wip: switch from BRANCH_TAKEN to BRANCH_RIGHT

Author: nedbat

coverage/control.py

@@ -534,7 +534,7 @@ def _init_for_start(self) -> None:
 
         self._core = Core(
             warn=self._warn,
-            timid=self.config.timid,
+            config=self.config,
             metacov=self._metacov,
         )
         self._collector = Collector(

coverage/core.py

@@ -10,6 +10,7 @@
 from typing import Any
 
 from coverage import env
+from coverage.config import CoverageConfig
 from coverage.disposition import FileDisposition
 from coverage.exceptions import ConfigError
 from coverage.misc import isolate_module
@@ -52,16 +53,9 @@ class Core:
     packed_arcs: bool
     systrace: bool
 
-    def __init__(self,
-        warn: TWarnFn,
-        timid: bool,
-        metacov: bool,
-    ) -> None:
-        # Defaults
-        self.tracer_kwargs = {}
-
+    def __init__(self, warn: TWarnFn, config: CoverageConfig, metacov: bool) -> None:
         core_name: str | None
-        if timid:
+        if config.timid:
             core_name = "pytrace"
         else:
             core_name = os.getenv("COVERAGE_CORE")
@@ -70,6 +64,13 @@ def __init__(self,
                 warn("sys.monitoring isn't available, using default core", slug="no-sysmon")
                 core_name = None
 
+            # if core_name == "sysmon" and config.branch and not env.PYBEHAVIOR.branch_right_left:
+            #     warn(
+            #         "sys.monitoring can't yet measure branches well, using default core",
+            #         slug="no-sysmon",
+            #     )
+            #     core_name = None
+
             if not core_name:
                 # Once we're comfortable with sysmon as a default:
                 # if env.PYBEHAVIOR.pep669 and self.should_start_context is None:
@@ -79,9 +80,11 @@ def __init__(self,
                 else:
                     core_name = "pytrace"
 
+        self.tracer_kwargs = {}
+
         if core_name == "sysmon":
             self.tracer_class = SysMonitor
-            self.tracer_kwargs = {"tool_id": 3 if metacov else 1}
+            self.tracer_kwargs["tool_id"] = 3 if metacov else 1
             self.file_disposition_class = FileDisposition
             self.supports_plugins = False
             self.packed_arcs = False

coverage/env.py

@@ -156,6 +156,12 @@ class PYBEHAVIOR:
     # PEP649 and PEP749: Deferred annotations
     deferred_annotations = (PYVERSION >= (3, 14))
 
+    # Does sys.monitoring support BRANCH_RIGHT and BRANCH_LEFT?
+    branch_right_left = (
+        pep669 and
+        hasattr(sys.monitoring.events, "BRANCH_RIGHT") # type:ignore[attr-defined,unused-ignore]
+    )
+
 
 # Coverage.py specifics, about testing scenarios. See tests/testenv.py also.