bpo-41001: Add os.eventfd() (#20930)

Co-authored-by: Kyle Stanley <aeros167@gmail.com>

Lib/test/test_os.py

@@ -15,10 +15,12 @@ import locale
 import mmap
 import os
 import pickle
+import select
 import shutil
 import signal
 import socket
 import stat
+import struct
 import subprocess
 import sys
 import sysconfig
@@ -59,6 +61,7 @@ try:
 except ImportError:
     INT_MAX = PY_SSIZE_T_MAX = sys.maxsize
 
+
 from test.support.script_helper import assert_python_ok
 from test.support import unix_shell
 from test.support.os_helper import FakePath
@@ -3528,6 +3531,89 @@ class MemfdCreateTests(unittest.TestCase):
         self.assertFalse(os.get_inheritable(fd2))
 
 
+@unittest.skipUnless(hasattr(os, 'eventfd'), 'requires os.eventfd')
+@support.requires_linux_version(2, 6, 30)
+class EventfdTests(unittest.TestCase):
+    def test_eventfd_initval(self):
+        def pack(value):
+            """Pack as native uint64_t
+            """
+            return struct.pack("@Q", value)
+        size = 8  # read/write 8 bytes
+        initval = 42
+        fd = os.eventfd(initval)
+        self.assertNotEqual(fd, -1)
+        self.addCleanup(os.close, fd)
+        self.assertFalse(os.get_inheritable(fd))
+
+        # test with raw read/write
+        res = os.read(fd, size)
+        self.assertEqual(res, pack(initval))
+
+        os.write(fd, pack(23))
+        res = os.read(fd, size)
+        self.assertEqual(res, pack(23))
+
+        os.write(fd, pack(40))
+        os.write(fd, pack(2))
+        res = os.read(fd, size)
+        self.assertEqual(res, pack(42))
+
+        # test with eventfd_read/eventfd_write
+        os.eventfd_write(fd, 20)
+        os.eventfd_write(fd, 3)
+        res = os.eventfd_read(fd)
+        self.assertEqual(res, 23)
+
+    def test_eventfd_semaphore(self):
+        initval = 2
+        flags = os.EFD_CLOEXEC | os.EFD_SEMAPHORE | os.EFD_NONBLOCK
+        fd = os.eventfd(initval, flags)
+        self.assertNotEqual(fd, -1)
+        self.addCleanup(os.close, fd)
+
+        # semaphore starts has initval 2, two reads return '1'
+        res = os.eventfd_read(fd)
+        self.assertEqual(res, 1)
+        res = os.eventfd_read(fd)
+        self.assertEqual(res, 1)
+        # third read would block
+        with self.assertRaises(BlockingIOError):
+            os.eventfd_read(fd)
+        with self.assertRaises(BlockingIOError):
+            os.read(fd, 8)
+
+        # increase semaphore counter, read one
+        os.eventfd_write(fd, 1)
+        res = os.eventfd_read(fd)
+        self.assertEqual(res, 1)
+        # next read would block, too
+        with self.assertRaises(BlockingIOError):
+            os.eventfd_read(fd)
+
+    def test_eventfd_select(self):
+        flags = os.EFD_CLOEXEC | os.EFD_NONBLOCK
+        fd = os.eventfd(0, flags)
+        self.assertNotEqual(fd, -1)
+        self.addCleanup(os.close, fd)
+
+        # counter is zero, only writeable
+        rfd, wfd, xfd = select.select([fd], [fd], [fd], 0)
+        self.assertEqual((rfd, wfd, xfd), ([], [fd], []))
+
+        # counter is non-zero, read and writeable
+        os.eventfd_write(fd, 23)
+        rfd, wfd, xfd = select.select([fd], [fd], [fd], 0)
+        self.assertEqual((rfd, wfd, xfd), ([fd], [fd], []))
+        self.assertEqual(os.eventfd_read(fd), 23)
+
+        # counter at max, only readable
+        os.eventfd_write(fd, (2**64) - 2)
+        rfd, wfd, xfd = select.select([fd], [fd], [fd], 0)
+        self.assertEqual((rfd, wfd, xfd), ([fd], [], []))
+        os.eventfd_read(fd)
+
+
 class OSErrorTests(unittest.TestCase):
     def setUp(self):
         class Str(str):