That is, when build-repeat > 0, and the output of two rounds differ,
then print a warning rather than fail the build. This is primarily to
let Hydra check reproducibility of all packages.
These syscalls are only available in 32bit architectures, but libseccomp
should handle them correctly even if we're on native architectures that
do not have these syscalls.
Signed-off-by: aszlig <aszlig@redmoonstudios.org>
Commands such as "cp -p" also use fsetxattr() in addition to fchown(),
so we need to make sure these syscalls always return successful as well
in order to avoid nasty "Invalid value" errors.
Signed-off-by: aszlig <aszlig@redmoonstudios.org>
What we basically want is a seccomp mode 2 BPF program like this but for
every architecture:
BPF_STMT(BPF_LD+BPF_W+BPF_ABS, offsetof(struct seccomp_data, nr)),
BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, __NR_chown, 4, 0),
BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, __NR_fchown, 3, 0),
BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, __NR_fchownat, 2, 0),
BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, __NR_lchown, 1, 0),
BPF_STMT(BPF_RET+BPF_K, SECCOMP_RET_ALLOW),
BPF_STMT(BPF_RET+BPF_K, SECCOMP_RET_ERRNO)
However, on 32 bit architectures we do have chown32, lchown32 and
fchown32, so we'd need to add all the architecture blurb which
libseccomp handles for us.
So we only need to make sure that we add the 32bit seccomp arch while
we're on x86_64 and otherwise we just stay at the native architecture
which was set during seccomp_init(), which more or less replicates
setting 32bit personality during runChild().
The FORCE_SUCCESS() macro here could be a bit less ugly but I think
repeating the seccomp_rule_add() all over the place is way uglier.
Another way would have been to create a vector of syscalls to iterate
over, but that would make error messages uglier because we can either
only print the (libseccomp-internal) syscall number or use
seccomp_syscall_resolve_num_arch() to get the name or even make the
vector a pair number/name, essentially duplicating everything again.
Signed-off-by: aszlig <aszlig@redmoonstudios.org>
This reverts commit ff0c0b645c.
We're going to use seccomp to allow "cp -p" and force chown-related
syscalls to always return 0.
Signed-off-by: aszlig <aszlig@redmoonstudios.org>
For example, you can now set
build-sandbox-paths = /dev/nvidiactl?
to specify that /dev/nvidiactl should only be mounted in the sandbox
if it exists in the host filesystem. This is useful e.g. for EC2
images that should support both CUDA and non-CUDA instances.
We can now write
throw Error("file '%s' not found", path);
instead of
throw Error(format("file '%s' not found") % path);
and similarly
printError("file '%s' not found", path);
instead of
printMsg(lvlError, format("file '%s' not found") % path);
This largely reverts c68e5913c7. Running
builds as root breaks "cp -p", since when running as root, "cp -p"
assumes that it can succesfully chown() files. But that's not actually
the case since the user namespace doesn't provide a complete uid
mapping. So it barfs with a fatal error message ("cp: failed to
preserve ownership for 'foo': Invalid argument").
This fixes an assertion failure in "assert(goal);" in
Worker::waitForInput() after a substitution goal is cancelled by the
termination of another goal. The problem was the line
//worker.childTerminated(shared_from_this()); // FIXME
in the SubstitutionGoal destructor. This was disabled because
shared_from_this() obviously doesn't work from a destructor. So we now
use a real pointer for object identity.
For example, you can now say:
configureFlags = "--prefix=${placeholder "out"} --includedir=${placeholder "dev"}";
The strings returned by the ‘placeholder’ builtin are replaced at
build time by the actual store paths corresponding to the specified
outputs.
Previously, you had to work around the inability to self-reference by doing stuff like:
preConfigure = ''
configureFlags+=" --prefix $out --includedir=$dev"
'';
or rely on ad-hoc variable interpolation semantics in Autoconf or Make
(e.g. --prefix=\$(out)), which doesn't always work.
This makes it easier to create a diverted store, i.e.
NIX_REMOTE="local?root=/tmp/root"
instead of
NIX_REMOTE="local?real=/tmp/root/nix/store&state=/tmp/root/nix/var/nix" NIX_LOG_DIR=/tmp/root/nix/var/log
This way, all builds appear to have a uid/gid of 0 inside the
chroot. In the future, this may allow using programs like
systemd-nspawn inside builds, but that will require assigning a larger
UID/GID map to the build.
