to make the Refs table more space-efficient. For instance, this
reduces the size of the database on my laptop from 93 MiB to 18
MiB. (It was 72 MiB with the old schema on an ext3 disk with a 1
KiB block size.)
This prevents remote builders from being killed by the
`max-silent-time' inactivity monitor while they are waiting for a
long garbage collection to finish. This happens fairly often in the
Hydra build farm.
_FILE_OFFSET_BITS=64. Without it, functions like stat() fail on
large file sizes. This happened with a Nix store on squashfs:
$ nix-store --dump /tmp/mnt/46wzqnk4cbdwh1dclhrpqnnz1icak6n7-local-net-cmds > /dev/null
error: getting attributes of path `/tmp/mnt/46wzqnk4cbdwh1dclhrpqnnz1icak6n7-local-net-cmds': Value too large for defined data type
$ stat /tmp/mnt/46wzqnk4cbdwh1dclhrpqnnz1icak6n7-local-net-cmds
File: `/tmp/mnt/46wzqnk4cbdwh1dclhrpqnnz1icak6n7-local-net-cmds'
Size: 0 Blocks: 36028797018963968 IO Block: 1024 regular empty file
(This is a bug in squashfs or mksquashfs, but it shouldn't cause Nix
to fail.)
complete set of live and dead paths before starting the actual
deletion, but determines liveness on demand. I.e. for any path in
the store, it first tries to delete all the referrers, and then the
path itself. This means that the collector can start deleting paths
almost immediately.
(Linux) machines no longer maintain the atime because it's too
expensive, and on the machines where --use-atime is useful (like the
buildfarm), reading the atimes on the entire Nix store takes way too
much time to make it practical.
intersectAttrs returns the (right-biased) intersection between two
attribute sets, e.g. every attribute from the second set that also
exists in the first. functionArgs returns the set of attributes
expected by a function.
The main goal of these is to allow the elimination of most of
all-packages.nix. Most package instantiations in all-packages.nix
have this form:
foo = import ./foo.nix {
inherit a b c;
};
With intersectAttrs and functionArgs, this can be written as:
foo = callPackage (import ./foo.nix) { };
where
callPackage = f: args:
f ((builtins.intersectAttrs (builtins.functionArgs f) pkgs) // args);
I.e., foo.nix is called with all attributes from "pkgs" that it
actually needs (e.g., pkgs.a, pkgs.b and pkgs.c). (callPackage can
do any other generic package-level stuff we might want, such as
applying makeOverridable.) Of course, the automatically supplied
arguments can be overriden if needed, e.g.
foo = callPackage (import ./foo.nix) {
c = c_version_2;
};
but for the vast majority of packages, this won't be needed.
The advantages are to reduce the amount of typing needed to add a
dependency (from three sites to two), and to reduce the number of
trivial commits to all-packages.nix. For the former, there have
been two previous attempts:
- Use "args: with args;" in the package's function definition.
This however obscures the actual expected arguments of a
function, which is very bad.
- Use "{ arg1, arg2, ... }:" in the package's function definition
(i.e. use the ellipis "..." to allow arbitrary additional
arguments), and then call the function with all of "pkgs" as an
argument. But this inhibits error detection if you call it with
an misspelled (or obsolete) argument.
NixOS evaluation errors in particular look intimidating and
generally aren't very useful. Ideally the builtins.throw messages
should be self-contained.
UTC) rather than 0 (00:00:00). 1 is a better choice because some
programs use 0 as a special value. For instance, the Template
Toolkit uses a timestamp of 0 to denote the non-existence of a file,
so it barfs on files in the Nix store (see
template-toolkit-nix-store.patch in Nixpkgs). Similarly, Maya 2008
fails to load script directories with a timestamp of 0 and can't be
patched because it's closed source.
This will also shut up those "implausibly old time stamp" GNU tar
warnings.
attributes of the rec are in scope of `e'. This is useful in
expressions such as
rec {
lib = import ./lib;
inherit (lib) concatStrings;
}
It does change the semantics of expressions such as
let x = {y = 1;}; in rec { x = {y = 2;}; inherit (x) y; }.y
This now returns 2 instead of 1. However, no code in Nixpkgs or
NixOS seems to rely on the old behaviour.
shorthand for {x = {y = {z = ...;};};}. This is especially useful
for NixOS configuration files, e.g.
