fix: address incompatibilities between ddtrace and the builtin multiprocessing library [backport 1.18] (#6756)

Backport 68becb8 from #6566 to 1.18.

## Description

- Resolves an import error raised in
`ddtrace/appsec/iast/taint_sinks/_base.py` when module unloading is
enabled.
- Removes inlined import in
`ddtrace/internal/remoteconfig/_connectors.py`. This will allow us to
unload all ddtrace references to multiprocessing library when
ddtrace-run is used.
- One nice side-effect of this PR is that it makes gevent compatible
with iast (I hope 🤞)

## Motivation

Resolves: #6511


[multiprocess.set_start_method(...)](https://docs.python.org/3/library/multiprocessing.html#multiprocessing.set_start_method)
should only be called in a process ONCE. Calling this method more than
once raises a [runtime
error](#6511). Using module
unloading and refactoring imports allows the ddtrace library to have a
unique reference to the multiprocessing library.

## Checklist

- [x] Change(s) are motivated and described in the PR description.
- [x] Testing strategy is described if automated tests are not included
in the PR.
- [x] Risk is outlined (performance impact, potential for breakage,
maintainability, etc).
- [x] Change is maintainable (easy to change, telemetry, documentation).
- [x] [Library release note
guidelines](https://ddtrace.readthedocs.io/en/stable/releasenotes.html)
are followed. If no release note is required, add label
`changelog/no-changelog`.
- [x] Documentation is included (in-code, generated user docs, [public
corp docs](https://github.com/DataDog/documentation/)).
- [x] Backport labels are set (if
[applicable](https://ddtrace.readthedocs.io/en/latest/contributing.html#backporting))

## Reviewer Checklist

- [x] Title is accurate.
- [x] No unnecessary changes are introduced.
- [x] Description motivates each change.
- [x] Avoids breaking
[API](https://ddtrace.readthedocs.io/en/stable/versioning.html#interfaces)
changes unless absolutely necessary.
- [x] Testing strategy adequately addresses listed risk(s).
- [x] Change is maintainable (easy to change, telemetry, documentation).
- [x] Release note makes sense to a user of the library.
- [x] Reviewer has explicitly acknowledged and discussed the performance
implications of this PR as reported in the benchmarks PR comment.
- [x] Backport labels are set in a manner that is consistent with the
[release branch maintenance
policy](https://ddtrace.readthedocs.io/en/latest/contributing.html#backporting)

Co-authored-by: Munir Abdinur <[email protected]>

Author: github-actions[bot]

ddtrace/internal/remoteconfig/_connectors.py

@@ -1,5 +1,6 @@
 from ctypes import c_char
 import json
+import multiprocessing
 import os
 import sys
 from typing import Any
@@ -38,8 +39,6 @@ class PublisherSubscriberConnector(object):
     """
 
     def __init__(self):
-        import multiprocessing
-
         self.data = multiprocessing.Array(c_char, SHARED_MEMORY_SIZE, lock=False)
         # Checksum attr validates if the Publisher send new data
         self.checksum = -1

releasenotes/notes/fix-monkey-patch-on-import-5cfb79bf2eea7c7e.yaml

@@ -0,0 +1,5 @@
+---
+issues:
+  - |
+    There are known issues configuring python's builtin multiprocessing library when ddtrace is installed.
+    To use the multiprocessing library with ddtrace ensure ``DD_UNLOAD_MODULES_FROM_SITECUSTOMIZE`` is set to ``True``.

tests/appsec/test_remoteconfiguration_e2e.py

@@ -374,3 +374,55 @@ def test_load_testing_appsec_1click_and_ip_blocking_gunicorn_block_and_kill_chil
         _unblock_ip(token)
 
         _request_200(gunicorn_client, debug_mode=False)
+
+
+@pytest.mark.parametrize(
+    "module_unloading_env",
+    [
+        True,
+        pytest.param(
+            False,
+            marks=pytest.mark.xfail(
+                reason="FIXME: multiprocessing is only supported when DD_UNLOAD_MODULES_FROM_SITECUSTOMIZE is set"
+            ),
+        ),
+    ],
+    ids=[
+        "module_unloading_enabled",
+        "module_unloading_unset",
+    ],
+)
+@pytest.mark.skipif(sys.version_info[0] < 3, reason="Python2.7 is not supported")
+def test_compatiblity_with_multiprocessing(module_unloading_env, ddtrace_run_python_code_in_subprocess):
+    """This test validates that module unloading resolves multiprocessing context errors"""
+    code = """
+import multiprocessing
+from multiprocessing import Process, Value, Array
+
+def f(n, a):
+    n.value = 420
+    for i in range(len(a)):
+        a[i] = i*10
+
+if __name__ == '__main__':
+    multiprocessing.set_start_method('spawn')
+    num = Value('d', 0.0)
+    arr = Array('i', range(10))
+
+    p = Process(target=f, args=(num, arr))
+    p.start()
+    p.join()
+
+    assert arr[:] == [0, 10, 20, 30, 40, 50, 60, 70, 80, 90]
+    assert num.value == 420
+    print("success")
+"""
+    env = os.environ.copy()
+    env["DD_REMOTE_CONFIGURATION_ENABLED"] = "true"
+
+    if module_unloading_env is True:
+        env["DD_UNLOAD_MODULES_FROM_SITECUSTOMIZE"] = "true"
+
+    out, stderr, status, _ = ddtrace_run_python_code_in_subprocess(code, env=env)
+    assert status == 0, stderr
+    assert out == b"success\n"