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Diffstat (limited to 'web')
| -rw-r--r-- | web/tvl/blog/2024-02-tvix-update.md | 59 |
1 files changed, 31 insertions, 28 deletions
diff --git a/web/tvl/blog/2024-02-tvix-update.md b/web/tvl/blog/2024-02-tvix-update.md index 6f7de626410c..0e011c07e08b 100644 --- a/web/tvl/blog/2024-02-tvix-update.md +++ b/web/tvl/blog/2024-02-tvix-update.md @@ -95,7 +95,7 @@ content-addressed systems, as it's a very Nix-specific format. ### The more granular Tvix model After experimenting with some concepts and ideas in Golang, mostly around how to -improve binary cache performance [^3], both on-disk as well as over the network, +improve binary cache performance[^3], both on-disk as well as over the network, we settled on a more granular, content-addressed and general-purpose format. Internally, it behaves very similar to how git handles tree objects, except @@ -138,7 +138,7 @@ storages, as well as on more granular seeking and content-defined chunking for blobs. ### FUSE/virtiofs -A tvix-store can be mounted via FUSE, or exposed through virtiofs [^4]. +A tvix-store can be mounted via FUSE, or exposed through virtiofs[^4]. While doing the obvious thing - allowing mounting and browsing the contents of the store, this will allow lazy substitution of builds on remote builders, be in containerized or virtualized workloads. @@ -161,7 +161,7 @@ The necessary parsers for NARInfo, signatures etc are also available in the ## EvalIO / builtins interacting with the store more closely tvix-eval itself is designed to be quite pure when it comes to IO - it doesn't do any IO directly on its own, but for the very little IO functionality it -does as part of "basic interaction with paths"[^2] (like importing other +does as part of "basic interaction with paths" (like importing other `.nix` files), it goes through an `EvalIO` interface, which is provided to the Evaluator struct on instantiation. @@ -170,10 +170,10 @@ which becomes interesting for specific store implementations that might not expose a POSIX filesystem directly, or targets where we don't have a filesystem at all (like WASM). -Using the `EvalIO` trait also allows avoiding the `tvix-eval` crate to get too -strongly coupled to a specific store implementation, hashing scheme etc [^2]. -As we can extend the set of builtins available to the evaluator with "foreign -builtins", these can live in other crates. +Using the `EvalIO` trait also lets `tvix-eval` avoid becoming too strongly +coupled to a specific store implementation, hashing scheme etc[^2]. As we can +extend the set of builtins available to the evaluator with "foreign builtins", +these can live in other crates. Following this pattern, we started implementing some of the "basic" builtins that deal with path access in `tvix-eval`, like: @@ -198,38 +198,40 @@ remaining changes should land quite soon. ## Catchables / tryEval -As you may know, Nix has a limited exception system for dealing with -user-generated errors: `builtins.tryEval` can be used to detect if an expression -fails (if `builtins.throw` or `assert` are used to generate it). This feature -requires extra support in any Nix implementation, as errors may not necessarily -cause the Nix program to abort. +Nix has a limited exception system for dealing with user-generated errors: +`builtins.tryEval` can be used to detect if an expression fails (if +`builtins.throw` or `assert` are used to generate it). This feature requires +extra support in any Nix implementation, as errors may not necessarily cause the +Nix program to abort. + +The C++ Nix implementation reuses the C++ language-provided Exception system for +`builtins.tryEval` which Tvix can't (even if Rust had an equivalent system): -The C++ Nix implementation just reuses the C++ language-provided Exception -system for `builtins.tryEval` which Tvix can't (even if Rust had an equivalent -system): In C++ Nix the runtime representation of the program in execution corresponds to the Nix expression tree of the relevant source files. This means that an exception raised in C++ code will automatically bubble up correctly since the C++ and Nix call stacks are equivalent to each other. -Tvix compiles the Nix expressions to a byte code program which may be mutated -by extra optimization rules (for example, we hope to eliminate as many thunks as -possible in the future). This means that such a correspondence between Nix and -the (in this case) VM runtime is not guaranteed. + +Tvix compiles the Nix expressions to a byte code program which may be mutated by +extra optimization rules (for example, we hope to eliminate as many thunks as +possible in the future). This means that such a correspondence between the state +of the runtime and the original Nix code is not guaranteed. Previously, `builtins.tryEval` (which is implemented in Rust and can access VM internals) just allowed the VM to recover from certain kinds of errors. This -proved to be insufficient as it [blew up as soon as a `builtins.tryEval`-ed thunk -is forced again][tryeval-infrec]–extra bookkeeping was needed. As a -solution, we now store thunk evaluation errors that can be recovered from as -`Value::Catchable` which mitigates this problem. +proved to be insufficient as it [blew up as soon as a `builtins.tryEval`-ed +thunk is forced again][tryeval-infrec] – extra bookkeeping was needed. As a +solution, we now store recoverable errors as a separate runtime value type. As you can imagine, storing evaluation failures as "normal" values quickly leads to all sorts of bugs because most VM/builtins code is written with only ordinary values like attribute sets, strings etc. in mind. + While ironing those out, we made sure to supplement those fixes with as many test cases for `builtins.tryEval` as possible. This will hopefully prevent any regressions if or rather when we touch this system again. We already have some -ideas for replacing the `Catchable` value type with a cleaner representation. +ideas for replacing the `Catchable` value type with a cleaner representation, +but first we want to pin down all the unspoken behaviour. ## String contexts @@ -248,12 +250,12 @@ and added support for string contexts into our `NixString` implementation, implemented the context-related builtins, and added more unit tests that verify string context behaviour of various builtins. -## Strings as bstr +## Strings as byte strings C++ Nix uses C-style zero-terminated strings internally - however, until -recently, Tvix has used Rust `String` and `str` for string values. Since those -are required to be valid utf-8, we haven't been able to properly represent all -the string values that Nix supports. +recently, Tvix has used standard Rust strings for string values. Since those are +required to be valid UTF-8, we haven't been able to properly represent all the +string values that Nix supports. We recently converted our internal representation to byte strings, which allows us to treat a `Vec<u8>` as a "string-like" value. @@ -309,6 +311,7 @@ you run into any snags, or have any questions. [^4]: Strictly speaking, not limited to tvix-store - literally anything providing a listing into tvix-castore nodes. +[Tvix]: https://tvix.dev [aterm]: http://program-transformation.org/Tools/ATermFormat.html [bazel-remote]: https://github.com/buchgr/bazel-remote/pull/715 [castore-docs]: https://cs.tvl.fyi/depot/-/blob/tvix/castore/docs |