Issue #625.
This allows an unprivileged user to perform builds on a diverted store
(i.e. where the physical store location differs from the logical
location).
Example:
$ NIX_LOG_DIR=/tmp/log NIX_REMOTE="local?real=/tmp/store&state=/tmp/var" nix-build -E \
'with import <nixpkgs> {}; runCommand "foo" { buildInputs = [procps nettools]; } "id; ps; ifconfig; echo $out > $out"'
will do a build in the Nix store physically in /tmp/store but
logically in /nix/store (and thus using substituters for the latter).
This is primarily to subsume the functionality of the
copy-from-other-stores substituter. For example, in the NixOS
installer, we can now do (assuming we're in the target chroot, and the
Nix store of the installation CD is bind-mounted on /tmp/nix):
$ nix-build ... --option substituters 'local?state=/tmp/nix/var&real=/tmp/nix/store'
However, unlike copy-from-other-stores, this also allows write access
to such a store. One application might be fetching substitutes for
/nix/store in a situation where the user doesn't have sufficient
privileges to create /nix, e.g.:
$ NIX_REMOTE="local?state=/home/alice/nix/var&real=/home/alice/nix/store" nix-build ...
If --no-build-output is given (which will become the default for the
"nix" command at least), show the last 10 lines of the build output if
the build fails.
Caching path info is generally useful. For instance, it speeds up "nix
path-info -rS /run/current-system" (i.e. showing the closure sizes of
all paths in the closure of the current system) from 5.6s to 0.15s.
This also eliminates some APIs like Store::queryDeriver() and
Store::queryReferences().
These are content-addressed paths or outputs of locally performed
builds. They are trusted even if they don't have signatures, so "nix
verify-paths" won't complain about them.
Also, move a few free-standing functions into StoreAPI and Derivation.
Also, introduce a non-nullable smart pointer, ref<T>, which is just a
wrapper around std::shared_ptr ensuring that the pointer is never
null. (For reference-counted values, this is better than passing a
"T&", because the latter doesn't maintain the refcount. Usually, the
caller will have a shared_ptr keeping the value alive, but that's not
always the case, e.g., when passing a reference to a std::thread via
std::bind.)
For example,
$ nix-build --hash -A nix-repl.src
will build the fixed-output derivation nix-repl.src (a fetchFromGitHub
call), but instead of *verifying* the hash given in the Nix
expression, it prints out the resulting hash, and then moves the
result to its content-addressed location in the Nix store. E.g
build produced path ‘/nix/store/504a4k6zi69dq0yjc0bm12pa65bccxam-nix-repl-8a2f5f0607540ffe56b56d52db544373e1efb980-src’ with sha256 hash ‘0cjablz01i0g9smnavhf86imwx1f9mnh5flax75i615ml71gsr88’
The goal of this is to make all nix-prefetch-* scripts unnecessary: we
can just let Nix run the real thing (i.e., the corresponding fetch*
derivation).
Another example:
$ nix-build --hash -E 'with import <nixpkgs> {}; fetchgit { url = "https://github.com/NixOS/nix.git"; sha256 = "ffffffffffffffffffffffffffffffffffffffffffffffffffff"; }'
...
git revision is 9e7c1a4bbd
...
build produced path ‘/nix/store/gmsnh9i7x4mb7pyd2ns7n3c9l90jfsi1-nix’ with sha256 hash ‘1188xb621diw89n25rifqg9lxnzpz7nj5bfh4i1y3dnis0dmc0zp’
(Having to specify a fake sha256 hash is a bit annoying...)
E.g.
$ nix-build pkgs/stdenv/linux/ -A stage1.pkgs.perl --check
nix-store: src/libstore/build.cc:1323: void nix::DerivationGoal::tryToBuild(): Assertion `buildMode != bmCheck || validPaths.size() == drv->outputs.size()' failed.
when perl.out exists but perl.man doesn't. The fix is to only check
the outputs that exist. Note that "nix-build -A stage1.pkgs.all
--check" will still give a (proper) error in this case.
This was observed in the deb_debian7x86_64 build:
http://hydra.nixos.org/build/29973215
Calling c_str() on a temporary should be fine because the temporary
shouldn't be destroyed until after the execl() call, but who knows...
