Drain the pty masters in ScreenTests so endwin() does not hang on macOS

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.  A background reader thread does not help: endwin() runs with the
GIL held, so the thread cannot read after its first chunk.

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

Author: serhiy-storchaka

Lib/test/test_curses.py

@@ -1773,11 +1773,28 @@ 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.
+        # A background reader thread cannot help: endwin() runs with the GIL
+        # held, so the thread could never read after its first chunk.
+        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 +1803,8 @@ 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