This can be used to import a dynamic shared object and return an
arbitrary value, including new primops. This can be used both to test
new primops without having to recompile nix every time, and to build
specialized primops that probably don't belong upstream (e.g. a function
that calls out to gpg to decrypt a nixops secret as-needed).
The imported function should initialize the Value & as needed. A single
import can define multiple values by creating an attrset or list, of
course.
An example initialization function might look like:
extern "C" void initialize(nix::EvalState & state, nix::Value & v)
{
v.type = nix::tPrimOp;
v.primOp = NEW nix::PrimOp(myFun, 1, state.symbols.create("myFun"));
}
Then `builtins.importNative ./example.so "initialize"` will evaluate to
the primop defined in the myFun function.
Nix search path lookups like <nixpkgs> are now desugared to ‘findFile
nixPath <nixpkgs>’, where ‘findFile’ is a new primop. Thus you can
override the search path simply by saying
let
nixPath = [ { prefix = "nixpkgs"; path = "/my-nixpkgs"; } ];
in ... <nixpkgs> ...
In conjunction with ‘scopedImport’ (commit
c273c15cb1), the Nix search path can be
propagated across imports, e.g.
let
overrides = {
nixPath = [ ... ] ++ builtins.nixPath;
import = fn: scopedImport overrides fn;
scopedImport = attrs: fn: scopedImport (overrides // attrs) fn;
builtins = builtins // overrides;
};
in scopedImport overrides ./nixos
‘scopedImport’ works like ‘import’, except that it takes a set of
attributes to be added to the lexical scope of the expression,
essentially extending or overriding the builtin variables. For
instance, the expression
scopedImport { x = 1; } ./foo.nix
where foo.nix contains ‘x’, will evaluate to 1.
This has a few applications:
* It allows getting rid of function argument specifications in package
expressions. For instance, a package expression like:
{ stdenv, fetchurl, libfoo }:
stdenv.mkDerivation { ... buildInputs = [ libfoo ]; }
can now we written as just
stdenv.mkDerivation { ... buildInputs = [ libfoo ]; }
and imported in all-packages.nix as:
bar = scopedImport pkgs ./bar.nix;
So whereas we once had dependencies listed in three places
(buildInputs, the function, and the call site), they now only need
to appear in one place.
* It allows overriding builtin functions. For instance, to trace all
calls to ‘map’:
let
overrides = {
map = f: xs: builtins.trace "map called!" (map f xs);
# Ensure that our override gets propagated by calls to
# import/scopedImport.
import = fn: scopedImport overrides fn;
scopedImport = attrs: fn: scopedImport (overrides // attrs) fn;
# Also update ‘builtins’.
builtins = builtins // overrides;
};
in scopedImport overrides ./bla.nix
* Similarly, it allows extending the set of builtin functions. For
instance, during Nixpkgs/NixOS evaluation, the Nixpkgs library
functions could be added to the default scope.
There is a downside: calls to scopedImport are not memoized, unlike
import. So importing a file multiple times leads to multiple parsings
/ evaluations. It would be possible to construct the AST only once,
but that would require careful handling of variables/environments.
This allows error messages like:
error: the anonymous function at `/etc/nixos/configuration.nix:1:1'
called without required argument `foo', at
`/nix/var/nix/profiles/per-user/root/channels/nixos/nixpkgs/lib/modules.nix:77:59'
This will allow e.g. channel expressions to use builtins.storePath IFF
it is safe to do so without knowing if the path is valid yet.
Signed-off-by: Shea Levy <shea@shealevy.com>
It kept temporary data in STL containers that were not scanned by
Boehm GC, so Nix programs using genericClosure could randomly crash if
the garbage collector kicked in at a bad time.
Also make it a bit more efficient by copying points to values rather
than values.
We already have some primops for determining the type of a value, such
as isString, but they're incomplete: for instance, there is no isPath.
Rather than adding more isBla functions, the generic typeOf function
returns a string representing the type of the argument (e.g. "int").