{
services = {
sshd = {
enable = true;
port = 2022;
};
};
}
can now be written as
{
services.sshd.enable = true;
services.sshd.port = 2022;
}
However, it is currently not permitted to write
{
services.sshd = {enable = true;};
services.sshd.port = 2022;
}
as this is considered a duplicate definition of `services.sshd'.
broken, but now the evaluator checks for it to prevent Nix
expressions from relying on undefined behaviour. Equality tests are
implemented using a shallow pointer equality test between ATerms.
However, because attribute sets are lazy and contain position
information, this can give false positives. For instance,
previously
let y = {x = 1;}; in y == y
evaluated to true, while the equivalent expression
{x = 1;} == {x = 1;}
evaluated to false. So disallow these tests for now. (Eventually
we may want to implement deep equality tests for attribute sets,
like lib.eqStrict.)
* Idem: disallow comparisons between functions.
* Implemented deep comparisons of lists. This had the same problem as
attribute sets - the elements in the list weren't evaluated. For
instance,
["xy"] == [("x" + "y")]
evaluated to false. Now it works properly.
(that is, call the build hook with a certain interval until it
accepts the build).
* build-remote.pl was totally broken: for all system types other than
the local system type, it would send all builds to the *first*
machine of the appropriate type.
poll for it (i.e. if we can't acquire the lock, then let the main
select() loop wait for at most a few seconds and then try again).
This improves parallelism: if two nix-store processes are both
trying to build a path at the same time, the second one shouldn't
block; it should first see if it can build other goals. Also, it
prevents the deadlocks that have been occuring in Hydra lately,
where a process waits for a lock held by another process that's
waiting for a lock held by the first.
The downside is that polling isn't really elegant, but POSIX doesn't
provide a way to wait for locks in a select() loop. The only
solution would be to spawn a thread for each lock to do a blocking
fcntl() and then signal the main thread, but that would require
pthreads.
would just silently store only (fileSize % 2^32) bytes.
* Use posix_fallocate if available when unpacking archives.
* Provide a better error message when trying to unpack something that
isn't a NAR archive.
sure that it works as expected when you pass it a derivation. That
is, we have to make sure that all build-time dependencies are built,
and that they are all in the input closure (otherwise remote builds
might fail, for example). This is ensured at instantiation time by
adding all derivations and their sources to inputDrvs and inputSrcs.
hook. This fixes a problem with log files being partially or
completely filled with 0's because another nix-store process
truncates the log file. It should also be more efficient.
the DerivationGoal runs. Otherwise, if a goal is a top-level goal,
then the lock won't be released until nix-store finishes. With
--keep-going and lots of top-level goals, it's possible to run out
of file descriptors (this happened sometimes in the build farm for
Nixpkgs). Also, for failed derivation, it won't be possible to
build it again until the lock is released.
* Idem for locks on build users: these weren't released in a timely
manner for failed top-level derivation goals. So if there were more
than (say) 10 such failed builds, you would get an error about
having run out of build users.
scan for runtime dependencies (i.e. the local machine shouldn't do a
scan that the remote machine has already done). Also pipe directly
into `nix-store --import': don't use a temporary file.
(e.g. an SSH connection problem) and permanent failures (i.e. the
builder failed). This matters to Hydra (it wants to know whether it
makes sense to retry a build).
allocate memory, which is verboten in signal handlers. This caused
random failures in the test suite on Mac OS X (triggered by the spurious
SIGPOLL signals on Mac OS X, which should also be fixed).
closure of the inputs. This really enforces that there can't be any
undeclared dependencies on paths in the store. This is done by
creating a fake Nix store and creating bind-mounts or hard-links in
the fake store for all paths in the closure. After the build, the
build output is moved from the fake store to the real store. TODO:
the chroot has to be on the same filesystem as the Nix store for
this to work, but this isn't enforced yet. (I.e. it only works
currently if /tmp is on the same FS as /nix/store.)
bind-mounts we do are only visible to the builder process and its
children. So accidentally doing "rm -rf" on the chroot directory
won't wipe out /nix/store and other bind-mounted directories
anymore. Also, the bind-mounts in the private namespace disappear
automatically when the builder exits.
necessary that at least one build hook doesn't return "postpone",
otherwise nix-store will barf ("waiting for a build slot, yet there
are no running children"). So inform the build hook when this is
the case, so that it can start a build even when that would exceed
the maximum load on a machine.
nix-store -r (or some other operation) is started via ssh, it will
at least have a chance of terminating quickly when the connection is
killed. Right now it just runs to completion, because it never
notices that stderr is no longer connected to anything. Of course
it would be better if sshd would just send a SIGHUP, but it doesn't
(https://bugzilla.mindrot.org/show_bug.cgi?id=396).
derivation should be a source rather than a derivation dependency of
the call to the NAR derivation. Otherwise the derivation (and all
its dependencies) will be built as a side-effect, which may not even
succeed.
