add tests for upper limits

README.md

@@ -130,9 +130,10 @@ Now that we've sketched the implementation of `id(3)`, it's clearer to
 understand which operations need to be fast; in order of importance:
 
 1. lookup gid -> group info (this is on hot path in id) without members.
-2. lookup uid -> user.
-3. lookup groupname -> group.
-4. lookup username -> user.
+2. lookup username -> user's group memberships.
+3. lookup uid -> user.
+4. lookup groupname -> group.
+5. lookup username -> user.
 
 These indices can use perfect hashing like [bdz from cmph][cmph]: a perfect
 hash hashes a list of bytes to a sequential list of integers. Perfect hashing
@@ -292,8 +293,25 @@ Varint is an efficiently encoded integer (packed for small values). Same as
 [protocol buffer varints][varint], except the largest possible value is `u64`.
 They compress integers well.
 
-`groupmembers`, `additional_gids`
----------------------------------
+Group memberships
+-----------------
+
+There are two group memberships at play:
+
+1. given a username, resolve user's group gids (for `initgroups(3)`).
+2. given a group (gid/name), resolve the members' names (e.g. `getgrgid`).
+
+When user's groups are resolved in (1), the additional userdata is not
+requested (there is no way to return it). Therefore, it is reasonable to store
+the user's memberships completely out-of-bound, keyed by the hash of the
+username.
+
+When group's memberships are resolved in (2), the same call also requires other
+group information: gid and group name. Therefore it makes sense to store a
+pointer to the group members in the group information itself. However, the
+memberships are not *always* necessary (see remarks about `id(1)` in this
+document), therefore the memberships will be stored separately, outside of the
+groups section.
 
 `groupmembers` and `additional_gids` store group and user memberships
 respectively: for each group, a list of pointers (offsets) to User records, and
@@ -307,16 +325,12 @@ pseudo-code:
 ```
 const PackedList = struct {
     length: varint,
-    members: []varint
+    members: [length]varint
 }
 const Groupmembers = PackedList;
 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` are sorted the same way as in the input.
-
 A packed list is a list of varints.
 
 Complete file structure