remove 3 bytes from PackedUser
additional_gids will be stored separately.
This commit is contained in:
Motiejus Jakštys
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78dc63669d
commit
e06cac38b2
25
README.md
25
README.md
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@ -130,7 +130,7 @@ Now that we've sketched the implementation of `id(3)`, it's clearer to
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understand which operations need to be fast; in order of importance:
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1. lookup gid -> group info (this is on hot path in id) without members.
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2. lookup username -> user's group memberships.
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2. lookup username -> user's groups.
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3. lookup uid -> user.
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4. lookup groupname -> group.
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5. lookup username -> user.
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@ -325,7 +325,7 @@ pseudo-code:
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```
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const PackedList = struct {
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length: varint,
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members: [length]varint
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members: [length]varint,
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}
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const Groupmembers = PackedList;
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const AdditionalGids = PackedList;
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@ -333,13 +333,17 @@ const AdditionalGids = PackedList;
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A packed list is a list of varints.
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Section `AdditionalGidsIndex` stores an index from `hash(username)` to `offset`
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in AdditionalGids.
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Complete file structure
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-----------------------
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`idx_*` sections are of type `[]PackedIntArray(u29)` and are pointing to the
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respective `Groups` and `Users` entries (from the beginning of the respective
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section). Since User and Group records are 8-byte aligned, 3 bits are saved
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from every element.
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section). Since User and Group records are 8-byte aligned, 3 bits can be saved
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from every element. However, since the header easily fits to 64 bytes, we are
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storing plain `u32` for easier inspection.
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Each section is padded to 64 bytes.
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@ -350,16 +354,17 @@ bdz_gid2group ? gid->group bdz
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bdz_uid2user ? uid->user bdz
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bdz_groupname2group ? groupname->group bdz
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bdz_name2user ? username->user bdz
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idx_gid2group len(group)*4*29/32 bdz->offset gid2group
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idx_groupname2group len(group)*4*29/32 bdz->offset groupname2group
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idx_uid2user len(user)*4*29/32 bdz->offset uid2user
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idx_name2user len(user)*4*29/32 bdz->offset name2user
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idx_gid2group len(group)*32 bdz->offset gid2group
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idx_groupname2group len(group)*32 bdz->offset groupname2group
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idx_uid2user len(user)*32 bdz->offset uid2user
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idx_name2user len(user)*32 bdz->offset name2user
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idx_username2gids len(user)*32 Per-user gidlist index
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ShellIndex len(shells)*2 Shell index array
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ShellBlob <= 4032 Shell data blob (max 63*64 bytes)
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Groups ? packed Group entries (8b padding)
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Users ? packed User entries (8b padding)
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groupmembers ? per-group memberlist (32b padding)
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additional_gids ? per-user grouplist (8b padding)
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Groupmembers ? per-group memberlist (32b padding)
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AdditionalGids ? Per-user gidlist entries
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```
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[git-subtrac]: https://apenwarr.ca/log/20191109
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25
src/user.zig
25
src/user.zig
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@ -9,13 +9,13 @@ const cast = std.math.cast;
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pub const PackedUser = packed struct {
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uid: u32,
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gid: u32,
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additional_gids_offset: u29,
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shell_here: bool,
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shell_len_or_idx: u6,
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home_len: u6,
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name_is_a_suffix: bool,
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name_len: u5,
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gecos_len: u8,
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padding: u5,
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// blobSize returns the length of the blob storing string values.
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pub fn blobLength(self: *const PackedUser) usize {
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@ -93,13 +93,13 @@ pub const UserWriter = struct {
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var puser = PackedUser{
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.uid = user.uid,
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.gid = user.gid,
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.additional_gids_offset = std.math.maxInt(u29), // needs second pass
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.shell_here = self.shellIndexFn(user.shell) == null,
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.shell_len_or_idx = self.shellIndexFn(user.shell) orelse shell_len,
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.home_len = home_len,
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.name_is_a_suffix = std.mem.endsWith(u8, user.home, user.name),
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.name_len = name_len,
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.gecos_len = gecos_len,
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.padding = 0,
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};
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try self.appendTo.appendSlice(std.mem.asBytes(&puser));
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@ -144,12 +144,10 @@ pub const UserReader = struct {
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if (it.index == it.ur.blob.len) return null;
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assert(it.index < it.ur.blob.len);
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// https://github.com/ziglang/zig/issues/1095
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const packedUserSizeHere = @sizeOf(PackedUser);
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const endUser = it.index + packedUserSizeHere;
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const endUser = it.index + @sizeOf(PackedUser);
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var packedUser = std.mem.bytesAsValue(
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PackedUser,
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it.ur.blob[it.index..endUser][0..packedUserSizeHere],
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it.ur.blob[it.index..endUser][0..@sizeOf(PackedUser)],
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);
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const startBlob = endUser;
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const endBlob = startBlob + packedUser.blobLength();
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@ -217,19 +215,8 @@ pub const UserReader = struct {
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const testing = std.testing;
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test "PackedUser alignment" {
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// byte-aligned
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try testing.expectEqual(0, @rem(@bitSizeOf(PackedUser), 8));
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const bytes = @divExact(@bitSizeOf(PackedUser), 8);
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// External padding (PackedUserAlignmentBits) must be higher or equal to
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// the "internal" PackedUser alignment. By aligning PackedUser we are also
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// working around https://github.com/ziglang/zig/issues/10958 ; PackedUser
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// cannot be converted from/to [@bitSizeOf(PackedUser)/8]u8;
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// asBytes/bytesAsValue use @sizeOf, which is larger. Now we are putting no
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// more than 1, but it probably could be higher.
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try testing.expect(@sizeOf(PackedUser) - bytes <= 1);
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test "PackedUser internal and external alignment" {
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try testing.expectEqual(@bitSizeOf(PackedUser), @sizeOf(PackedUser) * 8);
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}
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fn testShellIndex(shell: []const u8) ?u6 {
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