Most functions now take a StorePath argument rather than a Path (which
is just an alias for std::string). The StorePath constructor ensures
that the path is syntactically correct (i.e. it looks like
<store-dir>/<base32-hash>-<name>). Similarly, functions like
buildPaths() now take a StorePathWithOutputs, rather than abusing Path
by adding a '!<outputs>' suffix.
Note that the StorePath type is implemented in Rust. This involves
some hackery to allow Rust values to be used directly in C++, via a
helper type whose destructor calls the Rust type's drop()
function. The main issue is the dynamic nature of C++ move semantics:
after we have moved a Rust value, we should not call the drop function
on the original value. So when we move a value, we set the original
value to bitwise zero, and the destructor only calls drop() if the
value is not bitwise zero. This should be sufficient for most types.
Also lots of minor cleanups to the C++ API to make it more modern
(e.g. using std::optional and std::string_view in some places).
Also, fetchGit now runs in O(1) memory since we pipe the output of
'git archive' directly into unpackTarball() (rather than first reading
it all into memory).
We can now convert Rust Errors to C++ exceptions. At the Rust->C++ FFI
boundary, Result<T, Error> will cause Error to be converted to and
thrown as a C++ exception.
E.g.
$ nix-build '<nixpkgs>' -A hello --experimental-features no-url-literals
error: URL literals are disabled, at /nix/store/vsjamkzh15r3c779q2711az826hqgvzr-nixpkgs-20.03pre194957.bef773ed53f/nixpkgs/pkgs/top-level/all-packages.nix:1236:11
Helps with implementing https://github.com/NixOS/rfcs/pull/45.
A corrupt entry in .links prevents adding a fixed version of that file
to the store in any path. The user experience is that corruption
present in the store 'spreads' to new paths added to the store:
(With store optimisation enabled)
1. A file in the store gets corrupted somehow (eg: filesystem bug).
2. The user tries to add a thing to the store which contains a good copy
of the corrupted file.
3. The file being added to the store is hashed, found to match the bad
.links entry, and is replaced by a link to the bad .links entry.
(The .links entry's hash is not verified during add -- this would
impose a substantial performance burden.)
4. The user observes that the thing in the store that is supposed to be
a copy of what they were trying to add is not a correct copy -- some
files have different contents! Running "nix-store --verify
--check-contents --repair" does not fix the problem.
This change makes "nix-store --verify --check-contents --repair" fix
this problem. Bad .links entries are simply removed, allowing future
attempts to insert a good copy of the file to succeed.
Derivations that want to use recursion should now set
requiredSystemFeatures = [ "recursive-nix" ];
to make the daemon socket appear.
Also, Nix should be configured with "experimental-features =
recursive-nix".
This allows Nix builders to call Nix to build derivations, with some
limitations.
Example:
let nixpkgs = fetchTarball channel:nixos-18.03; in
with import <nixpkgs> {};
runCommand "foo"
{
buildInputs = [ nix jq ];
NIX_PATH = "nixpkgs=${nixpkgs}";
}
''
hello=$(nix-build -E '(import <nixpkgs> {}).hello.overrideDerivation (args: { name = "hello-3.5"; })')
$hello/bin/hello
mkdir -p $out/bin
ln -s $hello/bin/hello $out/bin/hello
nix path-info -r --json $hello | jq .
''
This derivation makes a recursive Nix call to build GNU Hello and
symlinks it from its $out, i.e.
# ll ./result/bin/
lrwxrwxrwx 1 root root 63 Jan 1 1970 hello -> /nix/store/s0awxrs71gickhaqdwxl506hzccb30y5-hello-3.5/bin/hello
# nix-store -qR ./result
/nix/store/hwwqshlmazzjzj7yhrkyjydxamvvkfd3-glibc-2.26-131
/nix/store/s0awxrs71gickhaqdwxl506hzccb30y5-hello-3.5
/nix/store/sgmvvyw8vhfqdqb619bxkcpfn9lvd8ss-foo
This is implemented as follows:
* Before running the outer builder, Nix creates a Unix domain socket
'.nix-socket' in the builder's temporary directory and sets
$NIX_REMOTE to point to it. It starts a thread to process
connections to this socket. (Thus you don't need to have nix-daemon
running.)
