user packing

README.md

@@ -67,7 +67,7 @@ Tight packing places some constraints on the underlying data:
 
 - Maximum database size: 4GB.
 - Permitted length of username and groupname: 1-32 bytes.
-- Permitted length of shell and homedir: 1-64 bytes.
+- Permitted length of shell and home: 1-64 bytes.
 - Permitted comment ("gecos") length: 0-255 bytes.
 - User name, groupname and gecos must be utf8-encoded.
 
@@ -219,11 +219,11 @@ const User = struct {
     // pointer to a separate structure that contains a list of gids
     additional_gids_offset: u29,
     // shell is a different story, documented elsewhere.
-    shell_here: u1,
+    shell_here: bool,
     shell_len_or_idx: u6,
-    homedir_len: u6,
-    username_is_a_suffix: u1,
-    username_offset_or_len: u5,
+    home_len: u6,
+    name_is_a_suffix: bool,
+    name_len: u5,
     gecos_len: u8,
     // a variable-sized array that will be stored immediately after this
     // struct.
@@ -232,27 +232,27 @@ const User = struct {
 ```
 
 `stringdata` contains a few string entries:
-- homedir.
-- username.
+- home.
+- name.
 - gecos.
 - shell (optional).
 
-First byte of the homedir is stored right after the `gecos_len` field, and it's
-length is `homedir_len`. The same logic applies to all the `stringdata` fields:
+First byte of the home is stored right after the `gecos_len` field, and it's
+length is `home_len`. The same logic applies to all the `stringdata` fields:
 there is a way to calculate their relative position from the length of the
 fields before them.
 
 Additionally, two optimizations for special fields are made:
 - shells are often shared across different users, see the "Shells" section.
-- username is frequently a suffix of the homedir. For example, `/home/motiejus`
-  and `motiejus`. In which case storing both username and homedir strings is
-  wasteful. For that cases, username has two options:
-  1. `username_is_a_suffix=true`: username is a suffix of the home dir. In that
-  case, the username starts at the `username_offset_or_len`'th byte of the
-  homedir, and ends at the same place as the homedir.
-  2. `username_is_a_suffix=false`: username is stored separately. In that case,
-  it begins one byte after homedir, and it's length is
-  `username_offset_or_len`.
+- name is frequently a suffix of the home. For example, `/home/motiejus`
+  and `motiejus`. In which case storing both name and home strings is
+  wasteful. For that cases, name has two options:
+  1. `name_is_a_suffix=true`: name is a suffix of the home dir. In that
+     case, the name starts at the `home_len - name_len`'th
+     byte of the home, and ends at the same place as the home.
+  2. `name_is_a_suffix=false`: name is stored separately. In that case,
+  it begins one byte after home, and it's length is
+  `name_len`.
 
 Shells
 ------
@@ -315,8 +315,7 @@ const AdditionalGids = PackedList;
 
 An entry in `members` field points to the offset into a respective `User` or
 `Group` entry (number of bytes relative to the first entry of the type).
-`members` in `PackedList` is sorted by the name (`username` or `groupname`) of
-the record it is pointing to.
+`members` in `PackedList` are sorted the same way as in the input.
 
 A packed list is a list of varints.
 

src/shell.zig

@@ -41,7 +41,7 @@ pub const ShellWriter = struct {
     const ShellSections = struct {
         index: BoundedArray(ShellIndex, MaxShells),
         blob: BoundedArray(u8, MaxShells * MaxShellLen),
-        indices: StringHashMap(u10),
+        indices: StringHashMap(u6),
 
         // initializes and populates shell sections. All strings are copied,
         // nothing is owned.
@@ -52,10 +52,10 @@ pub const ShellWriter = struct {
             var self = ShellSections{
                 .index = try BoundedArray(ShellIndex, MaxShells).init(shells.len),
                 .blob = try BoundedArray(u8, MaxShells * MaxShellLen).init(0),
-                .indices = StringHashMap(u10).init(allocator),
+                .indices = StringHashMap(u6).init(allocator),
             };
             var fullOffset: u12 = 0;
-            var idx: u10 = 0;
+            var idx: u6 = 0;
             while (idx < shells.len) {
                 const len = @intCast(u6, shells.get(idx).len);
                 try self.blob.appendSlice(shells.get(idx));
@@ -90,7 +90,7 @@ pub const ShellWriter = struct {
             self.* = undefined;
         }
 
-        pub fn getIndex(self: *const ShellSections, shell: []const u8) ?u10 {
+        pub fn getIndex(self: *const ShellSections, shell: []const u8) ?u6 {
             return self.indices.get(shell);
         }
     };

src/user.zig

@@ -1,17 +1,18 @@
 const std = @import("std");
 
 const Allocator = std.mem.Allocator;
+const cast = std.math.cast;
 
 pub const PackedUserSize = @sizeOf(PackedUser);
 pub const PackedUser = packed struct {
