[Nix-dev] nix store: hardlink vs symlink (was Re: python application vs python environment)

[shea at shealevy.com]

>From my understanding (which may be terribly incorrect), hard links would
cause two problems during the hash-scanning phase of a nix build:

1. At least as of Eelco's thesis, we were ignoring hard links in the
canonicalization of store paths. That means that after a build, any
hard-linked files will no longer be linked

2. Even if we did keep track of hard links during canonicalization, it
would be fairly difficult if not impossible to resolve runtime
dependencies. Nix would need to find the inode of the hard link and scan
all store outputs for files with that inode, and if it finds more than one
it will probably have to consider them all runtime dependencies of the
current output, unless there's a way to determine which file was the
earliest of a set of hard links. The canonicalization of symlinks, on the
other hand, contains the path pointed to so hash scanning finds the
runtime dependencies fine.

~Shea

>
> Isolating aspects of the original mail:
>
> On Sat, 24 Sep 2011 14:36:20 +0200, Florian Friesdorf <flo at chaoflow.net>
> wrote:
>> If ./bin/python is a symlink, we have currently a problem and PYTHONHOME
>> would be a solution, if python is a real file, we do not have a problem.
>>
>> So instead of symlinking the python executable, nix-env should be
>> instructed
>> to hard-link it - is this possible?
>
> Why is nix using symlinks to link files in the nix store?
> Would switching to hardlinks break something?
>
> - symlinks in case of whole directories
> - hardlinks for individual files
>
> --
> Florian Friesdorf <flo at chaoflow.net>
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