If repair found a corrupted/missing path that depended on a
multiple-output derivation, and some of the outputs of the latter were
not present, it failed with a message like
error: path ‘/nix/store/cnfn9d5fjys1y93cz9shld2xwaibd7nn-bash-4.3-p42-doc’ is not valid
This makes Darwin consistent with Linux: Nix expressions can't break
out of the sandbox unless relaxed sandbox mode is enabled.
For the normal sandbox mode this will require fixing #759 however.
Caused by 8063fc497a. If tmpDir !=
tmpDirInSandbox (typically when there are multiple concurrent builds
with the same name), the *Path attribute would not point to an
existing file. This caused Nixpkgs' writeTextFile to write an empty
file. In particular this showed up as hanging VM builds (because it
would run an empty run-nixos-vm script and then wait for it to finish
booting).
This is arguably nitpicky, but I think this new formulation is even
clearer. My thinking is that it's easier to comprehend when the
calculated hash value is displayed close to the output path. (I think it
is somewhat similar to eliminating double negatives in logic
statements.)
The formulation is inspired / copied from the OpenEmbedded build tool,
bitbake.
Rather than using $<host-TMPDIR>/nix-build-<drvname>-<number>, the
temporary directory is now always /tmp/nix-build-<drvname>-0. This
improves bitwise-exact reproducibility for builds that store $TMPDIR
in their build output. (Of course, those should still be fixed...)
Temporarily allow derivations to describe their full sandbox profile.
This will be eventually scaled back to a more secure setup, see the
discussion at #695
Nix reports a hash mismatch saying:
output path ‘foo’ should have sha256 hash ‘abc’, instead has ‘xyz’
That message is slightly ambiguous and some people read that statement
to mean the exact opposite of what it is supposed to mean. After this
patch, the message will be:
Nix expects output path ‘foo’ to have sha256 hash ‘abc’, instead it has ‘xyz’
- rename options but leav old names as lower-priority aliases,
also "-dirs" -> "-paths" to get closer to the meaning
- update docs to reflect the new names (old aliases are not documented),
including a new file with release notes
- tests need an update after corresponding changes to nixpkgs
- __noChroot is left as it is (after discussion on the PR)
Passing "--option build-repeat <N>" will cause every build to be
repeated N times. If the build output differs between any round, the
build is rejected, and the output paths are not registered as
valid. This is primarily useful to verify build determinism. (We
already had a --check option to repeat a previously succeeded
build. However, with --check, non-deterministic builds are registered
in the DB. Preventing that is useful for Hydra to ensure that
non-deterministic builds don't end up getting published at all.)
This reverts commit 79ca503332. Ouch,
never noticed this. We definitely don't want to allow builds to have
arbitrary access to /bin and /usr/bin, because then they can (for
instance) bring in a bunch of setuid programs. Also, we shouldn't be
encouraging the use of impurities in the default configuration.
The stack allocated for the builder was way too small (32 KB). This is
sufficient for normal derivations, because they just do some setup and
then exec() the actual builder. But for the fetchurl builtin
derivation it's not enough. Also, allocating the stack on the caller's
stack was fishy business.
Fixes https://github.com/NixOS/nixpkgs/issues/9504.
Note that this means we may have a non-functional /bin/sh in the
chroot while rebuilding Bash or one of its dependencies. Ideally those
packages don't rely on /bin/sh though.
This ensures that 1) the derivation doesn't change when Nix changes;
2) the derivation closure doesn't contain Nix and its dependencies; 3)
we don't have to rely on ugly chroot hacks.
In particular, hydra-queue-runner can now distinguish between remote
build / substitution / already-valid. For instance, if a path already
existed on the remote side, we don't want to store a log file.
Previously, to build a derivation remotely, we had to copy the entire
closure of the .drv file to the remote machine, even though we only
need the top-level derivation. This is very wasteful: the closure can
contain thousands of store paths, and in some Hydra use cases, include
source paths that are very large (e.g. Git/Mercurial checkouts).
So now there is a new operation, StoreAPI::buildDerivation(), that
performs a build from an in-memory representation of a derivation
(BasicDerivation) rather than from a on-disk .drv file. The only files
that need to be in the Nix store are the sources of the derivation
(drv.inputSrcs), and the needed output paths of the dependencies (as
described by drv.inputDrvs). "nix-store --serve" exposes this
interface.
