Fix derivation load assertion errors

[iFreilicht]

Display nice error messages instead and give some indication where the error happened.

Before:

$ echo 4 | nix derivation add
error: [json.exception.type_error.305] cannot use operator[] with a string argument with number

$ nix derivation show nixpkgs#hello | nix derivation add
Assertion failed: (it != m_value.object->end()), function operator[], file /nix/store/8h9pxgq1776ns6qi5arx08ifgnhmgl22-nlohmann_json-3.11.2/include/nlohmann/json.hpp, line 2135.

$ nix derivation show nixpkgs#hello | jq '.[] | .name = 5' | nix derivation add
error: [json.exception.type_error.302] type must be string, but is object

$ nix derivation show nixpkgs#hello | jq '.[] | .outputs = { out: "/nix/store/8j3f8j-hello" }' | nix derivation add
error: [json.exception.type_error.302] type must be object, but is string

After:

$ echo 4 | nix derivation add
error: Expected JSON of derivation to be of type 'object', but it is of type 'number'

$ nix derivation show nixpkgs#hello | nix derivation add
error: Expected JSON object to contain key 'name' but it doesn't

$ nix derivation show nixpkgs#hello | jq '.[] | .name = 5' | nix derivation add
error:
       … while reading value with key 'name'

       error: Expected JSON value to be of type 'string' but it is of type 'number'

$ nix derivation show nixpkgs#hello | jq '.[] | .outputs = { out: "/nix/store/8j3f8j-hello" }' | nix derivation add
error:
       … while reading key 'outputs'

       … while reading value with key 'out'

       error: Expected JSON value to be of type 'object' but it is of type 'string'

Motivation

When loading a derivation from a JSON, malformed input would trigger cryptic "assertion failed" errors. Simply replacing calls to operator [] with calls to .at() was not good enough IMO, as this would cause json.execptions to be printed verbatim.

Context

Fixes the complaint about bad error messages in #8747 as requested in this comment.

The PR adds a new method valueOfTypeAt that accesses the value via a key in a JSON object, ensures it is of a certain type and errors with a decent explanation text if the key does not exist or it is of the wrong type.
It specifically does not check whether map is actually an object. This should be done at the callsite, where information about the context of map is available.

Checklist for maintainers

Maintainers: tick if completed or explain if not relevant

• agreed on idea
• agreed on implementation strategy
• tests, as appropriate
  • functional tests - tests/**.sh
  • unit tests - src/*/tests
  • integration tests - tests/nixos/*
• documentation in the manual
• documentation in the internal API docs
• code and comments are self-explanatory
• commit message explains why the change was made
• new feature or incompatible change: updated release notes

Priorities

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[Ericson2314]

Thanks! This looks quite good now.

[Ericson2314]

BTW see nlohmann/json#4100