vfork() is just too weird.  For instance, in this build:

  http://hydra.nixos.org/build/3330487

the value fromHook.writeSide becomes corrupted in the parent, even
though the child only reads from it.  At -O0 the problem goes away.
Probably the child is overriding some spilled temporary variable.

If I get bored I may implement using posix_spawn() instead.

Hopefully this reduces the chance of hitting ‘unable to fork: Cannot
allocate memory’ errors.  vfork() is used for everything except
starting builders.

http://hydra.nixos.org/build/3031618

E.g. Darwin doesn't allow this.

Also use utimes() instead of utime() if lutimes() is not available.

XZ compresses significantly better than bzip2.  Here are the
compression ratios and execution times (using 4 cores in parallel) on
my /var/run/current-system (3.1 GiB):

  bzip2: total compressed size 849.56 MiB, 30.8% [2m08]
  xz -6: total compressed size 641.84 MiB, 23.4% [6m53]
  xz -7: total compressed size 621.82 MiB, 22.6% [7m19]
  xz -8: total compressed size 599.33 MiB, 21.8% [7m18]
  xz -9: total compressed size 588.18 MiB, 21.4% [7m40]

Note that compression takes much longer.  More importantly, however,
decompression is much faster:

  bzip2: 1m47.274s
  xz -6: 0m55.446s
  xz -7: 0m54.119s
  xz -8: 0m52.388s
  xz -9: 0m51.842s

The only downside to using -9 is that decompression takes a fair
amount (~65 MB) of memory.

Since the Perl bindings require shared libraries, this is required on
platforms such as Cygwin where we do a static build.

Nix needs SQLite's foreign key constraint feature, which was
introduced in 3.6.19.  Without it, the database won't be cleaned up
correctly when paths are deleted.  See
e.g. http://hydra.nixos.org/build/2494142.

http://hydra.nixos.org/build/2306540

Nix now requires SQLite and bzip2 to be pre-installed.  SQLite is
detected using pkg-config.  We required DBD::SQLite anyway, so
depending on SQLite is not a big problem.

The --with-bzip2, --with-openssl and --with-sqlite flags are gone.

I was bitten one time too many by Python modifying the Nix store by
creating *.pyc files when run as root.  On Linux, we can prevent this
by setting the immutable bit on files and directories (as in ‘chattr
+i’).  This isn't supported by all filesystems, so it's not an error
if setting the bit fails.  The immutable bit is cleared by the garbage
collector before deleting a path.  The only tricky aspect is in
optimiseStore(), since it's forbidden to create hard links to an
immutable file.  Thus optimiseStore() temporarily clears the immutable
bit before creating the link.

  scripts.
* Include the version and architecture in the -I flag so that there is
  at least a chance that a Nix binary built for one Perl version will
  run on another version.

* Refactor the nix-channel unpacker a bit.

  broke building on Cygwin and Solaris.

  will eat them.

  uname reports amd64.
* Drop the FreeBSD version number, e.g. "i686-freebsd" instead of
  "i686-freebsd8.2".

This should be more robust and also plays better with
cross-compilation---it uses the host name, instead of using the build
name.

  intermittent problems are gone by now.  WAL mode is preferrable
  because it does way fewer fsyncs.

  the Nix:: namespace.

  bindings to be used in Nix's own Perl scripts.

  The only downside is that Perl XS and Automake/libtool don't really
  like each other, so building is a bit tricky.

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  location to DBI and DBD::SQLite can be passed with --with-dbi and
  --with-dbd-sqlite.

  little RAM.  Even if the memory isn't actually used, it can cause
  problems with the overcommit heuristics in the kernel.  So use a VM
  space of 25% of RAM, up to 384 MB.