SHA-256 outputs of fixed-output derivations. I.e. they now produce
the same store path:
$ nix-store --add x
/nix/store/j2fq9qxvvxgqymvpszhs773ncci45xsj-x
$ nix-store --add-fixed --recursive sha256 x
/nix/store/j2fq9qxvvxgqymvpszhs773ncci45xsj-x
the latter being the same as the path that a derivation
derivation {
name = "x";
outputHashAlgo = "sha256";
outputHashMode = "recursive";
outputHash = "...";
...
};
produces.
This does change the output path for such fixed-output derivations.
Fortunately they are quite rare. The most common use is fetchsvn
calls with SHA-256 hashes. (There are a handful of those is
Nixpkgs, mostly unstable development packages.)
* Documented the computation of store paths (in store-api.cc).
dependency. `storePath /nix/store/bla' gives exactly the same
result as `toPath /nix/store/bla', except that the former includes
/nix/store/bla in the dependency context of the string.
Useful in some generated Nix expressions like nix-push, which now
finally does the right thing wrt distributed builds. (Previously
the path to be packed wasn't an explicit dependency, so it wouldn't
be copied to the remote machine.)
subtle and often hard-to-reproduce bugs where programs in pipes
either barf with a "Broken pipe" message or not, depending on the
exact timing conditions. This particularly happened in GNU M4 (and
Bison, which uses M4).
disasters involving `rm -rf' on bind mounts. Will try the
definitive fix (per-process mounts, apparently possible via the
CLONE_NEWNS flag in clone()) some other time.
accessed time of paths that may be deleted. Anything more recently
used won't be deleted. The time is specified in time_t,
e.g. seconds since 1970-01-01 00:00:00 UTC; use `date +%s' to
convert to time_t from the command line.
Example: to delete everything that hasn't been used in the last two
months:
$ nix-store --gc -v --max-atime $(date +%s -d "2 months ago")
order of ascending last access time. This is useful in conjunction
with --max-freed or --max-links to prefer deleting non-recently used
garbage, which is good (especially in the build farm) since garbage
may become live again.
The code could easily be modified to accept other criteria for
ordering garbage by changing the comparison operator used by the
priority queue in collectGarbage().
attributes from the meta attribute. Not doing so caused nix-env to
barf on the "psi" package, which has a meta.function attribute,
the textual serialisation of which causes a gigantic string to be
produced --- so big that it causes nix-env to run out of memory.
Note however that "meta" really only should contain strings.
meta.function should be passthru.function.
particular, dietlibc cannot figure out the cwd because the inode of
the current directory doesn't appear in .. (because getdents returns
the inode of the mount point).
(which means it can only be defined via "inherit"), otherwise we get
scoping bugs, since __overrides can't be recursive (or at least, it
would be hard).
need /etc in the chroot (in particular, /etc/resolv.conf for
fetchurl). Not having /etc/resolv.conf in the chroot is a good
thing, since we don't want normal derivations to download files.
~/.nix-defexpr, otherwise the attribute cannot be selected with the
`-A' option. Useful if you want to stick a Nix expression directly
in ~/.nix-defexpr.
a rec. This will be very useful to allow end-user customisation of
all-packages.nix, for instance globally overriding GCC or some other
dependency. The // operator doesn't cut it: you could replace the
"gcc" attribute, but all other attributes would continue to
reference the original value due to the substitution semantics of
rec.
The syntax is a bit hacky but this is to allow backwards
compatibility.
in attribute set pattern matches. This allows defining a function
that takes *at least* the listed attributes, while ignoring
additional attributes. For instance,
{stdenv, fetchurl, fuse, ...}:
stdenv.mkDerivation {
...
