testsuite: skip dir-symlink-basis when openat2/RESOLVE_BENEATH is absent

The issue #715 fix relies on openat2(RESOLVE_BENEATH); without it
secure_relative_open() uses the per-component fallback, which can't
follow a dir-symlink basedir, so #715 still applies. The test already
skips on the non-Linux fallback platforms by name, but Android/Termux is
a Linux kernel with openat2 blocked by seccomp, so it must skip too.
Probe openat2(RESOLVE_BENEATH) at runtime in a subprocess (so a seccomp
SIGSYS kills the child, not the test) and skip when it's unavailable.
Also handle Termux's platform.system() == 'Android'.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: tridge

testsuite/symlink-dirlink-basis_test.py

@@ -16,6 +16,7 @@
 import os
 import platform
 import subprocess
+import sys
 import time
 
 from rsyncfns import (
@@ -30,6 +31,37 @@
         f"{platform.system()}; issue #715 still affects this platform"
     )
 
+
+def _resolve_beneath_works() -> bool:
+    """The issue #715 fix relies on openat2(RESOLVE_BENEATH). Where that is
+    unavailable (kernel < 5.6) or blocked by a seccomp filter (the Android
+    app sandbox, hardened containers), secure_relative_open() uses the
+    per-component fallback, which can't follow a dir-symlink basedir -- so
+    #715 still applies and this test must skip, exactly as it does on the
+    non-Linux fallback platforms above. Probe in a subprocess so a seccomp
+    SIGSYS kills the child rather than this test."""
+    code = (
+        "import ctypes, sys\n"
+        "class H(ctypes.Structure):\n"
+        "    _fields_ = [('f', ctypes.c_uint64), ('m', ctypes.c_uint64),\n"
+        "                ('r', ctypes.c_uint64)]\n"
+        "libc = ctypes.CDLL(None, use_errno=True)\n"
+        "h = H(0, 0, 0x08)  # resolve = RESOLVE_BENEATH\n"
+        "fd = libc.syscall(437, -100, b'.', ctypes.byref(h), ctypes.sizeof(h))\n"
+        "sys.exit(0 if fd >= 0 else 1)\n"
+    )
+    return subprocess.run([sys.executable, '-c', code]).returncode == 0
+
+
+# Termux's Python reports 'Android', not 'Linux'; both run the Linux
+# kernel where syscall 437 is openat2, so probe on either. (Darwin and
+# FreeBSD use O_RESOLVE_BENEATH and must not run the Linux-specific probe.)
+if platform.system() in ('Linux', 'Android') and not _resolve_beneath_works():
+    test_skipped(
+        "openat2(RESOLVE_BENEATH) is unavailable here (old kernel or a "
+        "seccomp filter); issue #715's dir-symlink-basis fix relies on it"
+    )
+
 os.environ['RSYNC_RSH'] = str(SRCDIR / 'support' / 'lsh.sh')
 # HOME -> SCRATCHDIR is set up by rsyncfns import.