Sharing devenv.lock in monorepos

[clotodex]

Based on a comment I found in #2209 this does not seem possible (referenced below)

The more general question: How can I inherit the exact versions from one devenv in a monorepo in a second devenv.

Is there a way of doing this that i am missing (apart from ref-linking the urls in the devenv.yaml). And what are the technical blockers to make this possible?

My exact use case:
The monorepo structure (simplified):

• parent
  • devenv.nix (let's call it P)
  • frontend
    • devenv.nix (let's call it F)
  • backend
    • devenv.nix (let's call it B)

F sets up everything to run frontend, B backend.
P contains all the wiring to allow devenv up to run the whole repo locally. This is extremely valuable and reproducible.
However, now we still have a script in the parent to update all devenvs in lockstep, to make sure the nixpkgs version of F is the same as P, otherwise F often does not compile in P.
Since different teams might manage F and B, keeping P in sync is potentially brittle and the only non-manual workaround would be a github action to relock all devenvs collectively. Which is not really what we want.

The docs say:

!!! warn "Locked inputs and follows"

"Follows" are fetched and re-locked using the URI of the followed inputs.
The external project's lock file will not constrain the revision.

For example, it's common to reference nixpkgs-unstable without specifying a revision.
The input is locked to a specific revision when fetched and this information is saved to either a devenv.lock or flake.lock file.

Following this input from another project will re-fetch the latest revison, which may be different from the lcoked revision used in the original project.

This is the comment referencing the commit where this doc was added.

It's a feature of Nix Flakes. Instead of providing a url, you can use follows to reference any other input by name, including their nested inputs.

I've added this information to the docs: bee405d

Originally posted by @sandydoo in #2209

[sandydoo]

I'm going to be testing this today. I don't think the note I added about locking is entirely correct.

[sandydoo]

So!

For flake inputs, follows does respect the remote lock file. I need to amend the note.

For monorepo-style imports, devenv currently merges the devenv.yamls.
Using your naming scheme, if you import frontend into parent, the inputs from frontend will be merged into the inputs of parent.
imports bypass the flake machinery, so no lock files are read. It can also override the parent's inputs in surprising ways. I think inputs need special handling to make this work.

[domenkozar]

I think this issue is about that each devenv.nix creates it's own devenv.lock instead of reusing the one that we're importing.