Allow running tests in parallel with pytest-xdist

The blocker for this was that gevent monkeypatching broke xdist. By
skipping `subprocess` and `thread` monkeypatching, this can be avoided.

We also need to make sure that not all processes choose the same port.
At least in my tests, choosing a free port by binding to port 0 did not
cause problematic race conditions, although I'm not sure that it always
works.

.travis.yml

@@ -10,5 +10,5 @@ script:
   - pip install -r requirements-dev.txt
   - pip install -e .
   - make lint
-  - pytest -v --cov=./ tests/
+  - pytest -v --cov=./ tests/ -n auto
   - codecov

requirements-dev.txt

@@ -18,6 +18,7 @@ pytest==4.4.1
 pytest-runner
 pytest-cov
 pytest-structlog==0.1
+pytest-xdist
 coverage>=4.5.2
 
 ipython==4.2.1

src/pathfinding_service/cli.py

@@ -3,7 +3,7 @@
 
 # there were some issues with the 'thread' resolver, remove it from the options
 config.resolver = ["dnspython", "ares", "block"]  # noqa
-monkey.patch_all()  # isort:skip # noqa
+monkey.patch_all(subprocess=False, thread=False)  # isort:skip # noqa
 
 from typing import Dict
 

src/request_collector/cli.py

@@ -1,6 +1,6 @@
 from gevent import monkey  # isort:skip # noqa
 
-monkey.patch_all()  # isort:skip # noqa
+monkey.patch_all(subprocess=False, thread=False)  # isort:skip # noqa
 
 import click
 import structlog

tests/conftest.py

@@ -2,7 +2,7 @@
 
 # there were some issues with the 'thread' resolver, remove it from the options
 config.resolver = ["dnspython", "ares", "block"]  # noqa
-monkey.patch_all()  # isort:skip # noqa
+monkey.patch_all(subprocess=False, thread=False)  # isort:skip # noqa
 
 import gc
 

tests/pathfinding/fixtures/api.py

@@ -1,53 +1,21 @@
 # pylint: disable=redefined-outer-name
-from itertools import count
+import socket
 from typing import Iterator
 
-import psutil
 import pytest
 
 from pathfinding_service.api import ServiceApi
-from pathfinding_service.config import API_PATH, DEFAULT_API_PORT
+from pathfinding_service.config import API_PATH
 from raiden.utils.typing import Address
 
 
-def get_free_port(address: str, initial_port: int):
-    """Find an unused TCP port in a specified range. This should not
-      be used in misson-critical applications - a race condition may
-      occur if someone grabs the port before caller of this function
-      has chance to use it.
-      Parameters:
-          address : an ip address of interface to use
-          initial_port : port to start iteration with
-      Return:
-          Iterator that will return next unused port on a specified
-          interface
-    """
-
-    try:
-        # On OSX this function requires root privileges
-        psutil.net_connections()
-    except psutil.AccessDenied:
-        return count(initial_port)
-
-    def _unused_ports():
-        for port in count(initial_port):
-            # check if the port is being used
-            connect_using_port = (
-                conn
-                for conn in psutil.net_connections()
-                if hasattr(conn, "laddr") and conn.laddr[0] == address and conn.laddr[1] == port
-            )
-
-            # only generate unused ports
-            if not any(connect_using_port):
-                yield port
-
-    return _unused_ports()
-
-
 @pytest.fixture(scope="session")
 def free_port() -> int:
-    return next(get_free_port("localhost", DEFAULT_API_PORT))
+    sock = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
+    sock.bind(("localhost", 0))  # binding to port 0 will choose a free socket
+    port = sock.getsockname()[1]
+    sock.close()
+    return port
 
 
 @pytest.fixture(scope="session")