Combined with the previous changes, stack traces involving derivations
are now much less verbose, since something like
while evaluating the builtin function `getAttr':
while evaluating the builtin function `derivationStrict':
while instantiating the derivation named `gtk+-2.24.20' at `/home/eelco/Dev/nixpkgs/pkgs/development/libraries/gtk+/2.x.nix:11:3':
while evaluating the derivation attribute `propagatedNativeBuildInputs' at `/home/eelco/Dev/nixpkgs/pkgs/stdenv/generic/default.nix:78:17':
while evaluating the attribute `outPath' at `/nix/store/212ngf4ph63mp6p1np2bapkfikpakfv7-nix-1.6/share/nix/corepkgs/derivation.nix:18:9':
...
now reads
while evaluating the attribute `propagatedNativeBuildInputs' of the derivation `gtk+-2.24.20' at `/home/eelco/Dev/nixpkgs/pkgs/development/libraries/gtk+/2.x.nix:11:3':
...
Messages like
while evaluating the attribute `outPath' at `/nix/store/212ngf4ph63mp6p1np2bapkfikpakfv7-nix-1.6/share/nix/corepkgs/derivation.nix:18:9':
are redundant, because Nix already shows that it's evaluating a derivation:
while instantiating the derivation named `firefox-24.0' at `/home/eelco/Dev/nixpkgs/pkgs/applications/networking/browsers/firefox/default.nix:131:5':
while evaluating the derivation attribute `nativeBuildInputs' at `/home/eelco/Dev/nixpkgs/pkgs/stdenv/generic/default.nix:76:17':
Since we already cache files in normal form (fileEvalCache), caching
parse trees is redundant.
Note that getting rid of this cache doesn't actually save much memory
at the moment, because parse trees are currently not freed / GC'ed.
This allows adding attributes like
attr = if stdenv.system == "bla" then something else null;
without changing the resulting derivation on non-<bla> platforms.
We once considered adding a special "ignore" value for this purpose,
but using null seems more elegant.
The integer constant ‘langVersion’ denotes the current language
version. It gets increased every time a language feature is
added/changed/removed. It's currently 1.
The string constant ‘nixVersion’ contains the current Nix version,
e.g. "1.2pre2980_9de6bc5".
This reverts commit 2980d1fba9. It
causes a regression in NixOS evaluation:
string `/nix/store/ya3s5gmj3b28170fpbjhgsk8wzymkpa1-pommed-1.39/etc/pommed.conf' cannot refer to other paths
This can serve as a generic efficient list builder. For instance, the
function ‘catAttrs’ in Nixpkgs can be rewritten from
attr: l: fold (s: l: if hasAttr attr s then [(getAttr attr s)] ++ l else l) [] l
to
attr: l: builtins.concatLists (map (s: if hasAttr attr s then [(getAttr attr s)] else []) l)
Statistics before:
time elapsed: 1.08683
size of a value: 24
environments allocated: 1384376 (35809568 bytes)
list elements: 6946783 (55574264 bytes)
list concatenations: 37434
values allocated: 1760440 (42250560 bytes)
attribute sets allocated: 392040
right-biased unions: 186334
values copied in right-biased unions: 591137
symbols in symbol table: 18273
number of thunks: 1297673
number of thunks avoided: 1380759
number of attr lookups: 430802
number of primop calls: 628912
number of function calls: 1333544
Statistics after (including new catAttrs):
time elapsed: 0.959854
size of a value: 24
environments allocated: 1010198 (26829296 bytes)
list elements: 1984878 (15879024 bytes)
list concatenations: 30488
values allocated: 1589760 (38154240 bytes)
attribute sets allocated: 392040
right-biased unions: 186334
values copied in right-biased unions: 591137
symbols in symbol table: 18274
number of thunks: 1040925
number of thunks avoided: 1038428
number of attr lookups: 438419
number of primop calls: 474844
number of function calls: 959366
The one in Nixpkgs is O(n^2), this one is O(n). Big reduction in the
number of list allocations.
Statistics before (on a NixOS system config):
time elapsed: 1.17982
size of a value: 24
environments allocated: 1543334 (39624560 bytes)
list elements: 9612638 (76901104 bytes)
list concatenations: 37434
values allocated: 1854933 (44518392 bytes)
attribute sets allocated: 392040
right-biased unions: 186334
values copied in right-biased unions: 591137
symbols in symbol table: 18272
number of thunks: 1392467
number of thunks avoided: 1507311
number of attr lookups: 430801
number of primop calls: 691600
number of function calls: 1492502
Statistics after:
time elapsed: 1.08683
size of a value: 24
environments allocated: 1384376 (35809568 bytes)
list elements: 6946783 (55574264 bytes)
list concatenations: 37434
values allocated: 1760440 (42250560 bytes)
attribute sets allocated: 392040
right-biased unions: 186334
values copied in right-biased unions: 591137
symbols in symbol table: 18273
number of thunks: 1297673
number of thunks avoided: 1380759
number of attr lookups: 430802
number of primop calls: 628912
number of function calls: 1333544
Evaluation of a NixOS configuration spends quite a lot of time in the
"filter" function in Nixpkgs. As implemented in Nixpkgs, this is a
O(n^2) operation, so it's a good candidate for providing a more
efficient (i.e. primop) implementation. Using it gives a ~10% speed
increase and a significant reduction in the number of evaluations.
