chore(gevent): run after-in-child hooks after reinit (#4070)

## Description

Threads created too early in an application that uses gevent end up not running when 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.

### More details

In its implementation, `gevent` wraps around `os.fork` and calls `gevent.hub.reinit` to re-initialise the state of the child process after fork. The `forksafe` hooks that are registered by the library end up running after the call to the OS `fork` syscall, but *before* `gevent.hub.reinit`, which causes the threads to not work as expected in the child process. Other frameworks, like `gunicorn` make their own call to `os.fork` and then call `gevent.hub.reinit` in the child process to obtain the same effect. This poses the same problem as in plain `gevent`. With this change, we register an import hook on `gevent.hub` to detect the possibility that `gevent.hub.reinit` might be called, and we patch this function to re-run the after-in-child fork hook to ensure that they run *after* the gevent hub has been reinitialised fully in the child process after fork. In particular, this ensures that threads are recreated at the right time and can then work as expected in child processes.

## Checklist
- [x] Title must conform to [conventional commit](https://github.com/conventional-changelog/commitlint/tree/master/%40commitlint/config-conventional).
- [x] Add additional sections for `feat` and `fix` pull requests.
- [x] Ensure tests are passing for affected code.
- [x] [Library documentation](https://github.com/DataDog/dd-trace-py/tree/1.x/docs) and/or [Datadog's documentation site](https://github.com/DataDog/documentation/) is updated. Link to doc PR in description.

## Reviewer Checklist
- [ ] Title is accurate.
- [ ] Description motivates each change.
- [ ] No unnecessary changes were introduced in this PR.
- [ ] PR cannot be broken up into smaller PRs.
- [ ] Avoid breaking [API](https://ddtrace.readthedocs.io/en/stable/versioning.html#interfaces) changes unless absolutely necessary.
- [ ] Tests provided or description of manual testing performed is included in the code or PR.
- [ ] Release note has been added for fixes and features, or else `changelog/no-changelog` label added.
- [ ] All relevant GitHub issues are correctly linked.
- [ ] Backports are identified and tagged with Mergifyio.
- [ ] Add to milestone.

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

riotfile.py

@@ -246,6 +246,7 @@ def select_pys(min_version=MIN_PYTHON_VERSION, max_version=MAX_PYTHON_VERSION):
                         "structlog": latest,
                         # httpretty v1.0 drops python 2.7 support
                         "httpretty": "==0.9.7",
+                        "gevent": latest,
                     },
                 )
             ],

tests/tracer/test_forksafe.py

@@ -1,4 +1,5 @@
 import os
+import sys
 
 import pytest
 import six
@@ -285,3 +286,59 @@ def fn():
     pid, status = os.waitpid(child, 0)
     exit_code = os.WEXITSTATUS(status)
     assert exit_code == 42
+
+
+@pytest.mark.subprocess(
+    out="" if (3,) < sys.version_info < (3, 7) else ("CTCTCT" if sys.platform == "darwin" else "CCCTTT"),
+    err=None,
+)
+def test_gevent_reinit_patch():
+    import os
+    import sys
+
+    from ddtrace.internal import forksafe
+    from ddtrace.internal.periodic import PeriodicService
+
+    class TestService(PeriodicService):
+        def __init__(self):
+            super(TestService, self).__init__(interval=1.0)
+
+        def periodic(self):
+            sys.stdout.write("T")
+
+    service = TestService()
+    service.start()
+
+    def restart_service():
+        global service
+
+        service.stop()
+        service = TestService()
+        service.start()
+
+    forksafe.register(restart_service)
+
+    import gevent  # noqa
+
+    def run_child():
+        global service
+
+        # We mimic what gunicorn does in child processes
+        gevent.monkey.patch_all()
+        gevent.hub.reinit()
+
+        sys.stdout.write("C")
+
+        gevent.sleep(1.5)
+
+        service.stop()
+
+    def fork_workers(num):
+        for _ in range(num):
+            if os.fork() == 0:
+                run_child()
+                sys.exit(0)
+
+    fork_workers(3)
+
+    service.stop()