remove 3 bytes from PackedUser

additional_gids will be stored separately.

src/user.zig

@@ -9,13 +9,13 @@ const cast = std.math.cast;
 pub const PackedUser = packed struct {
     uid: u32,
     gid: u32,
-    additional_gids_offset: u29,
     shell_here: bool,
     shell_len_or_idx: u6,
     home_len: u6,
     name_is_a_suffix: bool,
     name_len: u5,
     gecos_len: u8,
+    padding: u5,
 
     // blobSize returns the length of the blob storing string values.
     pub fn blobLength(self: *const PackedUser) usize {
@@ -93,13 +93,13 @@ pub const UserWriter = struct {
         var puser = PackedUser{
             .uid = user.uid,
             .gid = user.gid,
-            .additional_gids_offset = std.math.maxInt(u29), // needs second pass
             .shell_here = self.shellIndexFn(user.shell) == null,
             .shell_len_or_idx = self.shellIndexFn(user.shell) orelse shell_len,
             .home_len = home_len,
             .name_is_a_suffix = std.mem.endsWith(u8, user.home, user.name),
             .name_len = name_len,
             .gecos_len = gecos_len,
+            .padding = 0,
         };
 
         try self.appendTo.appendSlice(std.mem.asBytes(&puser));
@@ -144,12 +144,10 @@ pub const UserReader = struct {
             if (it.index == it.ur.blob.len) return null;
             assert(it.index < it.ur.blob.len);
 
-            // https://github.com/ziglang/zig/issues/1095
-            const packedUserSizeHere = @sizeOf(PackedUser);
-            const endUser = it.index + packedUserSizeHere;
+            const endUser = it.index + @sizeOf(PackedUser);
             var packedUser = std.mem.bytesAsValue(
                 PackedUser,
-                it.ur.blob[it.index..endUser][0..packedUserSizeHere],
+                it.ur.blob[it.index..endUser][0..@sizeOf(PackedUser)],
             );
             const startBlob = endUser;
             const endBlob = startBlob + packedUser.blobLength();
@@ -217,19 +215,8 @@ pub const UserReader = struct {
 
 const testing = std.testing;
 
-test "PackedUser alignment" {
-    // byte-aligned
-    try testing.expectEqual(0, @rem(@bitSizeOf(PackedUser), 8));
-
-    const bytes = @divExact(@bitSizeOf(PackedUser), 8);
-
-    // External padding (PackedUserAlignmentBits) must be higher or equal to
-    // the "internal" PackedUser alignment. By aligning PackedUser we are also
-    // working around https://github.com/ziglang/zig/issues/10958 ; PackedUser
-    // cannot be converted from/to [@bitSizeOf(PackedUser)/8]u8;
-    // asBytes/bytesAsValue use @sizeOf, which is larger. Now we are putting no
-    // more than 1, but it probably could be higher.
-    try testing.expect(@sizeOf(PackedUser) - bytes <= 1);
+test "PackedUser internal and external alignment" {
+    try testing.expectEqual(@bitSizeOf(PackedUser), @sizeOf(PackedUser) * 8);
 }
 
 fn testShellIndex(shell: []const u8) ?u6 {