Note that this is a privileged operation, because you can construct a
derivation that builds any store path whatsoever. Fixing this will
require changing the hashing scheme (i.e., the output paths should be
computed from the other fields in BasicDerivation, allowing them to be
verified without access to other derivations). However, this would be
quite nice because it would allow .drv-free building (e.g. "nix-env
-i" wouldn't have to write any .drv files to disk).
Fixes#173.
The following patch is an attempt to address this bug (see
<http://bugs.gnu.org/18994>) by preserving the supplementary groups of
build users in the build environment.
In practice, I would expect that supplementary groups would contain only
one or two groups: the build users group, and possibly the “kvm” group.
[Changed &at(0) to data() and removed tabs - Eelco]
Not substituting builds with "preferLocalBuild = true" was a bad idea,
because it didn't take the cost of dependencies into account. For
instance, if we can't substitute a fetchgit call, then we have to
download/build git and all its dependencies.
Partially reverts 5558652709 and adds a
new derivation attribute "allowSubstitutes" to specify whether a
derivation may be substituted.
Nixpkgs' writeTextAsFile does this:
mv "$textPath" "$n"
Since $textPath was owned by root, if $textPath is on the same
filesystem as $n, $n will be owned as root. As a result, the build
result was rejected as having suspicious ownership.
http://hydra.nixos.org/build/22836807
This hook can be used to set system-specific per-derivation build
settings that don't fit into the derivation model and are too complex or
volatile to be hard-coded into nix. Currently, the pre-build hook can
only add chroot dirs/files through the interface, but it also has full
access to the chroot root.
The specific use case for this is systems where the operating system ABI
is more complex than just the kernel-support system calls. For example,
on OS X there is a set of system-provided frameworks that can reliably
be accessed by any program linked to them, no matter the version the
program is running on. Unfortunately, those frameworks do not
necessarily live in the same locations on each version of OS X, nor do
their dependencies, and thus nix needs to know the specific version of
OS X currently running in order to make those frameworks available. The
pre-build hook is a perfect mechanism for doing just that.
This hook can be used to set system specific per-derivation build
settings that don't fit into the derivation model and are too complex or
volatile to be hard-coded into nix. Currently, the pre-build hook can
only add chroot dirs/files.
The specific use case for this is systems where the operating system ABI
is more complex than just the kernel-supported system calls. For
example, on OS X there is a set of system-provided frameworks that can
reliably be accessed by any program linked to them, no matter the
version the program is running on. Unfortunately, those frameworks do
not necessarily live in the same locations on each version of OS X, nor
do their dependencies, and thus nix needs to know the specific version
of OS X currently running in order to make those frameworks available.
The pre-build hook is a perfect mechanism for doing just that.
This was causing NixOS VM tests to fail mysteriously since
5ce50cd99e. Nscd could (sometimes) no
longer read /etc/hosts:
open("/etc/hosts", O_RDONLY|O_CLOEXEC) = -1 EACCES (Permission denied)
Probably there was some wacky interaction between the guest kernel and
the 9pfs implementation in QEMU.
Thus, for example, to get /bin/sh in a chroot, you only need to
specify /bin/sh=${pkgs.bash}/bin/sh in build-chroot-dirs. The
dependencies of sh will be added automatically.
I'm seeing hangs in Glibc's setxid_mark_thread() again. This is
probably because the use of an intermediate process to make clone()
safe from a multi-threaded program (see
524f89f139) is defeated by the use of
vfork(), since the intermediate process will have a copy of Glibc's
threading data structures due to the vfork(). So use a regular fork()
again.
If ‘build-use-chroot’ is set to ‘true’, fixed-output derivations are
now also chrooted. However, unlike normal derivations, they don't get
a private network namespace, so they can still access the
network. Also, the use of the ‘__noChroot’ derivation attribute is
no longer allowed.
Setting ‘build-use-chroot’ to ‘relaxed’ gives the old behaviour.
chroot only changes the process root directory, not the mount namespace root
directory, and it is well-known that any process with chroot capability can
break out of a chroot "jail". By using pivot_root as well, and unmounting the
original mount namespace root directory, breaking out becomes impossible.
Non-root processes typically have no ability to use chroot() anyway, but they
can gain that capability through the use of clone() or unshare(). For security
reasons, these syscalls are limited in functionality when used inside a normal
chroot environment. Using pivot_root() this way does allow those syscalls to be
put to their full use.
I.e., not readable to the nixbld group. This improves purity a bit for
non-chroot builds, because it prevents a builder from enumerating
store paths (i.e. it can only access paths it knows about).