     uid: u32,
     gid: u32,
     additional_gids_offset: u29,
-    shell_here: u1,
+    shell_here: bool,
     shell_len_or_idx: u6,
-    homedir_len: u6,
-    username_is_a_suffix: u1,
-    username_offset_or_len: u5,
+    home_len: u6,
+    name_is_a_suffix: bool,
+    name_len: u5,
     gecos_len: u8,
 };
 
@@ -31,7 +32,7 @@ pub const UserWriter = struct {
     // string and returns it's index in the global shell section. Passing a
     // function makes tests easier, and removes the Shell dependency of this
     // module.
-    const shellIndexFnType = fn ([]const u8) ?u10;
+    const shellIndexFnType = fn ([]const u8) ?u6;
     allocator: Allocator,
     shellIndexFn: shellIndexFnType,
 
@@ -42,30 +43,65 @@ pub const UserWriter = struct {
         };
     }
 
-    pub fn fromUser(self: *UserWriter, user: User) !PackedUser {
-        var shell_here: u1 = undefined;
-        var shell_len_or_idx: u6 = undefined;
-        if (self.shellIndexFn(user.shell)) |idx| {
-            shell_here = false;
-            shell_len_or_idx = idx;
-        } else {
-            shell_here = true;
-            shell_len_or_idx = user.shell.len;
-        }
+    const fromUserErr = std.mem.Allocator.Error || error{InvalidRecord};
+
+    pub fn fromUser(self: *UserWriter, user: User) fromUserErr![]const u8 {
+        const home_len = std.math.cast(u6, user.home.len - 1) catch return error.InvalidRecord;
+        const name_len = cast(u5, user.name.len - 1) catch return error.InvalidRecord;
+        const shell_len = cast(u6, user.shell.len - 1) catch return error.InvalidRecord;
+        const gecos_len = cast(u8, user.gecos.len) catch return error.InvalidRecord;
+        var bindata_len: u32 = home_len;
 
         var puser = PackedUser{
-            .uid = user.uid,
-            .gid = user.gid,
-            .additional_gids_offset = 0, // second pass
-            .shell_here = shell_here,
-            .shell_len_or_idx = shell_len_or_idx,
-            .homedir_len = undefined,
-            .username_is_a_suffix = undefined,
-            .username_offset_or_len = undefined,
-            .gecos_len = undefined,
+            .uid = @as(u32, user.uid),
+            .gid = @as(u32, user.gid),
+            .additional_gids_offset = std.math.maxInt(u29), // needs second pass
+            .shell_here = undefined,
+            .shell_len_or_idx = undefined,
+            .home_len = home_len,
+            .name_is_a_suffix = undefined,
+            .name_len = name_len,
+            .gecos_len = gecos_len,
         };
 
-        return puser;
+        if (std.mem.endsWith(u8, user.home, user.name)) {
+            puser.name_is_a_suffix = true;
+        } else {
+            puser.name_is_a_suffix = false;
+            bindata_len += name_len;
+        }
+
+        bindata_len += gecos_len;
+
+        if (self.shellIndexFn(user.shell)) |idx| {
+            puser.shell_here = false;
+            puser.shell_len_or_idx = idx;
+        } else {
+            puser.shell_here = true;
+            puser.shell_len_or_idx = shell_len;
+            bindata_len += shell_len;
+        }
+
+        var result = try self.allocator.alloc(u8, PackedUserSize + bindata_len);
+
+        const userPointer = @ptrCast([*]const u8, &puser);
+        {
+            var i: u32 = 0;
+            while (i < PackedUserSize) {
+                result[i] = userPointer[i];
+                i += 1;
+            }
+        }
+        std.mem.copy(u8, result, user.home);
+        if (!puser.name_is_a_suffix) {
+            std.mem.copy(u8, result, user.name);
+        }
+        std.mem.copy(u8, result, user.gecos);
+        if (puser.shell_here) {
+            std.mem.copy(u8, result, user.shell);
+        }
+
+        return result;
     }
 };
 
@@ -74,3 +110,36 @@ const testing = std.testing;
 test "PackedUser is byte-aligned" {
     try testing.expectEqual(0, @rem(@bitSizeOf(PackedUser), 8));
 }
+
+fn testShellIndex(shell: []const u8) ?u6 {
+    if (std.mem.eql(u8, shell, "/bin/bash")) {
+        return 0;
+    } else if (std.mem.eql(u8, shell, "/bin/zsh")) {
+        return 1;
+    }
+    return null;
+}
+
+test "construct PackedUser blob" {
+    var writer = UserWriter.init(testing.allocator, testShellIndex);
+    const user1 = User{
+        .uid = 1000,
+        .gid = 1000,
+        .name = "vidmantas",
+        .gecos = "Vidmantas Kaminskas",
+        .home = "/home/vidmantas",
+        .shell = "/bin/bash",
+    };
+    const user2 = User{
+        .uid = 1001,
+        .gid = 1001,
+        .name = "svc-foo",
+        .gecos = "Service Account",
+        .home = "/home/service1",
+        .shell = "/usr/bin/nologin",
+    };
+    const puser1 = try writer.fromUser(user1);
+    const puser2 = try writer.fromUser(user2);
+    defer testing.allocator.free(puser1);
+    defer testing.allocator.free(puser2);
+}