Drain the pty masters in ScreenTests and release the GIL in endwin()

ScreenTests drives curses over pseudo-terminals whose master ends are
never read.  On macOS (unlike Linux) the tcdrain() that curses performs
inside endwin(), and even a plain write(), blocks once the unread output
fills the pty buffer, so the test hung until the timeout.

Drain the masters synchronously before endwin(), leaving room for its
output.  Also release the GIL around the endwin() call, so that it no
longer blocks other threads while it talks to the terminal.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: serhiy-storchaka

Lib/test/test_curses.py

@@ -1773,11 +1773,26 @@ def setUp(self):
         self.save_signals = SaveSignals()
         self.save_signals.save()
         self.addCleanup(self.save_signals.restore)
+        self.pty_masters = []
+
+    def drain_ptys(self):
+        # Discard whatever curses has written to the screens.  Nothing reads
+        # the master end, so on platforms such as macOS (but not Linux) the
+        # tcdrain() that curses performs inside endwin() -- and even a plain
+        # write() -- blocks once the unread output fills the pty buffer.
+        # Draining here, before endwin(), leaves room for its output to drain.
+        for master in self.pty_masters:
+            try:
+                while os.read(master, 65536):
+                    pass
+            except BlockingIOError:
+                pass
 
     def tearDown(self):
         # Leave visual mode and reclaim the screens the test created, while
         # their pseudo-terminals are still open (closing them happens later,
         # via the make_pty() cleanups).
+        self.drain_ptys()
         try:
             curses.endwin()
         except curses.error:
@@ -1786,6 +1801,8 @@ def tearDown(self):
 
     def make_pty(self):
         master, slave = os.openpty()
+        os.set_blocking(master, False)
+        self.pty_masters.append(master)
         self.addCleanup(os.close, master)
         self.addCleanup(os.close, slave)
         return slave

Modules/_cursesmodule.c

@@ -3723,7 +3723,18 @@ De-initialize the library, and return terminal to normal status.
 static PyObject *
 _curses_endwin_impl(PyObject *module)
 /*[clinic end generated code: output=c0150cd96d2f4128 input=e172cfa43062f3fa]*/
-NoArgNoReturnFunctionBody(endwin)
+{
+    PyCursesStatefulInitialised(module);
+
+    /* endwin() writes to the terminal and may call tcdrain(), which can block
+       (e.g. on a pty whose output is not being read); release the GIL so other
+       threads -- including one draining that terminal -- can run meanwhile. */
+    int code;
+    Py_BEGIN_ALLOW_THREADS
+    code = endwin();
+    Py_END_ALLOW_THREADS
+    return curses_check_err(module, code, "endwin", NULL);
+}
 
 /*[clinic input]
 _curses.erasechar