For the "stdenv accidentally referring to bootstrap-tools", it seems
easier to specify the path that we don't want to depend on, e.g.
disallowedRequisites = [ bootstrapTools ];
It turns out that using clone() to start a child process is unsafe in
a multithreaded program. It can cause the initialisation of a build
child process to hang in setgroups(), as seen several times in the
build farm:
The reason is that Glibc thinks that the other threads of the parent
exist in the child, so in setxid_mark_thread() it tries to get a futex
that has been acquired by another thread just before the clone(). With
fork(), Glibc runs pthread_atfork() handlers that take care of this
(in particular, __reclaim_stacks()). But clone() doesn't do that.
Fortunately, we can use fork()+unshare() instead of clone() to set up
private namespaces.
See also https://www.mail-archive.com/lxc-devel@lists.linuxcontainers.org/msg03434.html.
The Nixpkgs stdenv prints some custom escape sequences to denote
nesting and stuff like that. Most terminals (e.g. xterm, konsole)
ignore them, but some do not (e.g. xfce4-terminal). So for the benefit
of the latter, filter them out.
While running Python 3’s test suite, we noticed that on some systems
/dev/pts/ptmx is created with permissions 0 (that’s the case with my
Nixpkgs-originating 3.0.43 kernel, but someone with a Debian-originating
3.10-3 reported not having this problem.)
There’s still the problem that people without
CONFIG_DEVPTS_MULTIPLE_INSTANCES=y are screwed (as noted in build.cc),
but I don’t see how we could work around it.
Since the addition of build-max-log-size, a call to
handleChildOutput() can result in cancellation of a goal. This
invalidated the "j" iterator in the waitForInput() loop, even though
it was still used afterwards. Likewise for the maxSilentTime
handling.
Probably fixes#231. At least it gets rid of the valgrind warnings.
The daemon now creates /dev deterministically (thanks!). However, it
expects /dev/kvm to be present.
The patch below restricts that requirement (1) to Linux-based systems,
and (2) to systems where /dev/kvm already exists.
I’m not sure about the way to handle (2). We could special-case
/dev/kvm and create it (instead of bind-mounting it) in the chroot, so
it’s always available; however, it wouldn’t help much since most likely,
if /dev/kvm missing, then KVM support is missing.
We were relying on SubstitutionGoal's destructor releasing the lock,
but if a goal is a top-level goal, the destructor won't run in a
timely manner since its reference count won't drop to zero. So
release it explicitly.
Fixes#178.
The flag ‘--check’ to ‘nix-store -r’ or ‘nix-build’ will cause Nix to
redo the build of a derivation whose output paths are already valid.
If the new output differs from the original output, an error is
printed. This makes it easier to test if a build is deterministic.
(Obviously this cannot catch all sources of non-determinism, but it
catches the most common one, namely the current time.)
For example:
$ nix-build '<nixpkgs>' -A patchelf
...
$ nix-build '<nixpkgs>' -A patchelf --check
error: derivation `/nix/store/1ipvxsdnbhl1rw6siz6x92s7sc8nwkkb-patchelf-0.6' may not be deterministic: hash mismatch in output `/nix/store/4pc1dmw5xkwmc6q3gdc9i5nbjl4dkjpp-patchelf-0.6.drv'
The --check build fails if not all outputs are valid. Thus the first
call to nix-build is necessary to ensure that all outputs are valid.
The current outputs are left untouched: the new outputs are either put
in a chroot or diverted to a different location in the store using
hash rewriting.
*headdesk*
*headdesk*
*headdesk*
So since commit 22144afa8d, Nix hasn't
actually checked whether the content of a downloaded NAR matches the
hash specified in the manifest / NAR info file. Urghhh...
On Linux, Nix can build i686 packages even on x86_64 systems. It's not
enough to recognize this situation by settings.thisSystem, we also have
to consult uname(). E.g. we can be running on a i686 Debian with an
amd64 kernel. In that situation settings.thisSystem is i686-linux, but
we still need to change personality to i686 to make builds consistent.