* The daemon thread uses a wrapper store (RestrictedStore) to keep
track of paths added through recursive Nix calls, to implement some
restrictions (see below), and to do some censorship (e.g. for
purity, queryPathInfo() won't return impure information such as
signatures and timestamps).
* After the build finishes, the output paths are scanned for
references to the paths added through recursive Nix calls (in
addition to the inputs closure). Thus, in the example above, $out
has a reference to $hello.
The main restriction on recursive Nix calls is that they cannot do
arbitrary substitutions. For example, doing
nix-store -r /nix/store/kmwd1hq55akdb9sc7l3finr175dajlby-hello-2.10
is forbidden unless /nix/store/kmwd... is in the inputs closure or
previously built by a recursive Nix call. This is to prevent
irreproducible derivations that have hidden dependencies on
substituters or the current store contents. Building a derivation is
fine, however, and Nix will use substitutes if available. In other
words, the builder has to present proof that it knows how to build a
desired store path from scratch by constructing a derivation graph for
that path.
Probably we should also disallow instantiating/building fixed-output
derivations (specifically, those that access the network, but
currently we have no way to mark fixed-output derivations that don't
access the network). Otherwise sandboxed derivations can bypass
sandbox restrictions and access the network.
When sandboxing is enabled, we make paths appear in the sandbox of the
builder by entering the mount namespace of the builder and
bind-mounting each path. This is tricky because we do a pivot_root()
in the builder to change the root directory of its mount namespace,
and thus the host /nix/store is not visible in the mount namespace of
the builder. To get around this, just before doing pivot_root(), we
branch a second mount namespace that shares its /nix/store mountpoint
with the parent.
Recursive Nix currently doesn't work on macOS in sandboxed mode
(because we can't change the sandbox policy of a running build) and in
non-root mode (because setns() barfs).
Add missing docstring on InstallableCommand. Also, some of these were wrapped
when they're right next to a line longer than the unwrapped line, so we can just
unwrap them to save vertical space.
Fixes the following warning and the indicate potential issue:
src/libstore/worker-protocol.hh:66:1: warning: class 'Source' was previously declared as a struct; this is valid, but may result in linker errors
under the Microsoft C++ ABI [-Wmismatched-tags]
(cherry picked from commit 6e1bb04870b1b723282d32182af286646f13bf3c)
This adds a command 'nix make-content-addressable' that rewrites the
specified store paths into content-addressable paths. The advantage of
such paths is that 1) they can be imported without signatures; 2) they
can enable deduplication in cases where derivation changes do not
cause output changes (apart from store path hashes).
For example,
$ nix make-content-addressable -r nixpkgs.cowsay
rewrote '/nix/store/g1g31ah55xdia1jdqabv1imf6mcw0nb1-glibc-2.25-49' to '/nix/store/48jfj7bg78a8n4f2nhg269rgw1936vj4-glibc-2.25-49'
...
rewrote '/nix/store/qbi6rzpk0bxjw8lw6azn2mc7ynnn455q-cowsay-3.03+dfsg1-16' to '/nix/store/iq6g2x4q62xp7y7493bibx0qn5w7xz67-cowsay-3.03+dfsg1-16'
We can then copy the resulting closure to another store without
signatures:
$ nix copy --trusted-public-keys '' ---to ~/my-nix /nix/store/iq6g2x4q62xp7y7493bibx0qn5w7xz67-cowsay-3.03+dfsg1-16
In order to support self-references in content-addressable paths,
these paths are hashed "modulo" self-references, meaning that
self-references are zeroed out during hashing. Somewhat annoyingly,
this means that the NAR hash stored in the Nix database is no longer
necessarily equal to the output of "nix hash-path"; for
content-addressable paths, you need to pass the --modulo flag:
$ nix path-info --json /nix/store/iq6g2x4q62xp7y7493bibx0qn5w7xz67-cowsay-3.03+dfsg1-16 | jq -r .[].narHash
sha256:0ri611gdilz2c9rsibqhsipbfs9vwcqvs811a52i2bnkhv7w9mgw
$ nix hash-path --type sha256 --base32 /nix/store/iq6g2x4q62xp7y7493bibx0qn5w7xz67-cowsay-3.03+dfsg1-16
1ggznh07khq0hz6id09pqws3a8q9pn03ya3c03nwck1kwq8rclzs
$ nix hash-path --type sha256 --base32 /nix/store/iq6g2x4q62xp7y7493bibx0qn5w7xz67-cowsay-3.03+dfsg1-16 --modulo iq6g2x4q62xp7y7493bibx0qn5w7xz67
0ri611gdilz2c9rsibqhsipbfs9vwcqvs811a52i2bnkhv7w9mgw
Experimental features are now opt-in. There is currently one
experimental feature: "nix-command" (which enables the "nix"
command. This will allow us to merge experimental features more
quickly, without committing to supporting them indefinitely.
