snix/tvix/docs/src/TODO.md

TODO

This contains a rough collection of ideas on the TODO list, trying to keep track of it somewhere.

Of course, there's no guarantee these things will get addressed, but it helps dumping the backlog somewhere.

Feel free to add new ideas. Before picking something, ask in #tvix-dev to make sure noone is working on this, or has some specific design in mind already.

Cleanups

Nix language test suite

• Think about how to merge, but "categorize" tvix_tests in glue and eval. We currently only have this split as they need a different feature set / builtins.
• move some of the rstest cases in tvix-glue to the .nix/.exp mechanism. Some of them need test fixtures, which cannot be represented in git (special file types in the import tests for example). Needs some support from the test suite to create these fixtures on demand.
• extend verify-lang-tests/default.nix mechanism to validate tvix-eval and tvix-glue test cases (or the common structure above).
• absorb eval/tests/nix_oracle.rs into tvix_tests, or figure out why it's not possible (and document) it. It looks like it's only as nix is invoked with a different level of --strict, but the toplevel doc-comment suggests its generic?

Error cleanup

• Currently, all services use tvix_castore::Error, which only has two kinds (invalid request, storage error), containing an (owned) string. This is quite primitive. We should have individual error types for BS, DS, PS. Maybe these should have some generics to still be able to carry errors from the underlying backend, similar to IngestionError.

Fixes towards correctness

• builtins.toXML is missing string context. See b/398.
• builtins.toXML self-closing tags need to be configurable in a more granular fashion, requires third-party crate support. See b/399.
• rnix only supports string source files, but NixString uses bytes (and Nix source code might be no valid UTF-8).

Documentation

Extend the other pages in here. Some ideas on what should be tackled:

• Document what Tvix is, and what it is not yet. What it is now, what it is not (yet), explaining some of the architectural choices (castore, more hermetic Build repr), while still being compatible. Explain how it's possible to plug in other frontends, and use tvix-{[ca]store,build} without Nixlang even. And how nix-compat is a useful crate for all sorts of formats and data types of Nix.
• Update the Architecture diagram to model the current state of things. There's no gRPC between Coordinator and Evaluator.
• Add a dedicated section/page explaining the separation between tvix-glue and tvix-eval, and how more annoying builtins get injected into tvix-eval through tvix-glue. Maybe restructure to only explain the component structure potentially crossing process boundaries (those with gRPC), and make the rest more crate and trait-focused?
• Restructure docs on castore vs store, this seems to be duplicated a bit and is probably still not too clear.
• Describe store composition(s) in more detail. There's some notes on granular fetching which probably can be repurposed.
• Absorb the rest of //tvix/website into this.

Features

CLI

• nix repl can set variables and effectively mutates a global scope. We should update the existing / add another repl that allows the same. We don't want to mutate the evaluator, but should construct a new one, passing in the root scope returned from the previous evaluation.

Fetchers

Some more fetcher-related builtins need work:

• fetchGit
• fetchTree (hairy, seems there's no proper spec and the URL syntax seems subject to change/underdocumented)

Convert builtins:fetchurl to Fetches

We need to convert builtins:fetchurl-style calls to builtins.derivation to fetches, not Derivations (tracked in KnownPaths).

Derivation -> Build

While we have some support for structuredAttrs and fetchClosure (at least enough to calculate output hashes, aka produce identical ATerm), the code populating the Build struct doesn't exist it yet.

Similarly, we also don't properly populate the build environment for fetchClosure yet. (Note there already is ExportedPathInfo, so once structuredAttrs is there this should be easy.

Builders

Once builds are proven to work with real-world builds, and the corner cases there are ruled out, adding other types of builders might be interesting.

• bwrap
• gVisor
• Cloud Hypervisor (using similar technique as //tvix//boot).

Long-term, we want to extend traits and gRPC protocol to expose more telemetry, logs etc, but this is something requiring a lot of designing.

Store composition

• Combinators: list-by-priority, first-come-first-serve, cache
• How do describe hierarchies. URL format too one-dimensional, but we might get quite far with a similar "substituters" concept that Nix uses, to construct the composed stores.

Store Config

There's already serde for some store options (bigtable uses serde_qs). We might also have common options global over all backends, like chunking parameters for chunking blobservices. Think where this would fit in.

• Rework the URL syntax for object_store. We should support the default s3/gcs URLs at least.

BlobService

• On the trait side, currently there's no way to distinguish reading a known-chunk vs blob, so we might be calling .chunks() unnecessarily often. At least for the object_store backend, this might be a problem.
• While object_store recently got support for Content-Type (https://github.com/apache/arrow-rs/pull/5650), there's no support on the local filesystem yet. We'd need to add support to this (through xattrs).

DirectoryService

• Add an object_store variant, storing a Directory closure keyed by the root Directory digest. This won't allow indexing intermediate Directory nodes, but once we have DirectoryService composition, it shouldn't be an issue.
• redb backend

PathInfoService

• redb backend
• sqlite backend (different schema than the Nix one, we need the root nodes data!)

Nix Daemon protocol

• Some work ongoing on the worker operation parsing (griff, picnoir)

O11Y

• gRPC trace propagation (cl/10532)
• tracing-tracy (cl/10952)
• [tracing-]indicatif for progress/log reporting (floklis stash)
• unification into tvix-tracing crate, currently a lot of boilerplate in tvix-store CLI entrypoint, and half of the boilerplate copied over to tvix-cli.