Statistics before (on a NixOS system config):
time elapsed: 1.3258
size of a value: 24
environments allocated: 1980939 (50127080 bytes)
list elements: 14679308 (117434464 bytes)
list concatenations: 50828
values allocated: 2098938 (50374512 bytes)
attribute sets allocated: 392040
right-biased unions: 186334
values copied in right-biased unions: 591137
symbols in symbol table: 18271
number of thunks: 1645752
number of thunks avoided: 1921196
number of attr lookups: 430798
number of primop calls: 838807
number of function calls: 1930107
Statistics after:
time elapsed: 1.17982
size of a value: 24
environments allocated: 1543334 (39624560 bytes)
list elements: 9612638 (76901104 bytes)
list concatenations: 37434
values allocated: 1854933 (44518392 bytes)
attribute sets allocated: 392040
right-biased unions: 186334
values copied in right-biased unions: 591137
symbols in symbol table: 18272
number of thunks: 1392467
number of thunks avoided: 1507311
number of attr lookups: 430801
number of primop calls: 691600
number of function calls: 1492502
Setting the environment variable NIX_COUNT_CALLS to 1 enables some
basic profiling in the evaluator. It will count calls to functions
and primops as well as evaluations of attributes.
For example, to see where evaluation of a NixOS configuration spends
its time:
$ NIX_SHOW_STATS=1 NIX_COUNT_CALLS=1 ./src/nix-instantiate/nix-instantiate '<nixos>' -A system --readonly-mode
...
calls to 39 primops:
239532 head
233962 tail
191252 hasAttr
...
calls to 1595 functions:
224157 `/nix/var/nix/profiles/per-user/root/channels/nixos/nixpkgs/pkgs/lib/lists.nix:17:19'
221767 `/nix/var/nix/profiles/per-user/root/channels/nixos/nixpkgs/pkgs/lib/lists.nix:17:14'
221767 `/nix/var/nix/profiles/per-user/root/channels/nixos/nixpkgs/pkgs/lib/lists.nix:17:10'
...
evaluations of 7088 attributes:
167377 undefined position
132459 `/nix/var/nix/profiles/per-user/root/channels/nixos/nixpkgs/pkgs/lib/attrsets.nix:119:41'
47322 `/nix/var/nix/profiles/per-user/root/channels/nixos/nixpkgs/pkgs/lib/attrsets.nix:13:21'
...
The generated attrset has drvPath and outPath with the right string context, type 'derivation', outputName with
the right name, all with a list of outputs, and an attribute for each output.
I see three uses for this (though certainly there may be more):
* Using derivations generated by something besides nix-instantiate (e.g. guix)
* Allowing packages provided by channels to be used in nix expressions. If a channel installed a valid deriver
for each package it provides into the store, then those could be imported and used as dependencies or installed
in environment.systemPackages, for example.
* Enable hydra to be consistent in how it treats inputs that are outputs of another build. Right now, if an
input is passed as an argument to the job, it is passed as a derivation, but if it is accessed via NIX_PATH
(i.e. through the <> syntax), then it is a path that can be imported. This is problematic because the build
being depended upon may have been built with non-obvious arguments passed to its jobset file. With this
feature, hydra can just set the name of that input to the path to its drv file in NIX_PATH
For each output, this adds a corresponding attribute to the derivation that is
the same as the derivation except for outPath, which is set to the path specific
to that output. Additionally, an "all" attribute is added that is a list of all
of the output derivations. This has to be done outside of derivationStrict as
each output is itself a derivation that contains itself (and all other outputs)
as an attribute. The derivation itself is equivalent to the first output in the
outputs list (or "out" if that list isn't set).
This should also fix:
nix-instantiate: ./../boost/shared_ptr.hpp:254: T* boost::shared_ptr<T>::operator->() const [with T = nix::StoreAPI]: Assertion `px != 0' failed.
which was caused by hashDerivationModulo() calling the ‘store’
object (during store upgrades) before openStore() assigned it.