};
defines a function that requires an attribute set that contains the
specified attributes but ignores others. The main advantage is that
we can then write in all-packages.nix
aefs = import ../bla/aefs pkgs;
instead of
aefs = import ../bla/aefs {
inherit stdenv fetchurl fuse;
};
This saves a lot of typing (not to mention not having to update
all-packages.nix with purely mechanical changes). It saves as much
typing as the "args: with args;" style, but has the advantage that
the function arguments are properly declared (not implicit in what
the body of the "with" uses).
functions that take a single argument (plain lambdas) into one AST
node (Function) that contains a Pattern node describing the
arguments. Current patterns are single lazy arguments (VarPat) and
matching against an attribute set (AttrsPat).
This refactoring allows other kinds of patterns to be added easily,
such as Haskell-style @-patterns, or list pattern matching.
`-u'. Instead of acquiring an exclusive lock on the profile for the
entire duration of the operation, we just perform the operation
optimistically (without an exclusive lock), and check at the end
whether the profile changed while we were busy (i.e., the symlink
target changed). If so, the operation is restarted. Restarting is
generally cheap, since the build results are still in the Nix store.
Most of the time, only the user environment has to be rebuilt.
again. (After the previous substituter mechanism refactoring I
didn't update the code that obtains the references of substitutable
paths.) This required some refactoring: the substituter programs
are now kept running and receive/respond to info requests via
stdin/stdout.
it only does something if $NIX_OTHER_STORES (not really a good
name...) is set.
* Do globbing on the elements of $NIX_OTHER_STORES. E.g. you could
set it to /mnts/*/nix or something.
* Install substituters in libexec/nix/substituters.
logic through the `parseDrvName' and `compareVersions' primops.
This will allow expressions to easily check whether some dependency
is a specific needed version or falls in some version range. See
tests/lang/eval-okay-versions.nix for examples.
bytes have been freed, `--max-links' to stop when the Nix store
directory has fewer than N hard links (the latter being important
for very large Nix stores on filesystems with a 32000 subdirectories
limit).
store under the reference relation, since that means that the
garbage collector will need a long time to start deleting paths.
Instead just delete the referrers of a path first.
This isn't usually a problem, except that it causes tests to fail
when performed in a directory with a very long path name. So chdir
to the socket directory and use a relative path name.
/tmp/nix-<pid>-<counter> for temporary build directories. This
increases purity a bit: many packages store the temporary build path
in their output, causing (generally unimportant) binary differences.
undefined variables by definition. This matters for the
implementation of "with", which does a call to checkVarDefs to see
if the body of the with has no undefined variables. (It can't be
checked at parse time because you don't know which variables are in
the "with" attribute set.) If we check closed terms, then we check
not just the with body but also the substituted terms, which are
typically very large. This is the cause of the poor nix-env
performance on Nixpkgs lately. It didn't happen earlier because
"with" wasn't used very often in the past.
This fix improves nix-env performance roughly 60x on current Nixpkgs.
nix-env -qa is down from 29.3s to 0.5s on my laptop, and nix-env -qa
--out-path is down from 229s to 3.39s. Not bad for a 1-line fix :-)
single quotes. Example (from NixOS):
job = ''
start on network-interfaces
start script
rm -f /var/run/opengl-driver
${if videoDriver == "nvidia"
then "ln -sf ${nvidiaDrivers} /var/run/opengl-driver"
else if cfg.driSupport
then "ln -sf ${mesa} /var/run/opengl-driver"
else ""
}
rm -f /var/log/slim.log
end script
'';
This style has two big advantages:
- \, ' and " aren't special, only '' and ${. So you get a lot less
escaping in shell scripts / configuration files in Nixpkgs/NixOS.
The delimiter '' is rare in scripts (and can usually be written as
""). ${ is also fairly rare.
Other delimiters such as <<...>>, {{...}} and <|...|> were also
considered but this one appears to have the fewest drawbacks
(thanks Martin).
- Indentation is intelligently stripped so that multi-line strings
can follow the nesting structure of the containing Nix
expression. E.g. in the example above 6 spaces are stripped from
the start of each line. This prevents unnecessary indentation in
generated files (which sometimes even breaks things).
See tests/lang/eval-okay-ind-string.nix for some examples.
