wip groupmembers is 2-alloc away.

README.md

@@ -274,20 +274,25 @@ Similarly, when user's groups are resolved in (2), they are not always necessary
 (i.e. not part of `struct user*`), therefore the memberships themselves are
 stored out of bound.
 
-`groupmembers` and `additional_gids` store group and user memberships respectively.
-Membership IDs are packed — not necessitating random access, thus suitable for
-compression.
+`groupmembers` and `additional_gids` store group and user memberships
+respectively. Membership IDs are packed — not necessitating random access, thus
+suitable for compression.
 
-- `groupmembers` is a list of pointers (offsets) to User records, because
-  `getgr*_r` returns pointers to membernames, thus a name has to be immediately
-  resolvable.
-- `additional_gids` is a list of gids, because `initgroups_dyn` (and friends) returns
-  an array of gids.
+- `groupmembers` consists of a number X followed by a list of offsets to User
+  records, because `getgr*_r` returns pointers to membernames, thus a name has
+  to be immediately resolvable.
+- `additional_gids` is a list of gids, because `initgroups_dyn` (and friends)
+  returns an array of gids.
 
-Each entry of `groupmembers` and `additional_gids` starts with a varint N, which is
-the number of upcoming elements, followed by N delta-compressed varints. These
-N delta-compressed varints are sorted the same way entries in `users` (in
-`groupmembers`) and `groups`.
+Each entry of `groupmembers` and `additional_gids` starts with a varint N,
+which is the number of upcoming elements. Then depending on the type:
+
+- **additional_gids** stores N delta-compressed varints. These varints
+  correspond to a list of gids.
+- **groupmembers** stores a total length of the member names, followed by N,
+  followed by N offsets. These are byte-offsets to the User records in the
+  `users` section. Having the length of membernames saves some CPU cycles when
+  decompressing the members in the hot path.
 
 Indices
 -------

lib/DB.zig

@@ -218,26 +218,38 @@ pub fn fromBytes(buf: []align(8) const u8) InvalidHeader!DB {
     return result;
 }
 
+const GroupMemberNames = struct {
+    arr: [:null]const ?[*:0]const u8,
+
+    pub fn deinit(self: *GroupMemberNames, allocator: Allocator) void {
+        allocator.free(self.arr[0]);
+        allocator.free(self.arr);
+    }
+};
+
 // returns a list of group member names starting at the given offset of
 // groupmembers blob.
 fn groupMemberNames(
     self: *const DB,
     allocator: Allocator,
     offset: u64,
-) error{OutOfMemory}![:null]const [*:0]const u8 {
+) error{OutOfMemory}!GroupMemberNames {
+    const v = compress.uvarintMust(self.groupmembers[offset..]);
+    const total_members_len = v.value;
+    offset += v.bytes_read;
     var vit = compress.VarintSliceIteratorMust(self.groupmembers[offset..]);
-    if (vit.remaining == 0) return null;
-    const total_members_len = vit.nextMust().?;
+    const num_members = vit.remaining;
+    if (num_members == 0) return null;
 
     // TODO (zig 0.10+) make result type sentinel-aware and stop
-    // the terminating-null-pointer-dances.
-    var result = try allocator.alloc(?[:0]const u8, vit.remaining + 1);
+    // the terminating-null-pointer-dancing.
+    var result = try allocator.alloc(?[:0]const u8, num_members + 1);
     errdefer allocator.free(result);
-    result.len = vit.remaining + 1;
-    result[result.len] = null;
-    result.len = vit.remaining;
+    result.len = num_members + 1;
+    result[num_members] = null;
+    result.len = num_members;
 
-    var buf = try allocator.alloc(u8, total_members_len + vit.remaining);
+    var buf = try allocator.alloc(u8, total_members_len + num_members);
     errdefer allocator.free(buf);
     var it = compress.DeltaCompressionIterator(&vit);
     var i: usize = 0;
@@ -250,28 +262,31 @@ fn groupMemberNames(
         buf[buf.len - 1] = 0;
         result[i] = buf[old_len..buf.len];
     }
-    return result;
+    return GroupMemberNames{ .arr = result };
 }
 
 // getgrtnam returns a Group entry by name. The Group must be
 // deinit'ed by caller.
-fn getgrnam(self: *const DB, allocator: Allocator, name: []const u8) ?Group {
-    const idx = bdz.search(self.bdz_groupname);
+fn getgrnam(
+    self: *const DB,
+    allocator: Allocator,
+    name: []const u8,
+) error{OutOfMemory}!?Group {
+    const idx = bdz.search(self.bdz_groupname, name);
     const offset = self.idx_groupname2group[idx];
     const group = PackedGroup.fromBytes(self.groups[offset..]).group;
-    if (!mem.eql(name, group.name())) return null;
+    if (!mem.eql(u8, name, group.name())) return null;
 
-    const namez = allocator.dupeZ(u8, name);
+    var members = try self.groupMemberNames(allocator, group.members_offset);
+    errdefer members.deinit(allocator);
+
+    const namez = try allocator.dupeZ(u8, name);
     errdefer allocator.free(namez);
 
-    // this operation is the last in the function, so it doesn't have
-    // a complex errdefer to deallocate it if something later fails.
-    const members = try groupMemberNames(allocator, group.members_offset);
-
     return Group{
         .name = namez,
         .gid = group.gid(),
-        .members = members,
+        .members = members.arr,
     };
 }
 
@@ -416,6 +431,13 @@ fn groupMembers(
         compress.deltaCompress(u32, scratch.items) catch |err| switch (err) {
             error.NotSorted => unreachable,
         };
+        const total_members_len = blk: {
+            var sum: u32 = 0;
+            for (members) |user_idx|
+                sum += @intCast(u32, corpus.users.get(user_idx).name.len);
+            break :blk sum;
+        };
+        try compress.appendUvarint(&blob, total_members_len);
         try compress.appendUvarint(&blob, members.len);
         for (scratch.items) |elem|
             try compress.appendUvarint(&blob, elem);
@@ -590,6 +612,9 @@ test "high-level API" {
 
     var db = try DB.fromCorpus(allocator, &corpus);
     defer db.deinit(allocator);
+
+    const all = try db.getgrnam(allocator, "all");
+    _ = all;
 }
 
 test "additionalGids" {

lib/compress.zig

@@ -80,6 +80,12 @@ pub fn uvarint(buf: []const u8) error{Overflow}!Varint {
     };
 }
 
+pub fn uvarintMust(buf: []const u8) Varint {
+    return uvarint(buf) catch |err| switch (err) {
+        error.Overflow => unreachable,
+    };
+}
+
 // https://golang.org/pkg/encoding/binary/#PutUvarint
 pub fn putUvarint(buf: []u8, x: u64) usize {
     var i: usize = 0;