pythongh-124213: Skip tests failing with --suppress-sync=true when inside systemd-nspawn container

Add a helper functon that checks whether the test suite is running
inside a systemd-nspawn container, and skip the few tests failing
with `--suppress-sync=true` in that case.  The tests are failing because
`--suppress-sync=true` stubs out `fsync()`, `fdatasync()` and `msync()`
calls, and therefore they always return success without checking for
invalid arguments.

Technically, this means that the tests are also skipped when running
inside systemd-nspawn container without `--suppress-sync=true`.
However, to the best of my knowledge the only way to detect this option
is to actually reproduce one of the unexpectedly-succeeding syscalls,
and since this is precisely what we're testing for, the skip-test
and the actual test would be doing the same thing.

Signed-off-by: Michał Górny <mgorny@gentoo.org>

Author: mgorny

Lib/test/support/__init__.py

@@ -25,6 +25,7 @@
 import types
 import unittest
 import warnings
+from pathlib import Path
 
 from .testresult import get_test_runner
 
@@ -72,6 +73,7 @@
     "run_with_tz", "PGO", "missing_compiler_executable", "fd_count",
     "ALWAYS_EQ", "NEVER_EQ", "LARGEST", "SMALLEST",
     "LOOPBACK_TIMEOUT", "INTERNET_TIMEOUT", "SHORT_TIMEOUT", "LONG_TIMEOUT",
+    "in_systemd_nspawn",
     ]
 
 
@@ -3305,3 +3307,11 @@ def adjust_int_max_str_digits(max_digits):
         yield
     finally:
         sys.set_int_max_str_digits(current)
+
+
+def in_systemd_nspawn() -> bool:
+    try:
+        return (Path("/run/systemd/container").read_bytes().rstrip() ==
+                b"systemd-nspawn")
+    except FileNotFoundError:
+        return False

Lib/test/test_mmap.py

@@ -1,5 +1,6 @@
 from test.support import (TESTFN, import_module, unlink,
-                          requires, _2G, _4G, gc_collect, cpython_only)
+                          requires, _2G, _4G, gc_collect, cpython_only,
+                          in_systemd_nspawn,)
 import unittest
 import os
 import re
@@ -740,7 +741,7 @@ def test_flush_return_value(self):
         mm.write(b'python')
         result = mm.flush()
         self.assertIsNone(result)
-        if sys.platform.startswith('linux'):
+        if sys.platform.startswith('linux') and not in_systemd_nspawn():
             # '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

@@ -1965,8 +1965,12 @@ def test_chmod(self):
 
 
 class TestInvalidFD(unittest.TestCase):
-    singles = ["fchdir", "dup", "fdopen", "fdatasync", "fstat",
-               "fstatvfs", "fsync", "tcgetpgrp", "ttyname"]
+    singles = ["fchdir", "dup", "fdopen", "fstat",
+               "fstatvfs", "tcgetpgrp", "ttyname"]
+    # systemd-nspawn --suppress-sync=true does not verify fd passed
+    # fdatasync() and fsync(), and always returns success
+    if not support.in_systemd_nspawn():
+        singles += ["fdatasync", "fsync"]
     #singles.append("close")
     #We omit close because it doesn't raise an exception on some platforms
     def get_single(f):