Typical usage:
$ nix build --experimental-features 'nix-command flakes' nixpkgs#hello
(cherry picked from commit 8e478c2341,
without the "flakes" feature)
The tmpDirInSandbox is different when in sandboxed vs. non-sandboxed.
Since we don’t know ahead of time here whether sandboxing is enabled,
we need to reset all of the env vars we’ve set previously. This fixes
the issue encountered in https://github.com/NixOS/nixpkgs/issues/70856.
Previously, SANDBOX_SHELL was set to empty when unavailable. This
caused issues when actually generating the sandbox. Instead, just set
SANDBOX_SHELL when --with-sandbox-shell= is non-empty. Alternative
implementation to https://github.com/NixOS/nix/pull/3038.
This integrates the functionality of the index-debuginfo program in
nixos-channel-scripts to maintain an index of DWARF debuginfo files in
a format usable by dwarffs. Thus the debug info index is updated by
Hydra rather than by the channel mirroring script.
Example usage:
$ nix copy --to 'file:///tmp/binary-cache?index-debug-info=true' /nix/store/vr9mhcch3fljzzkjld3kvkggvpq38cva-nix-2.2.2-debug
$ cat /tmp/binary-cache/debuginfo/036b210b03bad75ab2d8fc80b7a146f98e7f1ecf.debug
{"archive":"../nar/0313h2kdhk4v73xna9ysiksp2v8xrsk5xsw79mmwr3rg7byb4ka8.nar.xz","member":"lib/debug/.build-id/03/6b210b03bad75ab2d8fc80b7a146f98e7f1ecf.debug"}
Fixes#3083.
Previously we allowed any length of name for Nix derivations. This is
bad because different file systems have different max lengths. To make
things predictable, I have picked a max. This was done by trying to
build this derivation:
derivation {
name = "aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa";
builder = "/no-such-path";
system = "x86_64-linux";
}
Take off one a and it will not lead to file name too long. That ends
up being 212 a’s. An even smaller max could be picked if we want to
support more file systems.
Working backwards, this is why:
aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa-${name}.drv.chroot
> 255 - 32 - 1 - 4 - 7 = 211
These are already handled separately. This fixes warnings like
warning: ignoring the user-specified setting 'max-jobs', because it is a restricted setting and you are not a trusted user
when using the -j flag.
Make curl's low speed limit configurable via stalled-download-timeout.
Before, this limit was five minutes without receiving a single byte.
This is much too long as if the remote end may not have even
acknowledged the HTTP request.
POSIX file locks are essentially incompatible with multithreading. BSD
locks have much saner semantics. We need this now that there can be
multiple concurrent LocalStore::buildPaths() invocations.
This currently fails because we're using POSIX file locks. So when the
garbage collector opens and closes its own temproots file, it causes
the lock to be released and then deleted by another GC instance.
Passing `--post-build-hook /foo/bar` to a nix-* command will cause
`/foo/bar` to be executed after each build with the following
environment variables set:
DRV_PATH=/nix/store/drv-that-has-been-built.drv
OUT_PATHS=/nix/store/...build /nix/store/...build-bin /nix/store/...build-dev
This can be useful in particular to upload all the builded artifacts to
the cache (including the ones that don't appear in the runtime closure
of the final derivation or are built because of IFD).
