files: allow target conflicts between recursively-linked directories (exclusively)

[niacdoial]

Description

Loosened the "no two home.file directives may have the same target" constraint: Multiple directives may share a target if and only if they are recursively-linked directories

Checklist

• [X] Change is backwards compatible. note: one bout of weirdness is the use of pathExists "${sourceStorePath v}/.." to detect if the source path is a directory. This might warrant a double check for cross-platform compatibility

• [X] Code formatted with ./format. note: due to the large changes caused by this, formatting was done in a separate, reproducible commit.

• [X] Code tested through nix-shell --pure tests -A run.all. (note: the line in the tests about neovim was disabled, as that test failed both before and after my changes. this is however not committed)

• [X] Test cases updated/added. See example.

• [X] Commit messages are formatted like

  {component}: {description}
  
  {long description}
  

  See CONTRIBUTING for more information and recent commit messages for examples.

[niacdoial]

If you saw the previous (closed WIP) pull request, this is a simpler version, which doesn't re-fix what was already fixed in the lest few months[?].

Also, I'm no longer trying to understand how force interacts with recursive, as there is no apparent breaking change in that area here. force seems to straight-up not work when recursive is active, both on per-file conflict and in terms of overriding the full directory, whatever the "ideal" behavior of that interaction is supposed to be

[rycee]

Thanks for the contribution! I was having a look at this and in general it seems fine. But I do feel a bit reluctant to make source a mergeable option since it seems quite unintuitive to me. If I needed to merge source directories and link them recursively then the first thing that comes to mind is to use symlinkJoin like so

home.file."test-recursive-joined" = {
  source = pkgs.symlinkJoin {
    name = "test-recursive-joined";
    paths = [ ./test1 ./test2 ];
  };
  recursive = true;
};

Do you really need the mergeability or is the symlinkJoin solution sufficient?

[niacdoial]

There would probably be some usecases where being able to have separate directives (if they are in different files which have completely different purposes and are both included by home.nix, maybe?) …But I have to admit I outright didn't know about symlinkJoin, and it would fit my usecase nicely. Should I do a pull request that simply mentions that function in the docs for home.file instead?

[niacdoial]

…@rycee should I?

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