Let it be so.
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Turbo NSS
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---------
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glibc nss library for passwd and group.
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Steps
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-----
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A known implementation runs id(1) at ~250 rps sequentially. Our goal is 10k
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ID/s.
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id(1) works as follows:
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- lookup user by name.
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- get all additional gids (an array attached to a member).
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- for each additional gid, return the group name.
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Assuming a member is in ~100 groups on average, that's 1M group lookups per
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second. We need to convert gid to a group index quickly.
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Data structures
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---------------
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Basic data structures that allow efficient storage:
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```lang=c
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// reminder:
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typedef uid_t uint32;
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typedef gid_t uint32;
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// 6*32b = 6*4B = 24B/user
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typedef struct {
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uid_t uid;
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gid_t gid;
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name_offset uint32; // offset into *usernames
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gecos_offset uint32; // offset into *gecos
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shell_offset uint32; // offset into *shells
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additional_groups_offset uint32; // offset into additional_groups
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} user;
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const char* usernames; // all concatenated usernames, fsst-compressed
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const char* gecoss; // all concatenated gecos, fsst-compressed
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const char* shells; // all concatenated home directories, fsst-compressed
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const uint8_t additional_groups; // all additional_groups, turbo compressed
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typedef struct {
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gid_t gid;
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name_offset uint32; // offset into *groupnames
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members_offset uint32; // offset into members
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}
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const char* groupnames; // all concatenated group names, fsst-compressed
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const uint8_8 members; // all concatenated members, turbo compressed
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```
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"turbo compression" encodes a list of uids/gids with this algorithm:
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1. sort ascending.
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2. extract deltas and subtract 1: `awk '{diff=$0-prev; prev=$0; print
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diff-1}'`.
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3. varint-encode these deltas into an uint32, like protobuf or utf8.
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With typical group memberships (as of writing) this requires ~1.3-1.5 byte per
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entry.
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Indexes
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-------
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The following operations need to be fast, in order of importance:
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1. lookup gid -> group (this is on hot path in id).
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2. lookup uid -> user.
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3. lookup username -> user.
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4. lookup groupname -> group.
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5. (optional) iterate users using a defined order (`getent passwd`).
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6. (optional) iterate groups using a defined order (`getent group`).
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