
             msg_status,
+            mapreduce_data,
         ) in pool.imap_unordered(_worker_check_single_file, files):
             linter.file_state.base_name = base_name
             linter.set_current_module(module, file_path)
             for msg in messages:
                 msg = Message(*msg)
                 linter.reporter.handle_message(msg)
-
             all_stats.append(stats)
+            all_mapreduce_data[worker_idx].append(mapreduce_data)
             linter.msg_status |= msg_status
     finally:
         pool.close()
         pool.join()
 
+    _merge_mapreduce_data(linter, all_mapreduce_data)
     linter.stats = _merge_stats(all_stats)
 
     # Insert stats data to local checkers.
diff --git a/tests/test_check_parallel.py b/tests/test_check_parallel.py
index e8f67f4b6..c45b0b3b9 100644
--- a/tests/test_check_parallel.py
+++ b/tests/test_check_parallel.py
@@ -103,9 +103,17 @@ class TestCheckParallelFramework:
     def test_worker_check_single_file_no_checkers(self):
         linter = PyLinter(reporter=Reporter())
         worker_initialize(linter=linter)
-        (name, _, _, msgs, stats, msg_status) = worker_check_single_file(
-            _gen_file_data()
-        )
+
+        (
+            _,  # proc-id
+            name,
+            _,  # file_path
+            _,  # base_name
+            msgs,
+            stats,
+            msg_status,
+            _,  # mapreduce_data
+        ) = worker_check_single_file(_gen_file_data())
         assert name == "--test-file_data-name-0--"
         assert [] == msgs
         no_errors_status = 0
@@ -140,9 +148,16 @@ class TestCheckParallelFramework:
         # Add the only checker we care about in this test
         linter.register_checker(SequentialTestChecker(linter))
 
-        (name, _, _, msgs, stats, msg_status) = worker_check_single_file(
-            _gen_file_data()
-        )
+        (
+            _,  # proc-id
+            name,
+            _,  # file_path
+            _,  # base_name
+            msgs,
+            stats,
+            msg_status,
+            _,  # mapreduce_data
+        ) = worker_check_single_file(_gen_file_data())
 
         # Ensure we return the same data as the single_file_no_checkers test
         assert name == "--test-file_data-name-0--"
diff --git a/tests/test_self.py b/tests/test_self.py
index cd138e789..e8f9f848f 100644
--- a/tests/test_self.py
+++ b/tests/test_self.py
@@ -46,7 +46,7 @@ from unittest import mock
 
 import pytest
 
-from pylint.constants import MAIN_CHECKER_NAME
+from pylint.constants import MAIN_CHECKER_NAME, MSG_TYPES_STATUS
 from pylint.lint import Run
 from pylint.reporters import JSONReporter
 from pylint.reporters.text import BaseReporter, ColorizedTextReporter, TextReporter
@@ -243,13 +243,19 @@ class TestRunTC:
         )
 
     def test_parallel_execution(self):
+        out = StringIO()
         self._runtest(
             [
                 "-j 2",
                 join(HERE, "functional", "a", "arguments.py"),
-                join(HERE, "functional", "a", "arguments.py"),
             ],
-            code=2,
+            out=out,
+            # We expect similarities to fail and an error
+            code=MSG_TYPES_STATUS["E"],
+        )
+        assert (
+            "Unexpected keyword argument 'fourth' in function call"
+            in out.getvalue().strip()
         )
 
     def test_parallel_execution_missing_arguments(self):