test(profiling): add a test for multiprocessing (#2934)

We test fork(), but we don't test higher level frameworks.
Most Python frameworks using multiple processes are based on `multiprocessing`.
Add a simple test that shows we do collect profiles for subprocesses when using
multiprocessing.

Co-authored-by: mergify[bot] <37929162+mergify[bot]@users.noreply.github.com>

Author: Julien Danjou

tests/profiling/_test_multiprocessing.py

@@ -0,0 +1,16 @@
+import multiprocessing
+import sys
+import time
+
+
+def f():
+    time.sleep(1)
+
+
+if __name__ == "__main__":
+    multiprocessing.set_start_method(sys.argv[1])
+
+    p = multiprocessing.Process(target=f)
+    p.start()
+    print(p.pid)
+    p.join()

tests/profiling/test_main.py

@@ -1,4 +1,6 @@
+import multiprocessing
 import os
+import sys
 
 import pytest
 
@@ -62,3 +64,32 @@ def test_fork_gevent(monkeypatch):
     monkeypatch.setenv("DD_PROFILING_API_TIMEOUT", "0.1")
     stdout, stderr, exitcode, pid = call_program("python", os.path.join(os.path.dirname(__file__), "gevent_fork.py"))
     assert exitcode == 0
+
+
+atleast_py37 = sys.version_info[:2] >= (3, 7)
+
+if atleast_py37:
+    methods = multiprocessing.get_all_start_methods()
+else:
+    methods = []
+
+
+@pytest.mark.parametrize(
+    "method",
+    methods,
+)
+def test_multiprocessing(method, tmp_path, monkeypatch):
+    filename = str(tmp_path / "pprof")
+    monkeypatch.setenv("DD_PROFILING_OUTPUT_PPROF", filename)
+    monkeypatch.setenv("DD_PROFILING_ENABLED", "1")
+    monkeypatch.setenv("DD_PROFILING_UPLOAD_INTERVAL", "0.1")
+    stdout, stderr, exitcode, pid = call_program(
+        "ddtrace-run",
+        "python",
+        os.path.join(os.path.dirname(__file__), "_test_multiprocessing.py"),
+        method,
+    )
+    assert exitcode == 0, (stdout, stderr)
+    child_pid = stdout.decode().strip()
+    utils.check_pprof_file(filename + "." + str(pid) + ".1")
+    utils.check_pprof_file(filename + "." + str(child_pid) + ".1")