fix: resolve CI coverage failures for platform-specific stdin EOF monitor

Use pragma: lax no cover for select.poll()-based code paths that are only
exercised on non-Windows platforms. Remove incorrect pragma: no cover from
test finally blocks that are always executed.

Github-Issue:#2231

Author: Bortlesboat

src/mcp/server/stdio.py

@@ -55,12 +55,14 @@ def _create_stdin_eof_monitor(
     if not hasattr(select, "poll"):
         return None  # pragma: no cover
 
-    try:
+    # The remaining code uses select.poll() which is not available on Windows.
+    # Coverage is exercised on non-Windows platforms only.
+    try:  # pragma: lax no cover
         fd = sys.stdin.buffer.fileno()
-    except Exception:
+    except Exception:  # pragma: lax no cover
         return None
 
-    async def monitor() -> None:
+    async def monitor() -> None:  # pragma: lax no cover
         poll_obj = select.poll()
         poll_obj.register(fd, select.POLLIN | select.POLLHUP)
         try:
@@ -74,7 +76,7 @@ async def monitor() -> None:
         finally:
             poll_obj.unregister(fd)
 
-    return monitor
+    return monitor  # pragma: lax no cover
 
 
 @asynccontextmanager
@@ -131,6 +133,6 @@ async def stdout_writer():
 
         eof_monitor = _create_stdin_eof_monitor(tg)
         if eof_monitor is not None:
-            tg.start_soon(eof_monitor)
+            tg.start_soon(eof_monitor)  # pragma: lax no cover
 
         yield read_stream, write_stream

tests/server/test_stdio.py

@@ -84,7 +84,7 @@ def test_create_stdin_eof_monitor_returns_none_when_fileno_fails():
 
 @pytest.mark.anyio
 @pytest.mark.skipif(sys.platform == "win32", reason="select.poll not available on Windows")
-async def test_stdin_eof_monitor_detects_hangup():
+async def test_stdin_eof_monitor_detects_hangup():  # pragma: lax no cover
     """The EOF monitor cancels the task group when stdin pipe closes."""
     read_fd, write_fd = os.pipe()
     try:
@@ -106,14 +106,14 @@ async def test_stdin_eof_monitor_detects_hangup():
                     while not tg.cancel_scope.cancel_called:
                         await anyio.sleep(0.05)
     finally:
-        os.close(read_fd)  # pragma: no cover
-        if write_fd != -1:  # pragma: no cover
+        os.close(read_fd)
+        if write_fd != -1:
             os.close(write_fd)
 
 
 @pytest.mark.anyio
 @pytest.mark.skipif(sys.platform == "win32", reason="select.poll not available on Windows")
-async def test_stdin_eof_monitor_ignores_pollin_events():
+async def test_stdin_eof_monitor_ignores_pollin_events():  # pragma: lax no cover
     """The monitor ignores POLLIN events (data available) and only reacts to hangup/error."""
     read_fd, write_fd = os.pipe()
     try:
@@ -144,6 +144,6 @@ async def test_stdin_eof_monitor_ignores_pollin_events():
                     while not tg.cancel_scope.cancel_called:  # pragma: no branch
                         await anyio.sleep(0.05)
     finally:
-        os.close(read_fd)  # pragma: no cover
-        if write_fd != -1:  # pragma: no cover
+        os.close(read_fd)
+        if write_fd != -1:
             os.close(write_fd)