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//! This crate is the core implementation of the feature flags (called "feattles", for short).
//!
//! Its main parts are the macro [`feattles!`] together with the trait [`Feattles`]. Please refer to
//! the [main package - `feattle`](https://crates.io/crates/feattle) for more information.
//!
//! # Usage example
//! ```
//! use std::sync::Arc;
//! use feattle_core::{feattles, Feattles};
//! use feattle_core::persist::NoPersistence;
//!
//! // Declare the struct
//! feattles! {
//! struct MyFeattles {
//! /// Is this usage considered cool?
//! is_cool: bool = true,
//! /// Limit the number of "blings" available.
//! /// This will not change the number of "blengs", though!
//! max_blings: i32,
//! /// List the actions that should not be available
//! blocked_actions: Vec<String>,
//! }
//! }
//!
//! // Create a new instance (`NoPersistence` is just a mock for the persistence layer)
//! let my_feattles = MyFeattles::new(Arc::new(NoPersistence));
//!
//! // Read values (note the use of `*`)
//! assert_eq!(*my_feattles.is_cool(), true);
//! assert_eq!(*my_feattles.max_blings(), 0);
//! assert_eq!(*my_feattles.blocked_actions(), Vec::<String>::new());
//! ```
//!
//! # How it works
//!
//! The macro will generate a struct with the given name and visibility modifier (assuming private
//! by default). The generated struct implements [`Feattles`] and also exposes one method for each
//! feattle.
//!
//! The methods created for each feattle allow reading their current value. For example, for a
//! feattle `is_cool: bool`, there will be a method like
//! `pub fn is_cool(&self) -> MappedRwLockReadGuard<bool>`. Note the use of
//! [`parking_lot::MappedRwLockReadGuard`] because the interior of the struct is stored behind a `RwLock` to
//! control concurrent access.
//!
//! A feattle is created with the syntax `$key: $type [= $default]`. You can use doc coments (
//! starting with `///`) to describe nicely what they do in your system. You can use any type that
//! implements [`FeattleValue`] and optionally provide a default. If not provided, the default
//! will be created with `Default::default()`.
//!
//! # Updating values
//! This crate only disposes of low-level methods to load current feattles with [`Feattles::reload()`]
//! and update their values with [`Feattles::update()`]. Please look for the crates
//! [feattle-sync](https://crates.io/crates/feattle-sync) and
//! [feattle-ui](https://crates.io/crates/feattle-ui) for higher-level functionalities.
//!
//! # Limitations
//! Due to some restrictions on how the macro is written, you can only use [`feattles!`] once per
//! module. For example, the following does not compile:
//!
//! ```compile_fail
//! use feattle_core::feattles;
//!
//! feattles! { struct A { } }
//! feattles! { struct B { } }
//! ```
//!
//! You can work around this limitation by creating a sub-module and then re-exporting the generated
//! struct. Note the use of `pub struct` in the second case.
//! ```
//! use feattle_core::feattles;
//!
//! feattles! { struct A { } }
//!
//! mod b {
//! use feattle_core::feattles;
//! feattles! { pub struct B { } }
//! }
//!
//! use b::B;
//! ```
//!
//! # Optional features
//!
//! - **uuid**: will add support for [`uuid::Uuid`].
#[doc(hidden)]
pub mod __internal;
mod definition;
mod feattle_value;
pub mod json_reading;
pub mod last_reload;
/// This module only contains exported macros, that are documented at the root level.
