Revert psutil dependency and introduce pytest_xdist_auto_num_workers …

…hook

This reverts the behavior we had before psutil was a hard dependency, but
opens up the possibility of customization through the
pytest_xdist_auto_num_workers hook, making things like pytest-dev#477 possible.

Fix pytest-dev#585

changelog/585.feature.rst

@@ -0,0 +1 @@
+New ``pytest_xdist_auto_num_workers`` hook can be implemented by plugins or ``conftets.py`` to control the number of workers when ``--numprocesses=auto`` is given in the command-line.

changelog/585.trivial.rst

@@ -0,0 +1,7 @@
+``psutil`` is no longer a dependency, as it has proven to make ``pytest-xdist`` installation in certain platforms and containers problematic.
+
+To those interested to continue to use ``psutil`` to detect the number of CPUs, the new ``pytest_xdist_auto_num_cpus`` hook can be used in the root ``conftest.py`` file of the project::
+
+    def pytest_xdist_auto_num_workers(config):
+        import psutil
+        return psutil.cpu_count(logical=False)

src/xdist/newhooks.py

@@ -55,3 +55,13 @@ def pytest_xdist_node_collection_finished(node, ids):
 @pytest.mark.firstresult
 def pytest_xdist_make_scheduler(config, log):
     """ return a node scheduler implementation """
+
+
+@pytest.mark.firstresult
+def pytest_xdist_auto_num_workers(config):
+    """
+    Return the number of workers to spawn when ``--numprocesses=auto`` is given in the
+    command-line.
+
+    .. versionadded:: 2.1
+    """

src/xdist/plugin.py

@@ -5,7 +5,7 @@
 import pytest
 
 
-def auto_detect_cpus():
+def pytest_xdist_auto_num_workers():
     try:
         from os import sched_getaffinity
     except ImportError:
@@ -28,13 +28,9 @@ def cpu_count():
     return n if n else 1
 
 
-class AutoInt(int):
-    """Mark value as auto-detected."""
-
-
 def parse_numprocesses(s):
     if s == "auto":
-        return AutoInt(auto_detect_cpus())
+        return "auto"
     elif s is not None:
         return int(s)
 
@@ -187,12 +183,13 @@ def pytest_configure(config):
 @pytest.mark.tryfirst
 def pytest_cmdline_main(config):
     usepdb = config.getoption("usepdb", False)  # a core option
-    if isinstance(config.option.numprocesses, AutoInt):
+    if config.option.numprocesses == "auto":
         if usepdb:
             config.option.numprocesses = 0
             config.option.dist = "no"
         else:
-            config.option.numprocesses = int(config.option.numprocesses)
+            auto_num_cpus = config.hook.pytest_xdist_auto_num_workers(config=config)
+            config.option.numprocesses = auto_num_cpus
 
     if config.option.numprocesses:
         if config.option.dist == "no":

testing/test_plugin.py

@@ -51,6 +51,7 @@ def test_auto_detect_cpus(testdir, monkeypatch):
     assert config.getoption("numprocesses") == 2
 
     config = testdir.parseconfigure("-nauto")
+    check_options(config)
     assert config.getoption("numprocesses") == 99
 
     config = testdir.parseconfigure("-nauto", "--pdb")
@@ -62,9 +63,24 @@ def test_auto_detect_cpus(testdir, monkeypatch):
     monkeypatch.delattr(os, "sched_getaffinity", raising=False)
     monkeypatch.setenv("TRAVIS", "true")
     config = testdir.parseconfigure("-nauto")
+    check_options(config)
     assert config.getoption("numprocesses") == 2
 
 
+def test_hook_auto_num_workers(testdir, monkeypatch):
+    from xdist.plugin import pytest_cmdline_main as check_options
+
+    testdir.makeconftest(
+        """
+        def pytest_xdist_auto_num_workers():
+            return 42
+    """
+    )
+    config = testdir.parseconfigure("-nauto")
+    check_options(config)
+    assert config.getoption("numprocesses") == 42
+
+
 def test_boxed_with_collect_only(testdir):
     from xdist.plugin import pytest_cmdline_main as check_options