[Nix-dev] "live builds" proposal

[Evgeny Egorochkin]

On Sunday 01 August 2010 02:20:28 Yury G. Kudryashov wrote:
> Hi!
> 
> I propose the following extension to the nix language that will allow
> native support for "live scm builds" etc.
> 
> If nix is called with --enable-exec, the new function builtins.exec is
> added. The proposed syntax is like the following:
> 
> src =
>   if builtins ? exec then
>     # fetch to ~/.nix-checkouts/name
>     builtins.exec [ "nix-prefetch-svn" .... ];
>     builtins.filterSource "~/.nix-checkouts/name"
>   else if pathExists "~/.nix-checkouts/name" then
>     builtins.filterSource ...
>   else
>     fetchsvn { # url and hash for some specific revision
>     };
> 
> Of course, all this logic should be hidden in new fetchsvn/fetchgit etc.
> functions.
> 
> The main drawback of this solution is that the result of a nix expression
> will depend on some things outside of the nix expression. That's why I
> propose to enable this builtin only if nix is called with --enable-exec: if
> you want to rebuild your system from the configuration written a year ago,
> just do not add --enable-exec.

I'm not sure I really understand the use case you are talking about, but I'll 
do my best to guess and explain how my approach for live builds can be used.

I assume you want to rebuild live packages not  everytime you do nixos-rebuild 
and it discovers that there's another commit in the trunk, but only if there's 
a commit and you feel like your live builds are getting out of date.

There are 3 approaches that I see:

* Specify revision via override and update it manually. Doesn't work well if 
you have many packages you want to constantly update. This is more appropriate 
to fix specific builds.

* Use nix-env and a separate profile, put all live builds in it and update 
only when necessary.

* modify gitfetchrevision function proposed in my previous e-mail by replacing     
dummy = builtins.currentTime with dummy = import /path/to/systemrevision. This 
way, if you want your live packages to be updated, you bump 
/path/to/systemrevision and do nixos-rebuild, which will trigger live rebuilds 
if there are any new commits. Of course this can be automated by wrapping 
nixos-rebuild in a script:
#!/bin/sh
date>/path/to/systemrevision; nixos-rebuild

The only drawback of this approach is that running garbage collector will 
cause live rebuilds regardless of systemrevision bump.

-- 
Evgeny