This allows you to easily set up a build environment containing the
specified packages from Nixpkgs. For example:
$ nix-shell -p sqlite xorg.libX11 hello
will start a shell in which the given packages are present.
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.
The tarball can now be unpacked anywhere. The installation script
uses "sudo" to create /nix if it doesn't exist. It also fetches the
nixpkgs-unstable channel.
Namely:
nix-store: derivations.cc:242: nix::Hash nix::hashDerivationModulo(nix::StoreAPI&, nix::Derivation): Assertion `store.isValidPath(i->first)' failed.
This happened because of the derivation output correctness check being
applied before the references of a derivation are valid.
This is useful when you do:
foo_SOURCES := $(wildcard *.cc) foo.cc
where foo.cc is a generated file. In this case, if foo.cc already
exists, you get foo.cc twice in foo_SOURCES, leading to a link error.
*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...
NAR info files in binary caches can now have a cryptographic signature
that Nix will verify before using the corresponding NAR file.
To create a private/public key pair for signing and verifying a binary
cache, do:
$ openssl genrsa -out ./cache-key.sec 2048
$ openssl rsa -in ./cache-key.sec -pubout > ./cache-key.pub
You should also come up with a symbolic name for the key, such as
"cache.example.org-1". This will be used by clients to look up the
public key. (It's a good idea to number keys, in case you ever need
to revoke/replace one.)
To create a binary cache signed with the private key:
$ nix-push --dest /path/to/binary-cache --key ./cache-key.sec --key-name cache.example.org-1
The public key (cache-key.pub) should be distributed to the clients.
They should have a nix.conf should contain something like:
signed-binary-caches = *
binary-cache-public-key-cache.example.org-1 = /path/to/cache-key.pub
If all works well, then if Nix fetches something from the signed
binary cache, you will see a message like:
*** Downloading ‘http://cache.example.org/nar/7dppcj5sc1nda7l54rjc0g5l1hamj09j-subversion-1.7.11’ (signed by ‘cache.example.org-1’) to ‘/nix/store/7dppcj5sc1nda7l54rjc0g5l1hamj09j-subversion-1.7.11’...
On the other hand, if the signature is wrong, you get a message like
NAR info file `http://cache.example.org/7dppcj5sc1nda7l54rjc0g5l1hamj09j.narinfo' has an invalid signature; ignoring
Signatures are implemented as a single line appended to the NAR info
file, which looks like this:
Signature: 1;cache.example.org-1;HQ9Xzyanq9iV...muQ==
Thus the signature has 3 fields: a version (currently "1"), the ID of
key, and the base64-encoded signature of the SHA-256 hash of the
contents of the NAR info file up to but not including the Signature
line.
Issue #75.
Eelco Dolstra