Add fchmodat() as an option to set symlink Unix access permissions

syscall.c

@@ -1735,6 +1735,8 @@ int do_chmod(const char *path, mode_t mode)
 				break;
 			if (errno == ENOTSUP)
 				code = 1;
+# elif defined AT_FDCWD && defined AT_SYMLINK_NOFOLLOW
+			code = fchmodat(AT_FDCWD, path, mode & CHMOD_BITS, AT_SYMLINK_NOFOLLOW);
 # else
 			code = 1;
 # endif

testsuite/skiplist/cygwin.txt

@@ -68,4 +68,5 @@ source-change-size-continues
 symlink-dest-backupdir
 symlink-race-dest
 symlink-race-relative-dest
+symlink-unix-perms  # platform does not support changing symlink permissions
 temp-dir-symlink-injection

testsuite/skiplist/linux.txt

@@ -8,3 +8,4 @@
 crtimes  # Rsync is configured without crtimes support
 partial-protected-regular-retry-policy  # deterministic partial EACCES recovery uses dyld interposing
 readonly-partial-abort-mode-regression  # 
+symlink-unix-perms  # platform does not support changing symlink permissions

testsuite/symlink-unix-perms_test.py

@@ -0,0 +1,117 @@
+#!/usr/bin/env python3
+
+import errno
+import os
+import stat
+import sys
+
+from rsyncfns import (
+    FROMDIR, SCRATCHDIR, TODIR,
+    assert_mode, rmtree, run_rsync, test_fail, test_skipped
+)
+
+#   Put some error messages into variable to make later code more readable.
+
+error = 'error occurred during test of symlink access change capability: '
+notimplemented = 'symlink Unix access change is not implemented on this system'
+notsupported = 'symlink Unix access change is not supported on this system'
+
+#   Make sure that the FROMDIR and TODIR directories exist. These are the
+#   source and destination directories, respectively, for the test.
+
+os.chdir(SCRATCHDIR)
+
+FROMDIR.mkdir(parents=True, exist_ok=True)
+TODIR.mkdir(parents=True, exist_ok=True)
+
+#   Create a test directory and a test file in FROMDIR for use later as symlink
+#   targets, saving the actual names of each in a variable that will also be
+#   used later.
+
+testdirectory = 'testdirectory'
+testfile = 'testfile'
+
+(FROMDIR / testdirectory).mkdir(parents=True)
+(FROMDIR / testfile).touch()
+
+#   See if the system permits setting Unix access permssions on symlinks.
+#   If not, there's no point in continuing.
+#
+#   To test this, the following code creates a symlink in FROMDIR that points
+#   to the test directory there. It then tries to change the Unix access
+#   permission for that symlink. If the result is a "not implemented" or a
+#   "not supported" error, the actual test is skipped. If any other error is
+#   returned, the test reports failure.
+
+os.chdir(FROMDIR)
+
+os.symlink(testdirectory, 'symlink', target_is_directory=True)
+
+try:
+    os.chmod('symlink', stat.S_IREAD + stat.S_IXOTH, follow_symlinks=False)
+
+except NotImplementedError:
+    test_skipped(notimplemented)
+
+except OSError as e:
+    if (e.errno == errno.ENOTSUP):
+        test_skipped(notsupported)
+    test_fail('OS' + error + f'{e}')
+
+except Exception as e:
+    test_fail(error + f'{e}')
+
+#   The symlink access change attempt reported success. Make sure that the
+#   Unix access permissions were actually changed. If not, the test fails.
+
+assert_mode('symlink', stat.S_IREAD + stat.S_IXOTH)
+
+#   The system can change symlink Unix access permissions. Continue ...
+
+os.remove('symlink')
+
+os.chdir(SCRATCHDIR)
+
+#   Create a number of symlinks to use for testing rsync.
+#
+#   Set the Unix access permissions for those symlinks to selected values.
+#   The selected values have no specific agenda. However, owner read access
+#   is always maintained to ensure that stat info is readable.
+
+#   The following array consists of a number of targets and associated Unix
+#   access permissions that will be used in the actual test. The symlink
+#   names will be of the form "symlink-{n}" where "{n}" is the array index
+#   for each symlink created.
+
+symlinks = [
+    [ testdirectory, 0o770 ],
+    [ testfile, 0o0444 ],
+    [ testfile, 0o0711 ],
+    [ testfile, 0o0777 ],
+    [ testdirectory, 0o7777 ],
+    [ testfile, 0o0400 ],
+    [ testdirectory, 0o1407 ],
+    [ testdirectory, 0o2470 ],
+    [ testdirectory, 0o4777 ],
+    [ testfile, 0o7777 ],
+]
+
+#   Actually create the test symlinks in the source directory.
+
+for n, symlink in enumerate(symlinks):
+    target, access = symlink
+
+    os.symlink(target, FROMDIR / f'symlink-{n}', target_is_directory=(target == testdirectory))
+    os.chmod(FROMDIR / f'symlink-{n}', access, follow_symlinks=False)
+
+#   Now, use rsync to copy the source directory to the destination.
+
+run_rsync('-av', f'{FROMDIR}/', f'{TODIR}/')
+
+#   Finally, check to make sure that the Unix access permissions on the
+#   symlinks in the destination match those in the source. The test fail
+#   if any one doesn't match.
+
+for n, symlink in enumerate(symlinks):
+    expected = stat.S_IMODE(os.stat(FROMDIR / f'symlink-{n}', follow_symlinks=False).st_mode)
+    assert_mode(TODIR / f'symlink-{n}', expected)