$ nix-env -e $(which firefox)
or
$ nix-env -e /nix/store/nywzlygrkfcgz7dfmhm5xixlx1l0m60v-pan-0.132
* nix-env -i: if an argument contains a slash anywhere, treat it as a
path and follow it through symlinks into the Nix store. This allows
things like
$ nix-build -A firefox
$ nix-env -i ./result
* nix-env -q/-i/-e: don't complain when the `*' selector doesn't match
anything. In particular, `nix-env -q \*' doesn't fail anymore on an
empty profile.
but installations/upgrades as well. So `nix-env -ub \*' will
upgrade only those packages for which a substitute is available (or
to be precise, it will upgrade each package to the highest version
for which a substitute is available).
executed in a chroot that contains just the Nix store, the temporary
build directory, and a configurable set of additional directories
(/dev and /proc by default). This allows a bit more purity
enforcement: hidden build-time dependencies on directories such as
/usr or /nix/var/nix/profiles are no longer possible. As an added
benefit, accidental network downloads (cf. NIXPKGS-52) are prevented
as well (because files such as /etc/resolv.conf are not available in
the chroot).
However the usefulness of chroots is diminished by the fact that
many builders depend on /bin/sh, so you need /bin in the list of
additional directories. (And then on non-NixOS you need /lib as
well...)
usage by finding identical files in the store and hard-linking them
to each other. It typically reduces the size of the store by
something like 25-35%. This is what the optimise-store.pl script
did, but the new command is faster and more correct (it's safe wrt
garbage collection and concurrent builds).
the given attribute path (just as -A does with other option)
(NIX-83). So you can now say
$ nix-env -qa -A nixpkgs_unstable.gnome \*
atk-1.12.4
esound-0.2.36
...
to see the packages in the "gnome" attribute in Nixpkgs.
To *print* the attribute path, you should now use "--attr-path" /
"-P" (running out of letters...).
Nix expressions in that directory are combined into an attribute set
{file1 = import file1; file2 = import file2; ...}, i.e. each Nix
expression is an attribute with the file name as the attribute
name. Also recurses into directories.
* nix-env: removed the "--import" (-I) option which set the
~/.nix-defexpr symlink.
* nix-channel: don't use "nix-env --import", instead symlink
~/.nix-defexpr/channels. So finally nix-channel --update doesn't
override any default Nix expressions but combines with them.
This means that you can have (say) a local Nixpkgs SVN tree and use
it as a default for nix-env:
$ ln -s .../path-to-nixpkgs-tree ~/.nix-defexpr/nixpkgs_svn
and be subscribed to channels (including Nixpkgs) at the same time.
(If there is any ambiguity, the -A flag can be used to
disambiguate, e.g. "nix-env -i -A nixpkgs_svn.pan".)
(/nix/var/nix/daemon-socket). This allows access to the Nix daemon
to be restricted by setting the mode/ownership on that directory as
desired, e.g.
$ chmod 770 /nix/var/nix/daemon-socket
$ chown root.wheel /nix/var/nix/daemon-socket
to allow only users in the wheel group to use Nix.
Setting the ownership on a socket is much trickier, since the socket
must be deleted and recreated every time the daemon is started
(which would require additional Nix configuration file directives to
specify the mode/ownership, and wouldn't support arbitrary ACLs),
some BSD variants appear to ignore permissions on sockets, and it's
not clear whether the umask is respected on every platform when
creating sockets.
fixed-output derivations or substitutions try to build the same
store path at the same time. Locking generally catches this, but
not between multiple goals in the same process. This happened
especially often (actually, only) in the build farm with fetchurl
downloads of the same file being executed on multiple machines and
then copied back to the main machine where they would clobber each
other (NIXBF-13).
Solution: if a goal notices that the output path is already locked,
then go to sleep until another goal finishes (hopefully the one
locking the path) and try again.
need any info on substitutable paths, we just call the substituters
(such as download-using-manifests.pl) directly. This means that
it's no longer necessary for nix-pull to register substitutes or for
nix-channel to clear them, which makes those operations much faster
(NIX-95). Also, we don't have to worry about keeping nix-pull
manifests (in /nix/var/nix/manifests) and the database in sync with
each other.
The downside is that there is some overhead in calling an external
program to get the substitutes info. For instance, "nix-env -qas"
takes a bit longer.
Abolishing the substitutes table also makes the logic in
local-store.cc simpler, as we don't need to store info for invalid
paths. On the downside, you cannot do things like "nix-store -qR"
on a substitutable but invalid path (but nobody did that anyway).
* Never catch interrupts (the Interrupted exception).
;-)
* Channels: fix channels that are plain lists of derivations (like
strategoxt-unstable) instead of functions (like nixpkgs-unstable).
