docs: add new sys.monitoring warnings

Author: nedbat

coverage/control.py

@@ -550,6 +550,7 @@ def _init_for_start(self) -> None:
             warn=self._warn,
             config=self.config,
             dynamic_contexts=(should_start_context is not None),
+            plugins=bool(self._plugins),
             metacov=self._metacov,
         )
         self._collector = Collector(
@@ -1399,9 +1400,6 @@ def process_startup() -> Coverage | None:
         # started coverage.py in this process.  Nothing to do.
         return None
 
-    # # This runs at the start of pytest xdist workers, but also at other times?
-    # warnings.filterwarnings("ignore", r".*no-sysmon")
-
     cov = Coverage(config_file=cps)
     process_startup.coverage = cov      # type: ignore[attr-defined]
     cov._warn_no_data = False

coverage/core.py

@@ -58,6 +58,7 @@ def __init__(
         warn: TWarnFn,
         config: CoverageConfig,
         dynamic_contexts: bool,
+        plugins: bool,
         metacov: bool,
     ) -> None:
         # Check the conditions that preclude us from using sys.monitoring.
@@ -68,6 +69,8 @@ def __init__(
             reason_no_sysmon = "can't measure branches in this version"
         elif dynamic_contexts:
             reason_no_sysmon = "doesn't yet support dynamic contexts"
+        elif plugins:
+            reason_no_sysmon = "doesn't yet support plugins"
 
         core_name: str | None = None
         if config.timid:

doc/cmd.rst

@@ -273,14 +273,31 @@ Conflicting dynamic contexts (dynamic-conflict)
   :meth:`.Coverage.switch_context` function to change the context. Only one of
   these mechanisms should be in use at a time.
 
-sys.monitoring isn't available, using default core (no-sysmon)
+sys.monitoring isn't available in this version, using default core (no-sysmon)
   You requested to use the sys.monitoring measurement core, but are running on
   Python 3.11 or lower where it isn't available.  A default core will be used
   instead.
 
+sys.monitoring can't measure branches in this version, using default core (no-sysmon)
+  You requested the sys.monitoring measurement core and also branch coverage.
+  This isn't supported until the later alphas of Python 3.14.  A default core
+  will be used instead.
+
+sys.monitoring doesn't yet support dynamic contexts, using default core (no-sysmon)
+  You requested the sys.monitoring measurement core and also dynamic contexts.
+  This isn't supported by coverage.py yet.  A default core will be used
+  instead.
+
+sys.monitoring doesn't yet support plugins, using default core (no-sysmon)
+  You requested the sys.monitoring measurement core and also are using some
+  plugins.  This isn't supported by coverage.py yet.  A default core will be
+  used instead.
+
 Individual warnings can be disabled with the :ref:`disable_warnings
-<config_run_disable_warnings>` configuration setting.  To silence "No data was
-collected," add this to your configuration file:
+<config_run_disable_warnings>` configuration setting.  It is a list of the
+short parenthetical nicknames in the warning messages.  For example, to silence
+"No data was collected (no-data-collected)", add this to your configuration
+file:
 
 .. [[[cog
     show_configs(

tests/test_process.py

@@ -1142,12 +1142,13 @@ def test_core_request_sysmon(self) -> None:
             assert core in ("core: CTracer", "core: PyTracer")
             assert warns
 
-    def test_core_request_nonsense(self) -> None:
+    def test_core_request_nosuchcore(self) -> None:
         self.del_environ("COVERAGE_TEST_CORES")
         self.set_environ("COVERAGE_CORE", "nosuchcore")
         self.make_file("numbers.py", "print(123, 456)")
         out = self.run_command("coverage run numbers.py")
         assert "Unknown core value: 'nosuchcore'\n" in out
+        assert "123 456" not in out
 
 
 class FailUnderNoFilesTest(CoverageTest):