feat(pytest): exception breakpoints when running with dap strategy (#25)

When running neotest with the DAP strategy (nvim-dap), e.g.

    require("neotest").run.run({ strategy = "dap" })

we make the pydebug debugger stop at "exception breakpoints"
where the exception is thrown out of a pytest method.

This will be very helpful for users to debug failing unit tests.

neotest_python/pytest.py

@@ -1,6 +1,6 @@
 from io import StringIO
 from pathlib import Path
-from typing import TYPE_CHECKING, Callable, Dict, List, Optional
+from typing import TYPE_CHECKING, Callable, Dict, List, Optional, Union
 
 from .base import NeotestAdapter, NeotestError, NeotestResult, NeotestResultStatus
 
@@ -18,7 +18,10 @@ class PytestNeotestAdapter(NeotestAdapter):
         import pytest
 
         result_collector = NeotestResultCollector(self, stream=stream)
-        pytest.main(args=args, plugins=[result_collector])
+        pytest.main(args=args, plugins=[
+            result_collector,
+            NeotestDebugpyPlugin(),
+        ])
         return result_collector.results
 
 
@@ -130,3 +133,43 @@ class NeotestResultCollector:
         if not params:
             self.stream(pos_id, result)
         self.results[pos_id] = result
+
+
+class NeotestDebugpyPlugin:
+    """A pytest plugin that would make debugpy stop at thrown exceptions."""
+
+    def pytest_exception_interact(
+        self,
+        node: Union['pytest.Item', 'pytest.Collector'],
+        call: 'pytest.CallInfo',
+        report: Union['pytest.CollectReport', 'pytest.TestReport'],
+    ):
+        # call.excinfo: _pytest._code.ExceptionInfo
+        self.maybe_debugpy_postmortem(call.excinfo._excinfo)
+
+    @staticmethod
+    def maybe_debugpy_postmortem(excinfo):
+        """Make the debugpy debugger enter and stop at a raised exception.
+
+        excinfo: A (type(e), e, e.__traceback__) tuple. See sys.exc_info()
+        """
+        # Reference: https://github.com/microsoft/debugpy/issues/723
+        import threading
+        try:
+            import pydevd
+        except ImportError:
+            return  # debugpy or pydevd not available, do nothing
+
+        py_db = pydevd.get_global_debugger()
+        if py_db is None:
+            # Do nothing if not running with a DAP debugger,
+            # e.g. neotest was invoked with {strategy = dap}
+            return
+
+        thread = threading.current_thread()
+        additional_info = py_db.set_additional_thread_info(thread)
+        additional_info.is_tracing += 1
+        try:
+            py_db.stop_on_unhandled_exception(py_db, thread, additional_info, excinfo)
+        finally:
+            additional_info.is_tracing -= 1