chore(gevent): run after-in-child hooks after reinit

Threads created too early in an application that uses gevent end up not
running after the gevent hub reinit is executed after fork in the child
process. This change ensures that we trigger the after-in-child fork
hooks after the call to gevent.hub.reinit to ensure that threads created
at any time will run as expected after fork.

ddtrace/__init__.py

@@ -1,4 +1,9 @@
-from ._logger import configure_ddtrace_logger
+from ddtrace.internal.module import ModuleWatchdog
+
+
+ModuleWatchdog.install()
+
+from ._logger import configure_ddtrace_logger  # noqa: E402
 
 
 # configure ddtrace logger before other modules log

ddtrace/internal/forksafe.py

@@ -7,6 +7,7 @@
 import typing
 import weakref
 
+from ddtrace.internal.module import ModuleWatchdog
 from ddtrace.vendor import wrapt
 
 
@@ -22,6 +23,25 @@
 _soft = False
 
 
+def patch_gevent_hub_reinit(module):
+    # The gevent hub is re-initialized *after* the after-in-child fork hooks are
+    # called, so we patch the gevent.hub.reinit function to ensure that the
+    # fork hooks run again after this further re-initialisation, if it is ever
+    # called.
+    from ddtrace.internal.wrapping import wrap
+
+    def wrapped_reinit(f, args, kwargs):
+        try:
+            return f(*args, **kwargs)
+        finally:
+            ddtrace_after_in_child()
+
+    wrap(module.reinit, wrapped_reinit)
+
+
+ModuleWatchdog.register_module_hook("gevent.hub", patch_gevent_hub_reinit)
+
+
 def ddtrace_after_in_child():
     # type: () -> None
     global _registry

tests/internal/test_module.py

@@ -9,6 +9,21 @@
 import tests.test_module
 
 
+@pytest.fixture(autouse=True, scope="module")
+def ensure_no_module_watchdog():
+    # DEV: The library might use the ModuleWatchdog and install it at a very
+    # early stage. This fixture ensures that the watchdog is not installed
+    # before the tests start.
+    was_installed = ModuleWatchdog.is_installed()
+    if was_installed:
+        ModuleWatchdog.uninstall()
+
+    yield
+
+    if was_installed:
+        ModuleWatchdog.install()
+
+
 @pytest.fixture
 def module_watchdog():
     ModuleWatchdog.install()
@@ -65,7 +80,10 @@ def test_import_origin_hook_for_module_not_yet_imported():
     path = os.getenv("MODULE_ORIGIN")
     hook = mock.Mock()
 
-    ModuleWatchdog.install()
+    try:
+        ModuleWatchdog.install()
+    except RuntimeError:
+        pass
 
     ModuleWatchdog.register_origin_hook(path, hook)
 
@@ -100,7 +118,10 @@ def test_import_module_hook_for_module_not_yet_imported():
     name = "tests.test_module"
     hook = mock.Mock()
 
-    ModuleWatchdog.install()
+    try:
+        ModuleWatchdog.install()
+    except RuntimeError:
+        pass
 
     ModuleWatchdog.register_module_hook(name, hook)
 
@@ -136,7 +157,10 @@ def test_module_deleted():
     path = os.getenv("MODULE_ORIGIN")
     hook = mock.Mock()
 
-    ModuleWatchdog.install()
+    try:
+        ModuleWatchdog.install()
+    except RuntimeError:
+        pass
 
     ModuleWatchdog.register_origin_hook(path, hook)
     ModuleWatchdog.register_module_hook(name, hook)