project tests: DummyExecutor for MSYS2 and OpenBSD

Added and tested on MSYS2/MinGW which doesn't implement the required
semaphore locks in the multiprocessing module:

Traceback (most recent call last):
  File "C:/msys64/mingw64/lib/python3.5\multiprocessing\synchronize.py", line 29, in <module>
    from _multiprocessing import SemLock, sem_unlink
ImportError: cannot import name 'sem_unlink'

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "run_project_tests.py", line 560, in <module>
    (passing_tests, failing_tests, skipped_tests) = run_tests(all_tests, 'meson-test-run', options.extra_args)
  File "run_project_tests.py", line 406, in run_tests
    executor = conc.ProcessPoolExecutor(max_workers=num_workers)
  File "F:/msys64/mingw64/lib/python3.5\concurrent\futures\process.py", line 390, in __init__
    EXTRA_QUEUED_CALLS)
  File "F:/msys64/mingw64/lib/python3.5\multiprocessing\context.py", line 101, in Queue
    return Queue(maxsize, ctx=self.get_context())
  File "F:/msys64/mingw64/lib/python3.5\multiprocessing\queues.py", line 42, in __init__
    self._rlock = ctx.Lock()
  File "F:/msys64/mingw64/lib/python3.5\multiprocessing\context.py", line 65, in Lock
    from .synchronize import Lock
  File "F:/msys64/mingw64/lib/python3.5\multiprocessing\synchronize.py", line 34, in <module>
    " function, see issue 3770.")
ImportError: This platform lacks a functioning sem_open implementation, therefore, the required synchronization primitives needed will not function, see issue 3770.

See also:
https://bugs.python.org/issue3770
https://github.com/mesonbuild/meson/issues/1323

According to 3770, the same problem also exists on OpenBSD, so this
will potentially also be useful there.

run_project_tests.py

@@ -34,6 +34,7 @@ import concurrent.futures as conc
 
 from mesonbuild.coredata import backendlist
 
+
 class BuildStep(Enum):
     configure = 1
     build = 2
@@ -41,6 +42,7 @@ class BuildStep(Enum):
     install = 4
     clean = 5
 
+
 class TestResult:
     def __init__(self, msg, step, stdo, stde, mlog, conftime=0, buildtime=0, testtime=0):
         self.msg = msg
@@ -52,6 +54,54 @@ class TestResult:
         self.buildtime = buildtime
         self.testtime = testtime
 
+class DummyFuture(conc.Future):
+    '''
+    Dummy Future implementation that executes the provided function when you
+    ask for the result. Used on platforms where sem_open() is not available:
+    MSYS2, OpenBSD, etc: https://bugs.python.org/issue3770
+    '''
+    def __init__(self, *args, **kwargs):
+        super().__init__(*args, **kwargs)
+
+    def set_function(self, fn, *args, **kwargs):
+        self.fn = fn
+        self.fn_args = args
+        self.fn_kwargs = kwargs
+
+    def result(self, **kwargs):
+        try:
+            result = self.fn(*self.fn_args, **self.fn_kwargs)
+        except BaseException as e:
+            self.set_exception(e)
+        else:
+            self.set_result(result)
+        return super().result(**kwargs)
+
+
+class DummyExecutor(conc.Executor):
+    '''
+    Dummy single-thread 'concurrent' executor for use on platforms where
+    sem_open is not available: https://bugs.python.org/issue3770
+    '''
+
+    def __init__(self):
+        from threading import Lock
+        self._shutdown = False
+        self._shutdownLock = Lock()
+
+    def submit(self, fn, *args, **kwargs):
+        with self._shutdownLock:
+            if self._shutdown:
+                raise RuntimeError('Cannot schedule new futures after shutdown')
+            f = DummyFuture()
+            f.set_function(fn, *args, **kwargs)
+            return f
+
+    def shutdown(self, wait=True):
+        with self._shutdownLock:
+            self._shutdown = True
+
+
 class AutoDeletedDir:
     def __init__(self, d):
         self.dir = d
@@ -421,7 +471,11 @@ def run_tests(all_tests, log_name_base, extra_args):
         print('Could not determine number of CPUs due to the following reason:' + str(e))
         print('Defaulting to using only one process')
         num_workers = 1
-    executor = conc.ProcessPoolExecutor(max_workers=num_workers)
+    try:
+        executor = conc.ProcessPoolExecutor(max_workers=num_workers)
+    except ImportError:
+        print('Platform doesn\'t ProcessPoolExecutor, falling back to single-threaded testing\n')
+        executor = DummyExecutor()
 
     for name, test_cases, skipped in all_tests:
         current_suite = ET.SubElement(junit_root, 'testsuite', {'name': name, 'tests': str(len(test_cases))})
@@ -441,6 +495,7 @@ def run_tests(all_tests, log_name_base, extra_args):
             result = executor.submit(run_test, skipped, t, extra_args, unity_flags + backend_flags, compile_commands, should_fail)
             futures.append((testname, t, result))
         for (testname, t, result) in futures:
+            sys.stdout.flush()
             result = result.result()
             if result is None or 'MESON_SKIP_TEST' in result.stdo:
                 print('Skipping:', t)