(even when it is interrupted by a signal).

  Berkeley DB from running out of locks.

  nix-store query options `--referer' and `--referer-closure' have
  been changed to `--referrer' and `--referrer-closure' (but the old
  ones are still accepted for compatibility).

  mapping.  The referer table is replaced by a referrer table (note
  spelling fix) that stores each referrer separately.  That is,
  instead of having

    referer[P] = {Q_1, Q_2, Q_3, ...}

  we store

    referer[(P, Q_1)] = ""
    referer[(P, Q_2)] = ""
    referer[(P, Q_3)] = ""
    ...

  To find the referrers of P, we enumerate over the keys with a value
  lexicographically greater than P.  This requires the referrer table
  to be stored as a B-Tree rather than a hash table.

  (The tuples (P, Q) are stored as P + null-byte + Q.)

  Old Nix databases are upgraded automatically to the new schema.

  (de)registration, in particular garbage collection (NIX-23).

  crashed Nix instances, and toss out our own recovery code.

* Checkpoint after an upgrade.

  much faster.

  Nix is properly shut down when it receives those signals.  In
  particular this ensures that killing the garbage collector doesn't
  cause a subsequent database recovery.

  result of parsing) can have very heavy sharing, causing exponential
  complexity if we naively recurse into them.  ATerms are graphs, not
  trees!

  simultaneously.  We do this using exclusive locks on uid files in
  /nix/var/nix/userpool, e.g., /nix/var/nix/userpool/123 for uid 123.

  critical to prevent certain kinds of 0wnage.

  running under that uid.

  nobody else has write permission to the build result.  This catches
  most hack attempts.

  builder.  Instead, require that the Nix store has sticky permission
  (S_ISVTX); everyone can created files in the Nix store, but they
  cannot delete, rename or modify files created by others.

  root (or setuid root), then builds will be performed under one of
  the users listed in the `build-users' configuration variables.  This
  is to make it impossible to influence build results externally,
  allowing locally built derivations to be shared safely between
  users (see ASE-2005 paper).

  To do: only one builder should be active per build user.

for NixOS, where we might not know our PATH in advance.

  versions to available versions, or vice versa.

  For example, the following compares installed versions to available
  versions:

    $ nix-env -qc
    autoconf-2.59            = 2.59
    automake-1.9.4           < 1.9.6
    f-spot-0.0.10            - ?
    firefox-1.0.4            < 1.0.7
    ...

  I.e., there are newer versions available (in the current default Nix
  expression) for Automake and Firefox, but not for Autoconf, and
  F-Spot is missing altogether.

  Conversely, the available versions can be compared to the installed
  versions:

    $ nix-env -qac
    autoconf-2.59                  = 2.59
    automake-1.9.6                 > 1.9.4
    bash-3.0                       - ?
    firefox-1.0.7                  > 1.0.4
    ...

  Note that bash is available but no version of it is installed.

  If multiple versions are available for comparison, then the highest
  is used.  E.g., if Subversion 1.2.0 is installed, and Subversion
  1.1.4 and 1.2.3 are available, then `nix-env -qc' will print `<
  1.2.3', not `> 1.1.4'.

  If higher versions are available, the version column is printed in
  red (using ANSI escape codes).