#[doc(hidden)]
pub mod macros;
pub mod persist;
use crate::__internal::{FeattlesStruct, InnerFeattles};
use crate::json_reading::FromJsonError;
use crate::last_reload::LastReload;
use async_trait::async_trait;
use chrono::Utc;
pub use definition::*;
pub use feattle_value::*;
use parking_lot::{MappedRwLockReadGuard, RwLockReadGuard, RwLockWriteGuard};
use persist::*;
use serde_json::Value;
use std::error::Error;
use std::fmt::Debug;
use std::sync::Arc;
use thiserror::Error;
/// Represents a type-erased error that comes from some external source
pub type BoxError = Box<dyn Error + Send + Sync>;
/// The error type returned by [`Feattles::update()`]
#[derive(Error, Debug)]
pub enum UpdateError {
/// Cannot update because current values were never successfully loaded from the persist layer
#[error("cannot update because current values were never successfully loaded from the persist layer")]
NeverReloaded,
/// The key is unknown
#[error("the key {0} is unknown")]
UnknownKey(String),
/// Failed to parse the value from JSON
#[error("failed to parse the value from JSON")]
Parsing(
#[source]
#[from]
FromJsonError,
),
/// Failed to persist new state
#[error("failed to persist new state")]
Persistence(#[source] BoxError),
}
/// The error type returned by [`Feattles::history()`]
#[derive(Error, Debug)]
pub enum HistoryError {
/// The key is unknown
#[error("the key {0} is unknown")]
UnknownKey(String),
/// Failed to load persisted state
#[error("failed to load persisted state")]
Persistence(#[source] BoxError),
}
/// The main trait of this crate.
///
/// The struct created with [`feattles!`] will implement this trait in addition to a method for each
/// feattle. Read more at the [crate documentation](crate).
#[async_trait]
pub trait Feattles: FeattlesPrivate {
/// Create a new feattles instance, using the given persistence layer logic.
///
/// All feattles will start with their default values. You can force an initial synchronization
/// with [`Feattles::update`].
fn new(persistence: Arc<dyn Persist>) -> Self;
/// Return a shared reference to the persistence layer.
fn persistence(&self) -> &Arc<dyn Persist>;
/// The list of all available keys.
fn keys(&self) -> &'static [&'static str];
/// Describe one specific feattle, returning `None` if the feattle with the given name does not
/// exist.
fn definition(&self, key: &str) -> Option<FeattleDefinition>;
/// Return details of the last time the data was synchronized by calling [`Feattles::reload()`].
fn last_reload(&self) -> LastReload {
self._read().last_reload
}
/// Return a reference to the last synchronized data. The reference is behind a
/// read-write lock and will block any update until it is dropped. `None` is returned if a
/// successful synchronization have never happened.
fn current_values(&self) -> Option<MappedRwLockReadGuard<CurrentValues>> {
let inner = self._read();
if inner.current_values.is_none() {
None
} else {
Some(RwLockReadGuard::map(inner, |x| {
x.current_values.as_ref().unwrap()
}))
}
}
/// Reload the current feattles' data from the persistence layer, propagating any errors
/// produced by it.
///
/// If any of the feattle values fail to be parsed from previously persisted values, their
/// updates will be skipped. Other feattles that parsed successfully will still be updated.
/// In this case, a [`log::error!`] will be generated for each time it occurs.
async fn reload(&self) -> Result<(), BoxError> {
let current_values = self.persistence().load_current().await?;
let mut inner = self._write();
let now = Utc::now();
match current_values {
None => {
inner.last_reload = LastReload::NoData { reload_date: now };
let empty = CurrentValues {
version: 0,
date: now,
feattles: Default::default(),
};
inner.current_values = Some(empty);
}
Some(current_values) => {
inner.last_reload = LastReload::Data {
reload_date: now,
version: current_values.version,
version_date: current_values.date,
};
for &key in self.keys() {
let value = current_values.feattles.get(key).cloned();
log::debug!("Will update {} with {:?}", key, value);
if let Err(error) = inner.feattles_struct.try_update(key, value) {
log::error!("Failed to update {}: {:?}", key, error);
}
}
inner.current_values = Some(current_values);
}
}
Ok(())
}
/// Update a single feattle, passing the new value (in JSON representation) and the user that
/// is associated with this change. The change will be persisted directly.
///
/// While the update is happening, the new value will already be observable from other
/// execution tasks or threads. However, if the update fails, the change will be rolled back.
///
/// # Consistency
///
/// To avoid operating on stale data, before doing an update the caller should usually call
/// [`Feattles::reload()`] to ensure data is current.
async fn update(
&self,
key: &str,
value: Value,
modified_by: String,
) -> Result<(), UpdateError> {
use UpdateError::*;
// The update operation is made of 4 steps, each of which may fail:
// 1. parse and update the inner generic struct
// 2. persist the new history entry
// 3. persist the new current values
// 4. update the copy of the current values
// If any step fails, the others will be rolled back
// Assert the key exists
if !self.keys().contains(&key) {
return Err(UnknownKey(key.to_owned()));
}
let new_value = CurrentValue {
modified_at: Utc::now(),
modified_by,
value,
};
let (new_values, old_value) = {
let mut inner = self._write();
// Check error condition for step 4 and prepare the new instance
let mut new_values = inner.current_values.clone().ok_or(NeverReloaded)?;
new_values
.feattles
.insert(key.to_owned(), new_value.clone());
new_values.version += 1;
// Step 1
let old_value = inner
.feattles_struct
.try_update(key, Some(new_value.clone()))?;
(new_values, old_value)
};
log::debug!("new_values = {:?}", new_values);
let rollback_step_1 = || {
// Note that if the old value was failing to parse, then the update will be final.
let _ = self
._write()
.feattles_struct
.try_update(key, old_value.clone());
};
// Step 2: load + modify + save history
let persistence = self.persistence();
let old_history = persistence
.load_history(key)
.await
.map_err(|err| {
rollback_step_1();
Persistence(err)
})?
.unwrap_or_default();
// Prepare updated history
let new_definition = self
.definition(key)
.expect("the key is guaranteed to exist");
let mut new_history = old_history.clone();
new_history.entries.push(HistoryEntry {
value: new_value.value.clone(),
value_overview: new_definition.value_overview,
modified_at: new_value.modified_at,
modified_by: new_value.modified_by.clone(),
});
persistence
.save_history(key, &new_history)
.await
.map_err(|err| {
rollback_step_1();
Persistence(err)
})?;
// Step 3
if let Err(err) = persistence.save_current(&new_values).await {
rollback_step_1();
if let Err(err) = self.persistence().save_history(key, &old_history).await {
log::warn!("Failed to rollback history for {}: {:?}", key, err);
}
return Err(Persistence(err));
}
// Step 4
self._write().current_values = Some(new_values);
Ok(())
}
/// Return the definition for all the feattles.
fn definitions(&self) -> Vec<FeattleDefinition> {
self.keys()
.iter()
.map(|&key| {
self.definition(key)
.expect("since we iterate over the list of known keys, this should always work")
})
.collect()
}
/// Return the history for a single feattle. It can be potentially empty (not entries).
async fn history(&self, key: &str) -> Result<ValueHistory, HistoryError> {
// Assert the key exists
if !self.keys().contains(&key) {
return Err(HistoryError::UnknownKey(key.to_owned()));
}
let history = self
.persistence()
.load_history(key)
.await
.map_err(HistoryError::Persistence)?;
Ok(history.unwrap_or_default())
}
}
/// This struct is `pub` because the macro must have access to it, but should be otherwise invisible
/// to the users of this crate.
