fix: fewer warnings, default to sys.monitoring on 3.14+

Don't emit no-ctracer warnings on Python versions where we don't ship
compiled wheels.

Author: nedbat

coverage/core.py

@@ -86,13 +86,12 @@ def __init__(
             core_name = None
 
         if core_name is None:
-            # Someday we will default to sysmon, but it's still experimental:
-            #   if not reason_no_sysmon:
-            #       core_name = "sysmon"
-            if CTRACER_FILE:
+            if env.SYSMON_DEFAULT and not reason_no_sysmon:
+                core_name = "sysmon"
+            elif CTRACER_FILE:
                 core_name = "ctrace"
             else:
-                if env.CPYTHON and IMPORT_ERROR:
+                if IMPORT_ERROR and env.SHIPPING_WHEELS:
                     warn(f"Couldn't import C tracer: {IMPORT_ERROR}", slug="no-ctracer", once=True)
                 core_name = "pytrace"
 

coverage/env.py

@@ -43,6 +43,12 @@
 # Do we have a GIL?
 GIL = getattr(sys, '_is_gil_enabled', lambda: True)()
 
+# Do we ship compiled coveragepy wheels for this version?
+SHIPPING_WHEELS = CPYTHON and PYVERSION[:2] <= (3, 13)
+
+# Should we default to sys.monitoring?
+SYSMON_DEFAULT = CPYTHON and PYVERSION >= (3, 14)
+
 # Python behavior.
 class PYBEHAVIOR:
     """Flags indicating this Python's behavior."""

tests/test_process.py

@@ -1133,11 +1133,12 @@ def test_core_default(self) -> None:
         self.make_file("numbers.py", "print(123, 456)")
         out = self.run_command("coverage run --debug=sys numbers.py")
         assert out.endswith("123 456\n")
-        core = re_line(r" core:", out).strip()
-        # if env.PYBEHAVIOR.pep669:
-        #     assert core == "core: SysMonitor"
         warns = re_lines(r"\(no-ctracer\)", out)
-        if self.has_ctracer:
+        core = re_line(r" core:", out).strip()
+        if env.SYSMON_DEFAULT:
+            assert core == "core: SysMonitor"
+            assert not warns
+        elif self.has_ctracer:
             assert core == "core: CTracer"
             assert not warns
         else: