Detect --suppress-sync=true and limit the skips to that

Call `os.open("", os.O_RDONLY | os.O_SYNC)` and check the errno to
detect whether `--suppress-sync=true` is actually used, and skip
the tests only in that scenario.

Lib/test/support/__init__.py

@@ -61,7 +61,7 @@
     "without_optimizer",
     "force_not_colorized",
     "BrokenIter",
-    "in_systemd_nspawn",
+    "in_systemd_nspawn", "in_systemd_nspawn_sync_suppressed",
     ]
 
 
@@ -2879,16 +2879,35 @@ def __iter__(self):
 def in_systemd_nspawn() -> bool:
     """
     Test whether the test suite is running inside of systemd-nspawn container
-
-    Return True if the test suite is being run inside a systemd-nspawn
-    container, False otherwise.  This can be used to skip tests that are
-    known to be reliable inside this kind of virtualization, for example
-    tests that are relying on fsync() not being stubbed out
-    (as ``systemd-nspawn --suppress-sync=true` does).
     """
 
     try:
         with open("/run/systemd/container", "rb") as fp:
             return fp.read().rstrip() == b"systemd-nspawn"
     except FileNotFoundError:
         return False
+
+
+def in_systemd_nspawn_sync_suppressed() -> bool:
+    """
+    Test whether the test suite is runing in systemd-nspawn with ``--suppress-sync=true``
+
+    Return True if the test suite is being run inside a systemd-nspawn
+    container and ``--suppress-sync=true`` option is detected to be used,
+    False otherwise.  This can be used to skip tests that rely
+    on ``fsync()`` calls and similar not being intercepted.
+    """
+
+    if hasattr(os, "O_SYNC") and in_systemd_nspawn():
+        import errno
+
+        # If systemd-nspawn is used, O_SYNC flag will immediately trigger
+        # EINVAL.  Otherwise, ENOENT will be given instead.
+        try:
+            with os.open("", os.O_RDONLY | os.O_SYNC):
+                pass
+        except OSError as err:
+            if err.errno == errno.EINVAL:
+                return True
+
+    return False

Lib/test/test_mmap.py

@@ -1,6 +1,6 @@
 from test.support import (
     requires, _2G, _4G, gc_collect, cpython_only, is_emscripten, is_apple,
-    in_systemd_nspawn,
+    in_systemd_nspawn_sync_suppressed,
 )
 from test.support.import_helper import import_module
 from test.support.os_helper import TESTFN, unlink
@@ -840,7 +840,7 @@ def test_flush_return_value(self):
         mm.write(b'python')
         result = mm.flush()
         self.assertIsNone(result)
-        if sys.platform.startswith(('linux', 'android')) and not in_systemd_nspawn():
+        if sys.platform.startswith(('linux', 'android')) and not in_systemd_nspawn_sync_suppressed():
             # 'offset' must be a multiple of mmap.PAGESIZE on Linux.
             # See bpo-34754 for details.
             self.assertRaises(OSError, mm.flush, 1, len(b'python'))

Lib/test/test_os.py

@@ -2355,7 +2355,7 @@ class TestInvalidFD(unittest.TestCase):
     singles_fildes = {"fchdir"}
     # systemd-nspawn --suppress-sync=true does not verify fd passed
     # fdatasync() and fsync(), and always returns success
-    if not support.in_systemd_nspawn():
+    if not support.in_systemd_nspawn_sync_suppressed():
         singles += ["fdatasync", "fsync"]
         singles_fildes |= {"fdatasync", "fsync"}
     #singles.append("close")

Misc/NEWS.d/next/Tests/2024-09-18-18-39-21.gh-issue-124213.AQq_xg.rst

@@ -1,3 +1,3 @@
-Skip ``test_os`` and ``test_mmap`` tests that are failing
-with ``--suppress-sync=true`` when running inside a systemd-nspawn
-container.
+Detect whether the test suite is running inside a systemd-nspawn container
+with ``--suppress-sync=true`` option, and skip the `test_os`` and ``test_mmap``
+tests that are failing in this scenario.