gh-110722: Make `-m test -T -j` use sys.monitoring (GH-111710)

Now all results from worker processes are aggregated and
displayed together as a summary at the end of a regrtest run.

The traditional trace is left in place for use with sequential
in-process test runs but now raises a warning that those
numbers are not precise.

`-T -j` requires `--with-pydebug` as it relies on `-Xpresite=`.

1 files changed, 11 insertions, 3 deletions

diff --git a/Lib/test/libregrtest/cmdline.py b/Lib/test/libregrtest/cmdline.py
index 1747511..a5f02d6 100644
--- a/Lib/test/libregrtest/cmdline.py
+++ b/Lib/test/libregrtest/cmdline.py
@@ -2,7 +2,7 @@ import argparse
 import os.path
 import shlex
 import sys
-from test.support import os_helper
+from test.support import os_helper, Py_DEBUG
 from .utils import ALL_RESOURCES, RESOURCE_NAMES
 
 
@@ -448,8 +448,16 @@ def _parse_args(args, **kwargs):
 
     if ns.single and ns.fromfile:
         parser.error("-s and -f don't go together!")
-    if ns.use_mp is not None and ns.trace:
-        parser.error("-T and -j don't go together!")
+    if ns.trace:
+        if ns.use_mp is not None:
+            if not Py_DEBUG:
+                parser.error("need --with-pydebug to use -T and -j together")
+        else:
+            print(
+                "Warning: collecting coverage without -j is imprecise. Configure"
+                " --with-pydebug and run -m test -T -j for best results.",
+                file=sys.stderr
+            )
     if ns.python is not None:
         if ns.use_mp is None:
             parser.error("-p requires -j!")