This new feature prints the stderr/stdout output to the `nix-build`
and `nix build` client, and the output is printed in a Nix 2
compatible format:
[nix]$ ./inst/bin/nix-build ./test.nix
these derivations will be built:
/nix/store/ishzj9ni17xq4hgrjvlyjkfvm00b0ch9-my-example-derivation.drv
building '/nix/store/ishzj9ni17xq4hgrjvlyjkfvm00b0ch9-my-example-derivation.drv'...
hello!
bye!
running post-build-hook '/home/grahamc/projects/github.com/NixOS/nix/post-hook.sh'...
post-build-hook: + sleep 1
post-build-hook: + echo 'Signing paths' /nix/store/qr213vjmibrqwnyp5fw678y7whbkqyny-my-example-derivation
post-build-hook: Signing paths /nix/store/qr213vjmibrqwnyp5fw678y7whbkqyny-my-example-derivation
post-build-hook: + sleep 1
post-build-hook: + echo 'Uploading paths' /nix/store/qr213vjmibrqwnyp5fw678y7whbkqyny-my-example-derivation
post-build-hook: Uploading paths /nix/store/qr213vjmibrqwnyp5fw678y7whbkqyny-my-example-derivation
post-build-hook: + sleep 1
post-build-hook: + printf 'very important stuff'
/nix/store/qr213vjmibrqwnyp5fw678y7whbkqyny-my-example-derivation
[nix-shell:~/projects/github.com/NixOS/nix]$ ./inst/bin/nix build -L -f ./test.nix
my-example-derivation> hello!
my-example-derivation> bye!
my-example-derivation (post)> + sleep 1
my-example-derivation (post)> + echo 'Signing paths' /nix/store/c263gzj2kb2609mz8wrbmh53l14wzmfs-my-example-derivation
my-example-derivation (post)> Signing paths /nix/store/c263gzj2kb2609mz8wrbmh53l14wzmfs-my-example-derivation
my-example-derivation (post)> + sleep 1
my-example-derivation (post)> + echo 'Uploading paths' /nix/store/c263gzj2kb2609mz8wrbmh53l14wzmfs-my-example-derivation
my-example-derivation (post)> Uploading paths /nix/store/c263gzj2kb2609mz8wrbmh53l14wzmfs-my-example-derivation
my-example-derivation (post)> + sleep 1
my-example-derivation (post)> + printf 'very important stuff'
[1 built, 0.0 MiB DL]
Co-authored-by: Graham Christensen <graham@grahamc.com>
Co-authored-by: Eelco Dolstra <edolstra@gmail.com>
startProcess does not appear to send the exit code to the helper
correctly. Not sure why this is, but it is probably safe to just
fallback on all sandbox errors.
When sandbox-fallback = true (the default), the Nix builder will fall
back to disabled sandbox mode when the kernel doesn’t allow users to
set it up. This prevents hard errors from occuring in tricky places,
especially the initial installer. To restore the previous behavior,
users can set:
sandbox-fallback = false
in their /etc/nix/nix.conf configuration.
Some kernels disable "unpriveleged user namespaces". This is
unfortunate, but we can still use mount namespaces. Anyway, since each
builder has its own nixbld user, we already have most of the benefits
of user namespaces.
This is a much simpler fix to the 'error 9 while decompressing xz
file' problem than 78fa47a7f0. We just
do a ranged HTTP request starting after the data that we previously
wrote into the sink.
Fixes#2952, #379.
$ sudo ./inst/bin/nix-instantiate -E '"${./.git}"'
error: The path name '.git' is invalid: it is illegal to start the
name with a period. Path names are alphanumeric and can include the
symbols +-._?= and must not begin with a period. Note: If '.git' is a
source file and you cannot rename it on disk,
builtins.path { name = ... } can be used to give it an alternative
name.
If multiple builds with fail with different errors it will be reflected
in the status code.
eg.
103 => timeout + hash mismatch
105 => timeout + check mismatch
106 => hash mismatch + check mismatch
107 => timeout + hash mismatch + check mismatch
Setting `http2 = false` in nix config (e.g. /etc/nix/nix.conf)
had no effect, and `nix-env -vvvvv -i hello` still downloaded .nar
packages using HTTP/2.
In `src/libstore/download.cc`, the `CURL_HTTP_VERSION_2TLS` option was
being explicitly set when `downloadSettings.enableHttp2` was `true`,
but, `CURL_HTTP_VERSION_1_1` option was not being explicitly set when
`downloadSettings.enableHttp2` was `false`.