On a system with multiple CPUs, running Nix operations through the
daemon is significantly slower than "direct" mode:
$ NIX_REMOTE= nix-instantiate '<nixos>' -A system
real 0m0.974s
user 0m0.875s
sys 0m0.088s
$ NIX_REMOTE=daemon nix-instantiate '<nixos>' -A system
real 0m2.118s
user 0m1.463s
sys 0m0.218s
The main reason seems to be that the client and the worker get moved
to a different CPU after every call to the worker. This patch adds a
hack to lock them to the same CPU. With this, the overhead of going
through the daemon is very small:
$ NIX_REMOTE=daemon nix-instantiate '<nixos>' -A system
real 0m1.074s
user 0m0.809s
sys 0m0.098s
This reverts commit 69b8f9980f.
The timeout should be enforced remotely. Otherwise, if the garbage
collector is running either locally or remotely, if will block the
build or closure copying for some time. If the garbage collector
takes too long, the build may time out, which is not what we want.
Also, on heavily loaded systems, copying large paths to and from the
remote machine can take a long time, also potentially resulting in a
timeout.
mount(2) with MS_BIND allows mounting a regular file on top of a regular
file, so there's no reason to only bind directories. This allows finer
control over just which files are and aren't included in the chroot
without having to build symlink trees or the like.
Signed-off-by: Shea Levy <shea@shealevy.com>
With C++ std::map, doing a comparison like ‘map["foo"] == ...’ has the
side-effect of adding a mapping from "foo" to the empty string if
"foo" doesn't exist in the map. So we ended up setting some
environment variables by accident.
In particular this means that "trivial" derivations such as writeText
are not substituted, reducing the number of GET requests to the binary
cache by about 200 on a typical NixOS configuration.
Before calling dumpPath(), we have to make sure the files are owned by
the build user. Otherwise, the build could contain a hard link to
(say) /etc/shadow, which would then be read by the daemon and
rewritten as a world-readable file.
This only affects systems that don't have hard link restrictions
enabled.
The assertion in canonicalisePathMetaData() failed because the
ownership of the path already changed due to the hash rewriting. The
solution is not to check the ownership of rewritten paths.
Issue #122.
Otherwise subsequent invocations of "--repair" will keep rebuilding
the path. This only happens if the path content differs between
builds (e.g. due to timestamps).
Don't pass --timeout / --max-silent-time to the remote builder.
Instead, let the local Nix process terminate the build if it exceeds a
timeout. The remote builder will be killed as a side-effect. This
gives better error reporting (since the timeout message from the
remote side wasn't properly propagated) and handles non-Nix problems
like SSH hangs.
I'm not sure if it has ever worked correctly. The line "lastWait =
after;" seems to mean that the timer was reset every time a build
produced log output.
Note that the timeout is now per build, as documented ("the maximum
number of seconds that a builder can run").
It turns out that in multi-user Nix, a builder may be able to do
ln /etc/shadow $out/foo
Afterwards, canonicalisePathMetaData() will be applied to $out/foo,
causing /etc/shadow's mode to be set to 444 (readable by everybody but
writable by nobody). That's obviously Very Bad.
Fortunately, this fails in NixOS's default configuration because
/nix/store is a bind mount, so "ln" will fail with "Invalid
cross-device link". It also fails if hard-link restrictions are
enabled, so a workaround is:
echo 1 > /proc/sys/fs/protected_hardlinks
The solution is to check that all files in $out are owned by the build
user. This means that innocuous operations like "ln
${pkgs.foo}/some-file $out/" are now rejected, but that already failed
in chroot builds anyway.
...where <XX> is the first two characters of the derivation.
Otherwise /nix/var/log/nix/drvs may become so large that we run into
all sorts of weird filesystem limits/inefficiences. For instance,
ext3/ext4 filesystems will barf with "ext4_dx_add_entry:1551:
Directory index full!" once you hit a few million files.
If a derivation has multiple outputs, then we only want to download
those outputs that are actuallty needed. So if we do "nix-build -A
openssl.man", then only the "man" output should be downloaded.
Likewise if another package depends on ${openssl.man}.
The tricky part is that different derivations can depend on different
outputs of a given derivation, so we may need to restart the
corresponding derivation goal if that happens.
For example, given a derivation with outputs "out", "man" and "bin":
$ nix-build -A pkg
produces ./result pointing to the "out" output;
$ nix-build -A pkg.man
produces ./result-man pointing to the "man" output;
$ nix-build -A pkg.all
produces ./result, ./result-man and ./result-bin;
$ nix-build -A pkg.all -A pkg2
produces ./result, ./result-man, ./result-bin and ./result-2.
