• Can you provide an example implementation of a fetcher?
• Are fetchers adding things to the store subject to hash checks?

re #1841

Fetchers aren't trivial... I'm not sure what value would be added by having another example here over what's in primops{.cc/*.cc} or in nixpkgs. Can you expand more?

Fetchers defined in extra-builtins can do hash checks or not, they can respect pure-eval or not, whatever. If you want a more limited interface you can build it on top of this. (of course, anything added to the store will be hash-addressed).

For the first point, I didn't mean to say fetcher, but also I completely missed (due to how short it is!) the example in the test. Very cool!

For the second point, I wanted to be sure a user builtin couldn't lie about the contents of a file and put data in the store where it doesn't belong. From IRC, the answer is no.

Nice!

I much prefer having a plugin config option (see #1841 (comment)). That's also more general because it supports other kinds of extensions (such as new Store subclasses, e.g. S3BinaryCacheStore could be moved into a plugin).

@edolstra it does seem like the laziness @shlevy is talking about in the comments below the one you linked to is appealing. My dream is for builtins.fetchGit to actually depend on git (so the relevant plugin and its git dependency are expressed in Nix), but for Nix not to. A static loadable plugins architecture would force the fetchGit plugin (and its git dependency) to be loaded whenever Nix is loaded.

Couldn't we just support both mechanisms? It does seem appealing to split out the S3BinaryCacheStore as you say, but nothing says that needs to work the same way as the extra builtins, if we don't make a mess in the code by supporting both.

Another issue is that there is no guarantee that the plugins from extra-builtins.nix are compatible. It's up to the user to ensure that the Nix version used by extra-builtins.nix is the same as the caller's. OTOH, with a NixOS option nix.plugins, it's guaranteed that Nix and the plugins come from the same Nixpkgs.

Working on plugins now, @copumpkin my first plugin will be one to support extraBuiltins 😉

@edolstra True, though that doesn't apply to extraBuiltins defined with builtins.exec.

And we have use cases to support besides NixOS

@copumpkin https://github.com/shlevy/nix-plugins/tree/nix-2x-plugins has an implementation of extra-builtins on top of #1855, though it doesn't add the setting (you have to use $NIX_CONF_DIR/extra-builtins.nix, but you can set NIX_CONF_DIR so...)

For anyone following along confused, we got this through #1854 instead.

Oops, sorry, thanks!