WIP Change -p so it is possible to early load setuptools plugins

-p accepts the option in the form of "module:plugin-name" which
can be used to specify the name of the entry point to early load

This fixes pytest-dev#4718

TODO: docs and tests

src/_pytest/config/__init__.py

@@ -482,19 +482,35 @@ def consider_preparse(self, args):
                     continue
                 self.consider_pluginarg(parg)
 
+    def _parse_early_load_arg(self, arg):
+        original_arg = arg
+        block = arg.startswith("no:")
+        if block:
+            arg = arg[len("no:") :]
+        fields = arg.split(":")
+        if len(fields) > 2:
+            raise UsageError(
+                '-p expected either plugin module or "mod:name" format, got {!r}'.format(
+                    original_arg
+                )
+            )
+        return block, fields[0], fields[1] if len(fields) == 2 else None
+
     def consider_pluginarg(self, arg):
-        if arg.startswith("no:"):
-            name = arg[3:]
+        block, modname, plugin_name = self._parse_early_load_arg(arg)
+        if block:
             # PR #4304 : remove stepwise if cacheprovider is blocked
-            if name == "cacheprovider":
+            if modname == "cacheprovider":
                 self.set_blocked("stepwise")
                 self.set_blocked("pytest_stepwise")
 
-            self.set_blocked(name)
-            if not name.startswith("pytest_"):
-                self.set_blocked("pytest_" + name)
+            self.set_blocked(modname)
+            if not modname.startswith("pytest_"):
+                self.set_blocked("pytest_" + modname)
+            if plugin_name:
+                self.set_blocked(plugin_name)
         else:
-            self.import_plugin(arg)
+            self.import_plugin(modname, plugin_name=plugin_name)
 
     def consider_conftest(self, conftestmodule):
         self.register(conftestmodule, name=conftestmodule.__file__)
@@ -510,7 +526,7 @@ def _import_plugin_specs(self, spec):
         for import_spec in plugins:
             self.import_plugin(import_spec)
 
-    def import_plugin(self, modname):
+    def import_plugin(self, modname, plugin_name=None):
         # most often modname refers to builtin modules, e.g. "pytester",
         # "terminal" or "capture".  Those plugins are registered under their
         # basename for historic purposes but must be imported with the
@@ -519,6 +535,8 @@ def import_plugin(self, modname):
             "module name as text required, got %r" % modname
         )
         modname = str(modname)
+        if not plugin_name:
+            plugin_name = modname
         if self.is_blocked(modname) or self.get_plugin(modname) is not None:
             return
         if modname in builtin_plugins:
@@ -548,7 +566,7 @@ def import_plugin(self, modname):
             )
         else:
             mod = sys.modules[importspec]
-            self.register(mod, modname)
+            self.register(mod, plugin_name)
 
 
 def _get_plugin_specs_as_list(specs):

testing/acceptance_test.py

@@ -108,6 +108,39 @@ def test_option(pytestconfig):
         assert result.ret == 0
         result.stdout.fnmatch_lines(["*1 passed*"])
 
+    def test_explicitly_early_load_existing_setuptools_plugin(
+        self, testdir, monkeypatch
+    ):
+        # WIP
+        pkg_resources = pytest.importorskip("pkg_resources")
+
+        def my_iter(name):
+            assert name == "pytest11"
+            return [EntryPoint()]
+
+        class EntryPoint:
+            name = "mytestplugin"
+            loaded = False
+
+            def load(self):
+                type(self).loaded = True
+                return sys.modules[self.name]
+
+            @property
+            def dist(self):
+                return self
+
+            def _get_metadata(self, *args):
+                return []
+
+        class Plugin(object):
+            pass
+
+        monkeypatch.setattr(pkg_resources, "iter_entry_points", my_iter)
+        monkeypatch.setitem(sys.modules, "mytestplugin", Plugin())
+        testdir.runpytest("-p", "mytestplugin")
+        assert EntryPoint.loaded
+
     def test_assertion_magic(self, testdir):
         p = testdir.makepyfile(
             """

testing/test_session.py

@@ -230,9 +230,6 @@ def test_minus_x_overridden_by_maxfail(self, testdir):
 def test_plugin_specify(testdir):
     with pytest.raises(ImportError):
         testdir.parseconfig("-p", "nqweotexistent")
-    # pytest.raises(ImportError,
-    #    "config.do_configure(config)"
-    # )
 
 
 def test_plugin_already_exists(testdir):