path (content hash, references, deriver) is now stored in
/nix/var/nix/db/meta/<base-path>.info. The referrers are stored
separately in /nix/var/nix/db/meta/<base-path>.referrers. This is
done because the referrers can change, which the info file is in
principle immutable. The referrers must also be updated efficiently
to prevent a regression of NIX-23 (quadratic complexity updating the
referrers during garbage collection).
This is just the conversion code, the rest of the code still uses
BDB.
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.