This may be because `https://curl.haxx.se/libcurl/c/libcurl-env.html` states:
"You have to set this option if you want to use libcurl's HTTP/2 support."
but, also, in the changelog, states:
"DEFAULT
Since curl 7.62.0: CURL_HTTP_VERSION_2TLS
Before that: CURL_HTTP_VERSION_1_1"
So, the default setting for `libcurl` is HTTP/2 for version >= 7.62.0.
In this commit, option `CURLOPT_HTTP_VERSION` is explicitly set to
`CURL_HTTP_VERSION_1_1` when `downloadSettings.enableHttp2` nix config
setting is `false`.
This can be tested by running `nix-env -vvvvv -i hello | grep HTTP`
The default nsswitch.conf(5) file in most distros can handle many
different things including host name, user names, groups, etc. In Nix,
we want to limit the amount of impurities that come from these things.
As a result, we should only allow nss to be used for gethostbyname(3)
and getservent(3).
/cc @Ericson2314
--no-net causes tarballTtl to be set to the largest 32-bit integer,
which causes comparison like 'time + tarballTtl < other_time' to
fail on 32-bit systems. So cast them to 64-bit first.
https://hydra.nixos.org/build/95076624
(cherry picked from commit 29ccb2e969)
This flag
* Disables substituters.
* Sets the tarball-ttl to infinity (ensuring e.g. that the flake
registry and any downloaded flakes are considered current).
* Disables retrying downloads and sets the connection timeout to the
minimum. (So it doesn't completely disable downloads at the moment.)
(cherry picked from commit 8ea842260b)
Once we've started writing data to a Sink, we can't restart a download
request, because then we end up writing duplicate data to the
Sink. Therefore we shouldn't handle retries in Downloader but at a
higher level (in particular, in copyStorePath()).
Fixes#2952.
(cherry picked from commit a67cf5a358)
Also, make fetchGit and fetchMercurial update allowedPaths properly.
(Maybe the evaluator, rather than the caller of the evaluator, should
apply toRealPath(), but that's a bigger change.)
(cherry picked from commit 5c34d66538)
This allows many programs (e.g. gcc, clang, cmake) to print colorized
log output (assuming $TERM is set to a value like "xterm").
There are other ways to get colors, in particular setting
CLICOLOR_FORCE, but they're less widely supported and can break
programs that parse tool output.
The value of useChroot is not set yet in the constructor, resulting in
hash rewriting being enabled in certain cases where it should not be.
Fixes#2801
To determine which seccomp filters to install, we were incorrectly
using settings.thisSystem, which doesn't denote the actual system when
--system is used.
Fixes#2791.
Scanning of /proc/<pid>/{exe,cwd} was broken because '{memory:' was
prepended twice. Also, get rid of the whole '{memory:...}' thing
because it's unnecessary, we can just list the file in /proc directly.
This new structure makes more sense as there may be many sources rooting
the same store path. Many profiles can reference the same path but this
is even more true with /proc/<pid>/maps where distinct pids can and
often do map the same store path.
This implementation is also more efficient as the `Roots` map contains
only one entry per rooted store path.
This reverts commit a0ef21262f. This
doesn't work in 'nix run' and nix-shell because setns() fails in
multithreaded programs, and Boehm GC mark threads are uncancellable.
Fixes#2646.
Use the same output ordering and format everywhere.
This is such a common issue that we trade the single-line error message for
more readability.
Old message:
```
fixed-output derivation produced path '/nix/store/d4nw9x2sy9q3r32f3g5l5h1k833c01vq-example.com' with sha256 hash '08y4734bm2zahw75b16bcmcg587vvyvh0n11gwiyir70divwp1rm' instead of the expected hash '1xzwnipjd54wl8g93vpw6hxnpmdabq0wqywriiwmh7x8k0lvpq5m'
```
New message:
```
hash mismatch in fixed-output derivation '/nix/store/d4nw9x2sy9q3r32f3g5l5h1k833c01vq-example.com':
wanted: sha256:1xzwnipjd54wl8g93vpw6hxnpmdabq0wqywriiwmh7x8k0lvpq5m
got: sha256:08y4734bm2zahw75b16bcmcg587vvyvh0n11gwiyir70divwp1rm
```
Without this information the content addressable state and hashes are
lost after the first request, this causes signatures to be required for
everything even tho the path could be verified without signing.
