//! The composition module allows composing different kinds of services based on a set of service
//! configurations _at runtime_.
//!
//! Store configs are deserialized with serde. The registry provides a stateful mapping from the
//! `type` tag of an internally tagged enum on the serde side to a Config struct which is
//! deserialized and then returned as a `Box<dyn ServiceBuilder<Output = dyn BlobService>>`
//! (the same for DirectoryService instead of BlobService etc).
//!
//! ### Example 1.: Implementing a new BlobService
//!
//! You need a Config struct which implements `DeserializeOwned` and
//! `ServiceBuilder<Output = dyn BlobService>`.
//! Provide the user with a function to call with
//! their registry. You register your new type as:
//!
//! ```
//! use std::sync::Arc;
//!
//! use tvix_castore::composition::*;
//! use tvix_castore::blobservice::BlobService;
//!
//! #[derive(serde::Deserialize)]
//! struct MyBlobServiceConfig {
//! }
//!
//! #[tonic::async_trait]
//! impl ServiceBuilder for MyBlobServiceConfig {
//! type Output = dyn BlobService;
//! async fn build(&self, _: &str, _: &CompositionContext) -> Result<Arc<Self::Output>, Box<dyn std::error::Error + Send + Sync + 'static>> {
//! todo!()
//! }
//! }
//!
//! impl TryFrom<url::Url> for MyBlobServiceConfig {
//! type Error = Box<dyn std::error::Error + Send + Sync>;
//! fn try_from(url: url::Url) -> Result<Self, Self::Error> {
//! todo!()
//! }
//! }
//!
//! pub fn add_my_service(reg: &mut Registry) {
//! reg.register::<Box<dyn ServiceBuilder<Output = dyn BlobService>>, MyBlobServiceConfig>("myblobservicetype");
//! }
//! ```
//!
//! Now, when a user deserializes a store config with the type tag "myblobservicetype" into a
//! `Box<dyn ServiceBuilder<Output = Arc<dyn BlobService>>>`, it will be done via `MyBlobServiceConfig`.
//!
//! ### Example 2.: Composing stores to get one store
//!
//! ```
//! use std::sync::Arc;
//! use tvix_castore::composition::*;
//! use tvix_castore::blobservice::BlobService;
//!
//! # fn main() -> Result<(), Box<dyn std::error::Error>> {
//! # tokio::runtime::Builder::new_current_thread().enable_all().build().unwrap().block_on(async move {
//! let blob_services_configs_json = serde_json::json!({
//! "blobstore1": {
//! "type": "memory"
//! },
//! "blobstore2": {
//! "type": "memory"
//! },
//! "root": {
//! "type": "combined",
//! "near": "blobstore1",
//! "far": "blobstore2"
//! }
//! });
//!
//! let blob_services_configs = with_registry(®, || serde_json::from_value(blob_services_configs_json))?;
//! let mut blob_service_composition = Composition::new(®);
//! blob_service_composition.extend_with_configs::<dyn BlobService>(blob_services_configs);
//! let blob_service: Arc<dyn BlobService> = blob_service_composition.build("root").await?;
//! # Ok(())
//! # })
//! # }
//! ```
//!
//! ### Example 3.: Creating another registry extending the default registry with third-party types
//!
//! ```
//! # pub fn add_my_service(reg: &mut tvix_castore::composition::Registry) {}
//! let mut my_registry = tvix_castore::composition::Registry::default();
//! tvix_castore::composition::add_default_services(&mut my_registry);
//! add_my_service(&mut my_registry);
//! ```
//!
//! Continue with Example 2, with my_registry instead of REG
//!
//! EXPERIMENTAL: If the xp-composition-url-refs feature is enabled,
//! entrypoints can also be URL strings, which are created as
//! anonymous stores. Instantiations of the same URL will
//! result in a new, distinct anonymous store each time, so creating
//! two `memory://` stores with this method will not share the same view.
//! This behavior might change in the future.
use erased_serde::deserialize;
use futures::future::BoxFuture;
use futures::FutureExt;
use serde::de::DeserializeOwned;
use serde_tagged::de::{BoxFnSeed, SeedFactory};
use serde_tagged::util::TagString;
use std::any::{Any, TypeId};
use std::cell::Cell;
use std::collections::BTreeMap;
use std::collections::HashMap;
use std::marker::PhantomData;
use std::sync::{Arc, LazyLock};
use tonic::async_trait;
/// Resolves tag names to the corresponding Config type.
// Registry implementation details:
// This is really ugly. Really we would want to store this as a generic static field:
//
// ```
// struct Registry<T>(BTreeMap<(&'static str), RegistryEntry<T>);
// static REG<T>: Registry<T>;
// ```
//
// so that one version of the static is generated for each Type that the registry is accessed for.
// However, this is not possible, because generics are only a thing in functions, and even there
// they will not interact with static items:
// https://doc.rust-lang.org/reference/items/static-items.html#statics--generics
//
// So instead, we make this lookup at runtime by putting the TypeId into the key.
// But now we can no longer store the `BoxFnSeed<T>` because we are lacking the generic parameter
// T, so instead store it as `Box<dyn Any>` and downcast to `&BoxFnSeed<T>` when performing the
// lookup.
// I said it was ugly...
#[derive(Default)]
pub struct Registry(BTreeMap<(TypeId, &'static str), Box<dyn Any + Sync>>);
pub type FromUrlSeed<T> =
Box<dyn Fn(url::Url) -> Result<T, Box<dyn std::error::Error + Send + Sync>> + Sync>;
pub struct RegistryEntry<T> {
serde_deserialize_seed: BoxFnSeed<DeserializeWithRegistry<T>>,
from_url_seed: FromUrlSeed<DeserializeWithRegistry<T>>,
}
struct RegistryWithFakeType<'r, T>(&'r Registry, PhantomData<T>);
impl<'r, 'de: 'r, T: 'static> SeedFactory<'de, TagString<'de>> for RegistryWithFakeType<'r, T> {
type Value = DeserializeWithRegistry<T>;
type Seed = &'r BoxFnSeed<Self::Value>;
// Required method
fn seed<E>(self, tag: TagString<'de>) -> Result<Self::Seed, E>
where
E: serde::de::Error,
{
// using find() and not get() because of https://github.com/rust-lang/rust/issues/80389
let seed: &Box<dyn Any + Sync> = self
.0
.0
.iter()
.find(|(k, _)| *k == &(TypeId::of::<T>(), tag.as_ref()))
.ok_or_else(|| serde::de::Error::custom(format!("Unknown type: {}", tag)))?
.1;
let entry: &RegistryEntry<T> = <dyn Any>::downcast_ref(&**seed).unwrap();
Ok(&entry.serde_deserialize_seed)
}
}
/// Wrapper type which implements Deserialize using the registry
///
/// Wrap your type in this in order to deserialize it using a registry, e.g.
/// `RegistryWithFakeType<Box<dyn MyTrait>>`, then the types registered for `Box<dyn MyTrait>`
/// will be used.
pub struct DeserializeWithRegistry<T>(pub T);
impl Registry {
/// Registers a mapping from type tag to a concrete type into the registry.
///
/// The type parameters are very important:
/// After calling `register::<Box<dyn FooTrait>, FooStruct>("footype")`, when a user
/// deserializes into an input with the type tag "myblobservicetype" into a
/// `Box<dyn FooTrait>`, it will first call the Deserialize imple of `FooStruct` and
/// then convert it into a `Box<dyn FooTrait>` using From::from.
pub fn register<
T: 'static,
C: DeserializeOwned
+ TryFrom<url::Url, Error = Box<dyn std::error::Error + Send + Sync>>
+ Into<T>,
>(
&mut self,
type_name: &'static str,
) {
self.0.insert(
(TypeId::of::<T>(), type_name),
Box::new(RegistryEntry {
serde_deserialize_seed: BoxFnSeed::new(|x| {
deserialize::<C>(x)
.map(Into::into)
.map(DeserializeWithRegistry)
}),
from_url_seed: Box::new(|url| {
C::try_from(url)
.map(Into::into)
.map(DeserializeWithRegistry)
}),
}),
);
}
}
impl<'de, T: 'static> serde::Deserialize<'de> for DeserializeWithRegistry<T> {
fn deserialize<D>(de: D) -> std::result::Result<Self, D::Error>
where
D: serde::Deserializer<'de>,
{
serde_tagged::de::internal::deserialize(
de,
"type",
RegistryWithFakeType(ACTIVE_REG.get().unwrap(), PhantomData::<T>),
)
}
}
#[derive(Debug, thiserror::Error)]
enum TryFromUrlError {
#[error("Unknown type: {0}")]
UnknownTag(String),
}
impl<T: 'static> TryFrom<url::Url> for DeserializeWithRegistry<T> {
type Error = Box<dyn std::error::Error + Send + Sync>;
fn try_from(url: url::Url) -> Result<Self, Self::Error> {
let tag = url.scheme().split('+').next().unwrap();
// same as in the SeedFactory impl: using find() and not get() because of https://github.com/rust-lang/rust/issues/80389
let seed = ACTIVE_REG
.get()
.unwrap()
.0
.iter()
.find(|(k, _)| *k == &(TypeId::of::<T>(), tag))
.ok_or_else(|| Box::new(TryFromUrlError::UnknownTag(tag.into())))?
.1;
let entry: &RegistryEntry<T> = <dyn Any>::downcast_ref(&**seed).unwrap();
(entry.from_url_seed)(url)
}
}
thread_local! {
/// The active Registry is global state, because there is no convenient and universal way to pass state
/// into the functions usually used for deserialization, e.g. `serde_json::from_str`, `toml::from_str`,
/// `serde_qs::from_str`.
static ACTIVE_REG: Cell<Option<&'static Registry>> = panic!("reg was accessed before initialization");
}
/// Run the provided closure with a registry context.
/// Any serde deserialize calls within the closure will use the registry to resolve tag names to
/// the corresponding Config type.
pub fn with_registry<R>(reg: &'static Registry, f: impl FnOnce() -> R) -> R {
ACTIVE_REG.set(Some(reg));
let result = f();
ACTIVE_REG.set(None);
result
}
/// The provided registry of tvix_castore, with all builtin BlobStore/DirectoryStore implementations
pub static REG: LazyLock<&'static Registry> = LazyLock::new(|| {
let mut reg = Default::default();
add_default_services(&mut reg);
// explicitly leak to get an &'static, so that we gain `&Registry: Send` from `Registry: Sync`
Box::leak(Box::new(reg))
});
// ---------- End of generic registry code --------- //
/// Register the builtin services of tvix_castore (blob services and directory
/// services) with the given registry.
/// This can be used outside to create your own registry with the builtin types
/// _and_ extra third party types.
pub fn add_default_services(reg: &mut Registry) {
crate::blobservice::register_blob_services(reg);
crate::directoryservice::register_directory_services(reg);
}
pub struct CompositionContext<'a> {
// The stack used to detect recursive instantiations and prevent deadlocks
// The TypeId of the trait object is included to distinguish e.g. the
// BlobService "root" and the DirectoryService "root".
stack: Vec<(TypeId, String)>,
registry: &'static Registry,
composition: Option<&'a Composition>,
}
impl<'a> CompositionContext<'a> {
/// Get a composition context for one-off store creation.
pub fn blank(registry: &'static Registry) -> Self {
Self {
registry,
stack: Default::default(),
composition: None,
}
}
pub async fn resolve<T: ?Sized + Send + Sync + 'static>(
&self,
entrypoint: String,
) -> Result<Arc<T>, Box<dyn std::error::Error + Send + Sync + 'static>> {
// disallow recursion
if self
.stack
.contains(&(TypeId::of::<T>(), entrypoint.clone()))
{
return Err(CompositionError::Recursion(
self.stack.iter().map(|(_, n)| n.clone()).collect(),
)
.into());
}
Ok(self.build_internal(entrypoint).await?)
}
#[cfg(feature = "xp-composition-url-refs")]
async fn build_anonymous<T: ?Sized + Send + Sync + 'static>(
&self,
entrypoint: String,
) -> Result<Arc<T>, Box<dyn std::error::Error + Send + Sync>> {
let url = url::Url::parse(&entrypoint)?;
let config: DeserializeWithRegistry<Box<dyn ServiceBuilder<Output = T>>> =
with_registry(self.registry, || url.try_into())?;
config.0.build("anonymous", self).await
}
fn build_internal<T: ?Sized + Send + Sync + 'static>(
&self,
entrypoint: String,
) -> BoxFuture<'_, Result<Arc<T>, CompositionError>> {
#[cfg(feature = "xp-composition-url-refs")]
if entrypoint.contains("://") {
// There is a chance this is a url. we are building an anonymous store
return Box::pin(async move {
self.build_anonymous(entrypoint.clone())
.await
.map_err(|e| CompositionError::Failed(entrypoint, Arc::from(e)))
});
}
let mut stores = match self.composition {
Some(comp) => comp.stores.lock().unwrap(),
None => return Box::pin(futures::future::err(CompositionError::NotFound(entrypoint))),
};
let entry = match stores.get_mut(&(TypeId::of::<T>(), entrypoint.clone())) {
Some(v) => v,
None => return Box::pin(futures::future::err(CompositionError::NotFound(entrypoint))),
};
// for lifetime reasons, we put a placeholder value in the hashmap while we figure out what
// the new value should be. the Mutex stays locked the entire time, so nobody will ever see
// this temporary value.
let prev_val = std::mem::replace(
entry,
Box::new(InstantiationState::<T>::Done(Err(
CompositionError::Poisoned(entrypoint.clone()),
))),
);
let (new_val, ret) = match *prev_val.downcast::<InstantiationState<T>>().unwrap() {
InstantiationState::Done(service) => (
InstantiationState::Done(service.clone()),
futures::future::ready(service).boxed(),
),
// the construction of the store has not started yet.
InstantiationState::Config(config) => {
let (tx, rx) = tokio::sync::watch::channel(None);
(
InstantiationState::InProgress(rx),
(async move {
let mut new_context = CompositionContext {
composition: self.composition,
registry: self.registry,
stack: self.stack.clone(),
};
new_context
.stack
.push((TypeId::of::<T>(), entrypoint.clone()));
let res =
config.build(&entrypoint, &new_context).await.map_err(|e| {
match e.downcast() {
Ok(e) => *e,
Err(e) => CompositionError::Failed(entrypoint, e.into()),
}
});
tx.send(Some(res.clone())).unwrap();
res
})
.boxed(),
)
}
// there is already a task driving forward the construction of this store, wait for it
// to notify us via the provided channel
InstantiationState::InProgress(mut recv) => {
(InstantiationState::InProgress(recv.clone()), {
(async move {
loop {
if let Some(v) =
recv.borrow_and_update().as_ref().map(|res| res.clone())
{
break v;
}
recv.changed().await.unwrap();
}
})
.boxed()
})
}
};
*entry = Box::new(new_val);
ret
}
}
#[async_trait]
/// This is the trait usually implemented on a per-store-type Config struct and
/// used to instantiate it.
pub trait ServiceBuilder: Send + Sync {
type Output: ?Sized;
async fn build(
&self,
instance_name: &str,
context: &CompositionContext,
) -> Result<Arc<Self::Output>, Box<dyn std::error::Error + Send + Sync + 'static>>;
}
impl<T: ?Sized, S: ServiceBuilder<Output = T> + 'static> From<S>
for Box<dyn ServiceBuilder<Output = T>>
{
fn from(t: S) -> Self {
Box::new(t)
}
}
enum InstantiationState<T: ?Sized> {
Config(Box<dyn ServiceBuilder<Output = T>>),
InProgress(tokio::sync::watch::Receiver<Option<Result<Arc<T>, CompositionError>>>),
Done(Result<Arc<T>, CompositionError>),
}
pub struct Composition {
registry: &'static Registry,
stores: std::sync::Mutex<HashMap<(TypeId, String), Box<dyn Any + Send + Sync>>>,
}
#[derive(thiserror::Error, Clone, Debug)]
pub enum CompositionError {
#[error("store not found: {0}")]
NotFound(String),
#[error("recursion not allowed {0:?}")]
Recursion(Vec<String>),
#[error("store construction panicked {0}")]
Poisoned(String),
#[error("instantiation of service {0} failed: {1}")]
Failed(String, Arc<dyn std::error::Error + Send + Sync>),
}
impl<T: ?Sized + Send + Sync + 'static>
Extend<(
String,
DeserializeWithRegistry<Box<dyn ServiceBuilder<Output = T>>>,
)> for Composition
{
fn extend<I>(&mut self, configs: I)
where
I: IntoIterator<
Item = (
String,
DeserializeWithRegistry<Box<dyn ServiceBuilder<Output = T>>>,
),
>,
{
self.stores
.lock()
.unwrap()
.extend(configs.into_iter().map(|(k, v)| {
(
(TypeId::of::<T>(), k),
Box::new(InstantiationState::Config(v.0)) as Box<dyn Any + Send + Sync>,
)
}))
}
}
impl Composition {
/// The given registry will be used for creation of anonymous stores during composition
pub fn new(registry: &'static Registry) -> Self {
Self {
registry,
stores: Default::default(),
}
}
pub fn extend_with_configs<T: ?Sized + Send + Sync + 'static>(
&mut self,
// Keep the concrete `HashMap` type here since it allows for type
// inference of what type is previously being deserialized.
configs: HashMap<String, DeserializeWithRegistry<Box<dyn ServiceBuilder<Output = T>>>>,
) {
self.extend(configs);
}
/// Looks up the entrypoint name in the composition and returns an instantiated service.
pub async fn build<T: ?Sized + Send + Sync + 'static>(
&self,
entrypoint: &str,
) -> Result<Arc<T>, CompositionError> {
self.context().build_internal(entrypoint.to_string()).await
}
pub fn context(&self) -> CompositionContext {
CompositionContext {
registry: self.registry,
stack: vec![],
composition: Some(self),
}
}
}
#[cfg(test)]
mod test {
use super::*;
use crate::blobservice::BlobService;
use std::sync::Arc;
/// Test that we return a reference to the same instance of MemoryBlobService (via ptr_eq)
/// when instantiating the same entrypoint twice. By instantiating concurrently, we also
/// test the channels notifying the second consumer when the store has been instantiated.
#[tokio::test]
async fn concurrent() {
let blob_services_configs_json = serde_json::json!({
"root": {
"type": "memory",
}
});
let blob_services_configs =
with_registry(®, || serde_json::from_value(blob_services_configs_json)).unwrap();
let mut blob_service_composition = Composition::new(®);
blob_service_composition.extend_with_configs::<dyn BlobService>(blob_services_configs);
let (blob_service1, blob_service2) = tokio::join!(
blob_service_composition.build::<dyn BlobService>("root"),
blob_service_composition.build::<dyn BlobService>("root")
);
assert!(Arc::ptr_eq(
&blob_service1.unwrap(),
&blob_service2.unwrap()
));
}
/// Test that we throw the correct error when an instantiation would recurse (deadlock)
#[tokio::test]
async fn reject_recursion() {
let blob_services_configs_json = serde_json::json!({
"root": {
"type": "combined",
"near": "other",
"far": "other"
},
"other": {
"type": "combined",
"near": "root",
"far": "root"
}
});
let blob_services_configs =
with_registry(®, || serde_json::from_value(blob_services_configs_json)).unwrap();
let mut blob_service_composition = Composition::new(®);
blob_service_composition.extend_with_configs::<dyn BlobService>(blob_services_configs);
match blob_service_composition
.build::<dyn BlobService>("root")
.await
{
Err(CompositionError::Recursion(stack)) => {
assert_eq!(stack, vec!["root".to_string(), "other".to_string()])
}
other => panic!("should have returned an error, returned: {:?}", other.err()),
}
}
}
