The fact that we were introducing a conversion from the output of `nix
path-info` into the input of `builtins.fetchTree` was the deciding
factor. We want scripting outputs into inputs like that to be easy.
Since JSON strings and objects are trivially distinguishable, we still
have the option of introducing the JSON format as an alternative input
scheme in the future, should we want to. (The output format would still
be SRI in that case, presumably.)
The previous commit hacked it into the output of `nix path-info --json`,
this cleans that up my making it an actual field of that data type, and
part of the canonical JSON serializers for it (and `ValidPathInfo` and
`NarInfo`).
Beyond cleaning up the JSON code, this also opens the doors to things
like:
- Binary caches that contain store objects that don't all belong in the
same store directory
- Relocatable store objects which carefully don't mention any store
directory by absolute path, and instead use relative paths for
anything. (#9549)
As discussed today at great length in the Nix meeting, we don't want to
break the format, but we also don't want to impede the improvement of
JSON formats. The solution is to add a new flag for control the output
format.
Note that prior to the release, we may want to replace `--json
--json-format N` with `--json=N`, but this is being left for a separate
PR, as we don't yet have `=` support for CLI flags.
