Run tests in parallel, whenever possible

using tests/run-tests-parallel.py script like:
MONERO_PARALLEL_TEST_JOBS=nproc tests/run-tests-parallel.py

.github/workflows/build.yml

@@ -153,7 +153,7 @@ jobs:
         CTEST_OUTPUT_ON_FAILURE: ON
       run: |
         ${{env.BUILD_DEFAULT_LINUX}}
-        cmake --build build --target test
+        cd build && MONERO_PARALLEL_TEST_JOBS=2 ../tests/run-tests-parallel.py
 
 # ARCH="default" (not "native") ensures, that a different execution host can execute binaries compiled elsewhere.
 # BUILD_SHARED_LIBS=ON speeds up the linkage part a bit, reduces size, and is the only place where the dynamic linkage is tested.

tests/run-tests-parallel.py

@@ -0,0 +1,109 @@
+#!/usr/bin/env python3
+# -*- coding: utf-8 -*-
+"""
+# Copyright (c) 2014-2021, The Monero Project
+#
+# All rights reserved.
+#
+# Redistribution and use in source and binary forms, with or without modification, are
+# permitted provided that the following conditions are met:
+#
+# 1. Redistributions of source code must retain the above copyright notice, this list of
+#    conditions and the following disclaimer.
+#
+# 2. Redistributions in binary form must reproduce the above copyright notice, this list
+#    of conditions and the following disclaimer in the documentation and/or other
+#    materials provided with the distribution.
+#
+# 3. Neither the name of the copyright holder nor the names of its contributors may be
+#    used to endorse or promote products derived from this software without specific
+#    prior written permission.
+#
+# THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS" AND ANY
+# EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF
+# MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL
+# THE COPYRIGHT HOLDER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+# SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO,
+# PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
+# INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT,
+# STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF
+# THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+
+# The parallelism celing can be controlled with MONERO_PARALLEL_TEST_JOBS env variable,
+# for example:
+#
+# MONERO_PARALLEL_TEST_JOBS=1     tests/run-tests-parallel.py
+# MONERO_PARALLEL_TEST_JOBS=nproc tests/run-tests-parallel.py
+#
+# The minimum between the ceiling and a currently selected reasonable number of threads is used in the end.
+# The reasonable number is selected as a number, that delivers the solution in the shortest time.
+"""
+
+import os
+import sys
+import subprocess
+import multiprocessing
+from timeit import default_timer as timer
+
+NUM_PROC_MAX_REASONABLE = 2 # This might be increased, once the core_tests are divided into many more independent pieces or simply sped up
+TESTS_EXCLUDED_REGEX = "unit_tests|functional_tests_rpc" # These tests collide with core_tests
+ERR_CODE = 1
+
+def get_forced_job_ceiling():
+    try:
+        ceiling_str = os.environ['MONERO_PARALLEL_TEST_JOBS']
+    except KeyError:
+        ceiling = multiprocessing.cpu_count()
+        print("No parallelism ceiling selected. Defaulting to nproc, so", ceiling)
+    else:
+        if ceiling_str == "nproc":
+            ceiling = multiprocessing.cpu_count()
+        else:
+            ceiling = int(ceiling_str)
+
+        print("Parallelism ceiling selected. Using", ceiling, "jobs.")
+
+    return ceiling
+
+
+def run(num_proc_final, fail_fast=False):
+    """
+    Run the excluded tests first, then everything but excluded.
+    In the current situation, the excluded tests are the most probable and quickest to fail,
+    giving an early feedback when fail_fast is set to true.
+    """
+    cmds = []
+    cmds.append(get_ctest_command(num_proc_final, '-R'))
+    cmds.append(get_ctest_command(num_proc_final, '-E'))
+    
+    error = False
+    for cmd in cmds:
+        status = run_cmd(cmd)
+        if status != 0:
+            error = True
+            if fail_fast:
+                return status
+
+    return ERR_CODE if error else 0
+
+def get_ctest_command(num_proc_final, option):
+    cmd = ['ctest', '-j{}'.format(num_proc_final), option, TESTS_EXCLUDED_REGEX]
+    return cmd
+
+def run_cmd(cmd):
+    result = subprocess.run(cmd, stderr=sys.stderr, stdout=sys.stdout)
+    return result.returncode
+
+
+job_ceiling = get_forced_job_ceiling()
+num_proc_final = min(NUM_PROC_MAX_REASONABLE, job_ceiling)
+print("Job number ceiling is" ,job_ceiling, "and the reasonable maximum is currently", NUM_PROC_MAX_REASONABLE, ".")
+print("Using the minimum of the two, so", num_proc_final, "jobs.")
+
+start = timer()
+ret = run(num_proc_final, fail_fast=False)
+tdiff = timer() - start
+print("Testing took:", round(tdiff / 60), "minutes.")
+print("Status:", ret)
+
+exit(ret)