Implement LWG issue 2720. Replace perms::resolve_symlinks with perms::symlink_nofollow.

This changes how filesystem::permissions(p, perms) handles symlinks. Previously
symlinks were not resolved by default instead only getting resolved when
"perms::resolve_symlinks" was used. After this change symlinks are resolved
by default and perms::symlink_nofollow must be given to change this.

This issue has not yet been moved to Ready status, and I will revert if it
doesn't get moved at the current meeting. However I feel confident that it
will and it's nice to have implementations when moving issues.

llvm-svn: 273328

libcxx/include/experimental/filesystem

@@ -292,7 +292,7 @@ enum class _LIBCPP_TYPE_VIS perms : unsigned
 
     add_perms        = 0x10000,
     remove_perms     = 0x20000,
-    resolve_symlinks = 0x40000
+    symlink_nofollow = 0x40000
 };
 
 _LIBCPP_INLINE_VISIBILITY

libcxx/src/experimental/filesystem/operations.cpp

@@ -594,7 +594,7 @@ void __last_write_time(const path& p, file_time_type new_time,
 void __permissions(const path& p, perms prms, std::error_code *ec)
 {
 
-    const bool resolve_symlinks = bool(perms::resolve_symlinks & prms);
+    const bool resolve_symlinks = !bool(perms::symlink_nofollow & prms);
     const bool add_perms = bool(perms::add_perms & prms);
     const bool remove_perms = bool(perms::remove_perms & prms);
 
@@ -605,6 +605,8 @@ void __permissions(const path& p, perms prms, std::error_code *ec)
     file_status st = detail::posix_lstat(p, &m_ec);
     if (m_ec) return set_or_throw(m_ec, ec, "permissions", p);
 
+    // AT_SYMLINK_NOFOLLOW can only be used on symlinks, using it on a regular
+    // file will cause fchmodat to report an error on some systems.
     const bool set_sym_perms = is_symlink(st) && !resolve_symlinks;
 
     if ((resolve_symlinks && is_symlink(st)) && (add_perms || remove_perms)) {
@@ -622,14 +624,16 @@ void __permissions(const path& p, perms prms, std::error_code *ec)
 # if defined(AT_SYMLINK_NOFOLLOW) && defined(AT_FDCWD)
     const int flags = set_sym_perms ? AT_SYMLINK_NOFOLLOW : 0;
     if (::fchmodat(AT_FDCWD, p.c_str(), real_perms, flags) == -1) {
+        return set_or_throw(ec, "permissions", p);
+    }
 # else
     if (set_sym_perms)
         return set_or_throw(make_error_code(errc::operation_not_supported),
                             ec, "permissions", p);
     if (::chmod(p.c_str(), real_perms) == -1) {
-# endif
         return set_or_throw(ec, "permissions", p);
     }
+# endif
     if (ec) ec->clear();
 }
 

libcxx/test/std/experimental/filesystem/fs.enum/enum.perms.pass.cpp

@@ -63,6 +63,6 @@ int main() {
         E::unknown      == ME(0xFFFF) &&
         E::add_perms        == ME(0x10000) &&
         E::remove_perms     == ME(0x20000) &&
-        E::resolve_symlinks == ME(0x40000),
+        E::symlink_nofollow == ME(0x40000),
         "Expected enumeration values do not match");
 }

libcxx/test/std/experimental/filesystem/fs.op.funcs/fs.op.permissions/permissions.pass.cpp

@@ -67,9 +67,9 @@ TEST_CASE(test_error_reporting)
     }
     {
         std::error_code ec;
-        fs::permissions(dne_sym, perms::resolve_symlinks, ec);
+        fs::permissions(dne_sym, fs::perms{}, ec);
         TEST_REQUIRE(ec);
-        TEST_CHECK(checkThrow(dne_sym, perms::resolve_symlinks, ec));
+        TEST_CHECK(checkThrow(dne_sym, fs::perms{}, ec));
     }
 }
 
@@ -82,7 +82,7 @@ TEST_CASE(basic_permissions_test)
     const path sym = env.create_symlink(file_for_sym, "sym");
     const perms AP = perms::add_perms;
     const perms RP = perms::remove_perms;
-    const perms RS = perms::resolve_symlinks;
+    const perms NF = perms::symlink_nofollow;
     struct TestCase {
       path p;
       perms set_perms;
@@ -98,11 +98,15 @@ TEST_CASE(basic_permissions_test)
         {dir, perms::owner_all, perms::owner_all},
         {dir, perms::group_all | AP, perms::owner_all | perms::group_all},
         {dir, perms::group_all | RP, perms::owner_all},
-        // test symlink with resolve symlinks on symlink
-        {sym, perms::none | RS, perms::none},
-        {sym, perms::owner_all | RS, perms::owner_all},
-        {sym, perms::group_all | AP | RS, perms::owner_all | perms::group_all},
-        {sym, perms::group_all | RP | RS, perms::owner_all}
+        // test symlink without symlink_nofollow
+        {sym, perms::none, perms::none},
+        {sym, perms::owner_all, perms::owner_all},
+        {sym, perms::group_all | AP, perms::owner_all | perms::group_all},
+        {sym, perms::group_all | RP , perms::owner_all},
+        // test non-symlink with symlink_nofollow. The last test on file/dir
+        // will have set their permissions to perms::owner_all
+        {file, perms::group_all | AP | NF, perms::owner_all | perms::group_all},
+        {dir,  perms::group_all | AP | NF, perms::owner_all | perms::group_all}
     };
     for (auto const& TC : cases) {
         TEST_CHECK(status(TC.p).permissions() != TC.expected);
@@ -144,7 +148,7 @@ TEST_CASE(test_no_resolve_symlink_on_symlink)
         std::error_code expected_ec = std::make_error_code(std::errc::operation_not_supported);
 #endif
         std::error_code ec = std::make_error_code(std::errc::bad_address);
-        permissions(sym, TC.set_perms, ec);
+        permissions(sym, TC.set_perms | perms::symlink_nofollow, ec);
         TEST_CHECK(ec == expected_ec);
         TEST_CHECK(status(file).permissions() == file_perms);
         TEST_CHECK(symlink_status(sym).permissions() == expected_link_perms);