#[doc(hidden)]
pub trait FeattlesPrivate {
type FeattleStruct: FeattlesStruct;
fn _read(&self) -> RwLockReadGuard<InnerFeattles<Self::FeattleStruct>>;
fn _write(&self) -> RwLockWriteGuard<InnerFeattles<Self::FeattleStruct>>;
}
#[cfg(test)]
mod tests {
use super::*;
use parking_lot::Mutex;
use serde_json::json;
use std::collections::BTreeMap;
use std::sync::Arc;
#[derive(Debug, thiserror::Error)]
#[error("Some error")]
struct SomeError;
#[derive(Default)]
struct MockPersistence(Mutex<MockPersistenceInner>);
#[derive(Default)]
struct MockPersistenceInner {
current: Option<CurrentValues>,
history: BTreeMap<String, ValueHistory>,
next_error: Option<BoxError>,
}
impl MockPersistence {
fn put_error(&self) {
let previous = self.0.lock().next_error.replace(Box::new(SomeError));
assert!(previous.is_none());
}
fn get_error(&self) -> Result<(), BoxError> {
match self.0.lock().next_error.take() {
None => Ok(()),
Some(e) => Err(e),
}
}
fn unwrap_current(&self) -> CurrentValues {
self.0.lock().current.clone().unwrap()
}
fn unwrap_history(&self, key: &str) -> ValueHistory {
self.0.lock().history.get(key).cloned().unwrap()
}
}
#[async_trait]
impl Persist for MockPersistence {
async fn save_current(&self, value: &CurrentValues) -> Result<(), BoxError> {
self.get_error().map(|_| {
self.0.lock().current = Some(value.clone());
})
}
async fn load_current(&self) -> Result<Option<CurrentValues>, BoxError> {
self.get_error().map(|_| self.0.lock().current.clone())
}
async fn save_history(&self, key: &str, value: &ValueHistory) -> Result<(), BoxError> {
self.get_error().map(|_| {
self.0.lock().history.insert(key.to_owned(), value.clone());
})
}
async fn load_history(&self, key: &str) -> Result<Option<ValueHistory>, BoxError> {
self.get_error()
.map(|_| self.0.lock().history.get(key).cloned())
}
}
#[tokio::test]
async fn test() {
feattles! {
struct Config {
/// A
a: i32,
b: i32 = 17
}
}
let persistence = Arc::new(MockPersistence::default());
let config = Config::new(persistence.clone());
// Initial state
assert_eq!(*config.a(), 0);
assert_eq!(*config.b(), 17);
assert_eq!(config.keys(), &["a", "b"]);
assert!(config.last_reload() == LastReload::Never);
assert!(config.current_values().is_none());
// Load from empty storage
config.reload().await.unwrap();
assert_eq!(*config.a(), 0);
assert_eq!(*config.b(), 17);
let last_reload = config.last_reload();
assert!(matches!(last_reload, LastReload::NoData { .. }));
assert!(config.current_values().is_some());
// Load from failing storage
persistence.put_error();
config.reload().await.unwrap_err();
assert_eq!(config.last_reload(), last_reload);
// Update value
config
.update("a", json!(27i32), "somebody".to_owned())
.await
.unwrap();
assert_eq!(*config.a(), 27);
let values = persistence.unwrap_current();
assert_eq!(values.version, 1);
let value = values.feattles.get("a").unwrap();
assert_eq!(value.modified_by, "somebody");
assert_eq!(value.value, json!(27i32));
let history = persistence.unwrap_history("a");
assert_eq!(history.entries.len(), 1);
assert_eq!(&history.entries[0].value, &json!(27i32));
assert_eq!(&history.entries[0].value_overview, "27");
assert_eq!(&history.entries[0].modified_by, "somebody");
// Failed to update
persistence.put_error();
config
.update("a", json!(207i32), "somebody else".to_owned())
.await
.unwrap_err();
assert_eq!(*config.a(), 27);
let values = persistence.unwrap_current();
assert_eq!(values.version, 1);
let value = values.feattles.get("a").unwrap();
assert_eq!(value.modified_by, "somebody");
assert_eq!(value.value, json!(27i32));
let history = persistence.unwrap_history("a");
assert_eq!(history.entries.len(), 1);
assert_eq!(&history.entries[0].value, &json!(27i32));
assert_eq!(&history.entries[0].value_overview, "27");
assert_eq!(&history.entries[0].modified_by, "somebody");
}
}