LibCore: Define and use a fallible, OS-independent getgrent(_r) wrapper

Rather than maintaining a list of #ifdef guards to check systems that do
not provide the reentrant version of getgrent, we can use C++ concepts
to let the compiler perform this check for us.

While we're at it, we can also provide this wrapper as fallible to let
the caller TRY calling it.

Userland/Libraries/LibCore/Group.cpp

@@ -19,32 +19,27 @@ namespace Core {
 ErrorOr<DeprecatedString> Group::generate_group_file() const
 {
     StringBuilder builder;
+    char buffer[1024] = { 0 };
 
     ScopeGuard grent_guard([] { endgrent(); });
     setgrent();
-    errno = 0;
-#ifndef AK_OS_MACOS
-    struct group group;
-    struct group* gr = nullptr;
-    char buffer[1024] = { 0 };
-    while (getgrent_r(&group, buffer, sizeof(buffer), &gr) == 0 && gr) {
-#else
-    for (auto const* gr = getgrent(); gr; gr = getgrent()) {
-#endif
-        if (gr->gr_name == m_name)
+
+    while (true) {
+        auto group = TRY(Core::System::getgrent({ buffer, sizeof(buffer) }));
+        if (!group.has_value())
+            break;
+
+        if (group->gr_name == m_name)
             builder.appendff("{}:x:{}:{}\n", m_name, m_id, DeprecatedString::join(',', m_members));
         else {
             Vector<DeprecatedString> members;
-            for (size_t i = 0; gr->gr_mem[i]; ++i)
-                members.append(gr->gr_mem[i]);
+            for (size_t i = 0; group->gr_mem[i]; ++i)
+                members.append(group->gr_mem[i]);
 
-            builder.appendff("{}:x:{}:{}\n", gr->gr_name, gr->gr_gid, DeprecatedString::join(',', members));
+            builder.appendff("{}:x:{}:{}\n", group->gr_name, group->gr_gid, DeprecatedString::join(',', members));
         }
     }
 
-    if (errno)
-        return Error::from_errno(errno);
-
     return builder.to_deprecated_string();
 }
 
@@ -123,28 +118,23 @@ ErrorOr<void> Group::add_group(Group& group)
 ErrorOr<Vector<Group>> Group::all()
 {
     Vector<Group> groups;
+    char buffer[1024] = { 0 };
 
     ScopeGuard grent_guard([] { endgrent(); });
     setgrent();
-    errno = 0;
-#ifndef AK_OS_MACOS
-    struct group group;
-    struct group* gr = nullptr;
-    char buffer[1024] = { 0 };
-    while (getgrent_r(&group, buffer, sizeof(buffer), &gr) == 0 && gr) {
-#else
-    for (auto const* gr = getgrent(); gr; gr = getgrent()) {
-#endif
+
+    while (true) {
+        auto group = TRY(Core::System::getgrent({ buffer, sizeof(buffer) }));
+        if (!group.has_value())
+            break;
+
         Vector<DeprecatedString> members;
-        for (size_t i = 0; gr->gr_mem[i]; ++i)
-            members.append(gr->gr_mem[i]);
+        for (size_t i = 0; group->gr_mem[i]; ++i)
+            members.append(group->gr_mem[i]);
 
-        groups.append({ gr->gr_name, gr->gr_gid, members });
+        groups.append({ group->gr_name, group->gr_gid, members });
     }
 
-    if (errno)
-        return Error::from_errno(errno);
-
     return groups;
 }
 

Userland/Libraries/LibCore/System.cpp

@@ -86,6 +86,40 @@ static ErrorOr<Optional<struct passwd>> getpwent_impl(Span<char> buffer)
     return Optional<struct passwd> {};
 }
 
+// clang-format off
+template<typename T>
+concept SupportsReentrantGetgrent = requires(T group, T* ptr)
+{
+    getgrent_r(&group, nullptr, 0, &ptr);
+};
+// clang-format on
+
+// Note: This has to be in the global namespace for the extern declaration to trick the compiler
+// into finding a declaration of getgrent_r when it doesn't actually exist.
+static ErrorOr<Optional<struct group>> getgrent_impl(Span<char> buffer)
+{
+    if constexpr (SupportsReentrantGetgrent<struct group>) {
+        struct group group;
+        struct group* ptr = nullptr;
+
+        extern int getgrent_r(struct group*, char*, size_t, struct group**);
+        auto result = getgrent_r(&group, buffer.data(), buffer.size(), &ptr);
+
+        if (result == 0 && ptr)
+            return group;
+        if (result != 0 && result != ENOENT)
+            return Error::from_errno(result);
+    } else {
+        errno = 0;
+        if (auto const* group = ::getgrent())
+            return *group;
+        if (errno)
+            return Error::from_errno(errno);
+    }
+
+    return Optional<struct group> {};
+}
+
 namespace Core::System {
 
 #ifndef HOST_NAME_MAX
@@ -676,6 +710,11 @@ ErrorOr<Optional<struct passwd>> getpwuid(uid_t uid)
     return Optional<struct passwd> {};
 }
 
+ErrorOr<Optional<struct group>> getgrent(Span<char> buffer)
+{
+    return getgrent_impl(buffer);
+}
+
 ErrorOr<Optional<struct group>> getgrgid(gid_t gid)
 {
     errno = 0;

Userland/Libraries/LibCore/System.h

@@ -126,6 +126,7 @@ ErrorOr<Optional<struct passwd>> getpwent(Span<char> buffer);
 ErrorOr<Optional<struct passwd>> getpwnam(StringView name);
 ErrorOr<Optional<struct group>> getgrnam(StringView name);
 ErrorOr<Optional<struct passwd>> getpwuid(uid_t);
+ErrorOr<Optional<struct group>> getgrent(Span<char> buffer);
 ErrorOr<Optional<struct group>> getgrgid(gid_t);
 ErrorOr<void> clock_settime(clockid_t clock_id, struct timespec* ts);
 ErrorOr<pid_t> posix_spawn(StringView path, posix_spawn_file_actions_t const* file_actions, posix_spawnattr_t const* attr, char* const arguments[], char* const envp[]);