This fixes the error message "error: the left-hand side of the
function call is neither a function nor a primop (built-in
operation) but a list".
get the basename of the channel URL (e.g., nixpkgs-unstable). The
top-level Nix expression of the channel is now an attribute set, the
attributes of which are the individual channels (e.g.,
{nixpkgs_unstable = ...; strategoxt_unstable = ...}). This makes
attribute paths ("nix-env -qaA" and "nix-env -iA") more sensible,
e.g., "nix-env -iA nixpkgs_unstable.subversion".
by priority and version install. That is, if there are multiple
packages with the same name, then pick the package with the highest
priority, and only use the version if there are multiple packages
with the same priority.
This makes it possible to mark specific versions/variant in Nixpkgs
more or less desirable than others. A typical example would be a
beta version of some package (e.g., "gcc-4.2.0rc1") which should not
be installed even though it is the highest version, except when it
is explicitly selected (e.g., "nix-env -i gcc-4.2.0rc1").
* Idem for nix-env -u, only the semantics are a bit trickier since we
also need to take into account the priority of the currently
installed package (we never upgrade to a lower priority, unless
--always is given).
a user environment by an install or upgrade action. This is
particularly useful if you have a version installed that you don't
want to upgrade (e.g., because the newer versions are broken).
Example:
$ nix-env -u zapping --dry-run
(dry run; not doing anything)
upgrading `zapping-0.9.6' to `zapping-0.10cvs6'
$ nix-env --set-flag keep true zapping
$ nix-env -u zapping --dry-run
(dry run; not doing anything)
However, "-e" will still uninstall the package. (Maybe we should
require the keep flag to be explicitly set to false before it can be
uninstalled.)
to show only those derivations whose output is already in the Nix
store or that can be substituted (i.e., downloaded from somewhere).
In other words, it shows the packages that can be installed “quickly”,
i.e., don’t need to be built from source.
evaluator. This was important because the NixOS expressions started
to hit 2 MB default stack size on Linux.
GCC is really dumb about stack space: it just adds up all the local
variables and temporaries of every scope into one huge stack frame.
This is really bad for deeply recursive functions. For instance,
every `throw Error(format("error message"))' causes a format object
of a few hundred bytes to be allocated on the stack. As a result,
every recursive call to evalExpr2() consumed 4680 bytes. By
splitting evalExpr2() and by moving the exception-throwing code out
of the main functions, evalExpr2() now only consumes 40 bytes.
Similar for evalExpr().
which paths specified on the command line are invalid (i.e., don't
barf when encountering an invalid path, just print it). This is
useful for build-remote.pl to figure out which paths need to be
copied to a remote machine. (Currently we use rsync, but that's
rather inefficient.)
--export' into the Nix store, and optionally check the cryptographic
signatures against /nix/etc/nix/signing-key.pub. (TODO: verify
against a set of public keys.)
path. This is like `nix-store --dump', only it also dumps the
meta-information of the store path (references, deriver). Will add
a `--sign' flag later to add a cryptographic signature, which we
will use for exchanging store paths between build farm machines in a
secure manner.
attribute) about installed packages in user environments. Thus, an
operation like `nix-env -q --description' shows useful information
not only on available packages but also on installed packages.
* nix-env now passes the entire manifest as an argument to the Nix
expression of the user environment builder (not just a list of
paths), so that in particular the user environment builder has
access to the meta attributes.
* New operation `--set-flag' in nix-env to change meta info of
installed packages. This will be useful to pass per-package
policies to the user environment builder (e.g., how to resolve
collision or whether to disable a package (NIX-80)) or upgrade
policies in nix-env (e.g., that a package should be "masked", that
is, left untouched by upgrade actions). Example:
$ nix-env --set-flag enabled false ghc-6.4
computing the store path (NIX-77). This is an important security
property in multi-user Nix stores.
Note that this changes the store paths of derivations (since the
derivation aterms are added using addTextToStore), but not most
outputs (unless they use builtins.toFile).
* `sub' to subtract two numbers.
* `stringLength' to get the length of a string.
* `substring' to get a substring of a string. These should be enough
to allow most string operations to be expressed.
programs, so if a builder uses TMPDIR, then it will fail when
executed through nix-setuid-helper. In fact Glibc clears a whole
bunch of variables (see sysdeps/generic/unsecvars.h in the Glibc
sources), but only TMPDIR should matter in practice. As a
workaround, we reinitialise TMPDIR from NIX_BUILD_TOP.
