sub-flake breaks when new input is added to parent flake

[adrian-gierakowski]

Describe the bug

In a repo setup as follows:

.
├── flake.lock
├── flake.nix
└── sub-flake
    ├── flake.lock
    └── flake.nix

where sub-flake/flake.nix refers to top level flake with:

inputs = {
    parent.url = ./..;
  };

when new input is added to top level flake the sub-flake breaks (fails to evaluate)

Steps To Reproduce

1. start with a repo structure as shown in this commit
2. make changes as shown in this commit
3. run:

cd sub-flake
nix eval .#lib.one

which should print the following error:

error:
       … in the left operand of the update (//) operator
         at «nix-internal»/call-flake.nix:77:9:
           76|         # This is shadowed in the next //
           77|         // sourceInfo
             |         ^
           78|         // {

       … from call site
         at «nix-internal»/call-flake.nix:68:17:
           67|
           68|       outputs = flake.outputs (inputs // { self = result; });
             |                 ^
           69|

       error: function 'outputs' called without required argument 'nixpkgs-lib-2'
       at /nix/store/k84vy37d2qlmagax150drp642klmlgc4-source/flake.nix:10:5:
            9|   outputs =
           10|     { nixpkgs-lib, nixpkgs-lib-2, ... }:
             |     ^
           11|     {

This can be fixed by updating sub-flake/flake.lock by running the following from sub-flake dir:

nix flake update parent

as seen in this commit

Expected behavior

The sub-flake should not need to be updated since all necessary details are already present in the parents flake, so it should simply read the info from there.

Metadata

Additional context

• Possible solution strategy: Reuse input lock files #7730
  • a simpler, small scope solution may also be possible instead (but wouldn't improve laziness in the remote input case)

Checklist

• checked latest Nix manual (source)
• checked open bug issues and pull requests for possible duplicates

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