This enables using for http for S3 request for debugging or
implementations that don't have https configured. This is not a problem
for binary caches since they should not contain sensitive information.
Both package signatures and AWS auth already protect against tampering.
Previously, config would only be read from XDG_CONFIG_HOME. This change
allows reading config from additional directories, which enables e.g.
per-project binary caches or chroot stores with the help of direnv.
The use of TransferManager has several issues, including that it
doesn't allow setting a Content-Encoding without a patch, and it
doesn't handle exceptions in worker threads (causing termination on
memory allocation failure).
Fixes#2493.
This commit partially reverts 48662d151b. When
copying from an older store (in my case a store running Nix 1.11.7), nix would
throw errors about there being no hash. This is fixed by recalculating the hash.
In structured-attributes derivations, you can now specify per-output
checks such as:
outputChecks."out" = {
# The closure of 'out' must not be larger than 256 MiB.
maxClosureSize = 256 * 1024 * 1024;
# It must not refer to C compiler or to the 'dev' output.
disallowedRequisites = [ stdenv.cc "dev" ];
};
outputChecks."dev" = {
# The 'dev' output must not be larger than 128 KiB.
maxSize = 128 * 1024;
};
Also fixed a bug in allowedRequisites that caused it to ignore
self-references.
Since its superclass RemoteStore::Connection contains 'to' and 'from'
fields that refer to the file descriptor maintained in the subclass,
it was possible for the flush() call in Connection::~Connection() to
write to a closed file descriptor (or worse, a file descriptor now
referencing another file). So make sure that the file descriptor
survives 'to' and 'from'.
This is primarily because Derivation::{can,will}BuildLocally() depends
on attributes like preferLocalBuild and requiredSystemFeatures, but it
can't handle them properly because it doesn't have access to the
structured attributes.
E.g. __noChroot and allowedReferences now work correctly. We also now
check that the attribute type is correct. For instance, instead of
allowedReferences = "out";
you have to write
allowedReferences = [ "out" ];
Fixes#2453.
This meant that making a typo in an s3:// URI would cause a bucket to
be created. Also it didn't handle eventual consistency very well. Now
it's up to the user to create the bucket.
* Don't wait forever for the client to remove data from the
buffer. This does mean that the buffer can grow without bounds
(e.g. when downloading is faster than writing to disk), but meh.
* Don't hold the state lock while calling the sink. The sink could
take any amount of time to process the data (in particular when it's
actually a coroutine), so we don't want to block the download
thread.
TransferManager allocates a lot of memory (50 MiB by default), and it
might leak but I'm not sure about that. In any case it was causing
OOMs in hydra-queue-runner. So allocate only one TransferManager per
S3BinaryCacheStore.
Hopefully fixes https://github.com/NixOS/hydra/issues/586.
This callback is executed on a different thread, so exceptions thrown
from the callback are not caught:
Aug 08 16:25:48 chef hydra-queue-runner[11967]: terminate called after throwing an instance of 'nix::Error'
Aug 08 16:25:48 chef hydra-queue-runner[11967]: what(): AWS error: failed to upload 's3://nix-cache/19dbddlfb0vp68g68y19p9fswrgl0bg7.ls'
Therefore, just check the transfer status after it completes. Also
include the S3 error message in the exception.
This didn't work anymore since decompression was only done in the
non-coroutine case.
Decompressors are now sinks, just like compressors.
Also fixed a bug in bzip2 API handling (we have to handle BZ_RUN_OK
rather than BZ_OK), which we didn't notice because there was a missing
'throw':
if (ret != BZ_OK)
CompressionError("error while compressing bzip2 file");
It adds a new operation, cmdAddToStoreNar, that does the same thing as
the corresponding nix-daemon operation, i.e. call addToStore(). This
replaces cmdImportPaths, which has the major issue that it sends the
NAR first and the store path second, thus requiring us to store the
incoming NAR either in memory or on disk until we decide what to do
with it.
For example, this reduces the memory usage of
$ nix copy --to 'ssh://localhost?remote-store=/tmp/nix' /nix/store/95cwv4q54dc6giaqv6q6p4r02ia2km35-blender-2.79
from 267 MiB to 12 MiB.
Probably fixes#1988.
