Reimplement the Config building mechanism

This patch rewrites the Config building mechanism using special objects
for tracking the config building state.

Transitions between states are done on the fly as required.

This is required so that the async sources can be stored inside the
configuration building objects, while keeping out the expenses in the
non-async case, so a user of the crate has only to pay for what they are
using (no async means no overhead for that).

Signed-off-by: Matthias Beyer <mail@beyermatthias.de>
Reviewed-by: Matthias Beyer <mail@beyermatthias.de>

3 files changed, 216 insertions, 25 deletions

diff --git a/src/builder.rs b/src/builder.rs
index a64040a..bb88f44 100644
--- a/src/builder.rs
+++ b/src/builder.rs
@@ -2,6 +2,7 @@ use std::str::FromStr;
 use std::{collections::HashMap, iter::IntoIterator};
 
 use crate::error::Result;
+use crate::source::AsyncSource;
 use crate::{config::Config, path::Expression, source::Source, value::Value};
 
 /// A configuration builder
@@ -22,15 +23,24 @@ use crate::{config::Config, path::Expression, source::Source, value::Value};
 /// It happens on demand when [`build`](Self::build) (or its alternative) is called.
 /// Therefore all errors, related to any of the [`Source`] will only show up then.
 ///
+/// # Sync and async builder
+///
+/// [`ConfigBuilder`] uses type parameter to keep track of builder state.
+///
+/// In [`DefaultState`] builder only supports [`Source`]s
+///
+/// In [`AsyncState`] it supports both [`Source`]s and [`AsyncSource`]s at the price of building using `async fn`.
+///
 /// # Examples
 ///
 /// ```rust
 /// # use config::*;
 /// # use std::error::Error;
 /// # fn main() -> Result<(), Box<dyn Error>> {
-/// let mut builder = ConfigBuilder::default()
+/// let mut builder = Config::builder()
 ///     .set_default("default", "1")?
 ///     .add_source(File::new("config/settings", FileFormat::Json))
+/// //  .add_async_source(...)
 ///     .set_override("override", "1")?;
 ///
 /// match builder.build() {
@@ -45,12 +55,15 @@ use crate::{config::Config, path::Expression, source::Source, value::Value};
 /// # }
 /// ```
 ///
+/// If any [`AsyncSource`] is used, the builder will transition to [`AsyncState`].
+/// In such case, it is required to _await_ calls to [`build`](Self::build) and its non-consuming sibling.
+///
 /// Calls can be not chained as well
 /// ```rust
 /// # use std::error::Error;
 /// # use config::*;
 /// # fn main() -> Result<(), Box<dyn Error>> {
-/// let mut builder = ConfigBuilder::default();
+/// let mut builder = Config::builder();
 /// builder = builder.set_default("default", "1")?;
 /// builder = builder.add_source(File::new("config/settings", FileFormat::Json));
 /// builder = builder.add_source(File::new("config/settings.prod", FileFormat::Json));
@@ -58,22 +71,86 @@ use crate::{config::Config, path::Expression, source::Source, value::Value};
 /// # Ok(())
 /// # }
 /// ```
+///
+/// Calling [`Config::builder`](Config::builder) yields builder in the default state.
+/// If having an asynchronous state as the initial state is desired, _turbofish_ notation needs to be used.
+/// ```rust
+/// # use config::{*, builder::AsyncState};
+/// let mut builder = ConfigBuilder::<AsyncState>::default();
+/// ```
+///
+/// If for some reason acquiring builder in default state is required without calling [`Config::builder`](Config::builder)
+/// it can also be achieved.
+/// ```rust
+/// # use config::{*, builder::DefaultState};
+/// let mut builder = ConfigBuilder::<DefaultState>::default();
+/// ```
 #[derive(Debug, Clone, Default)]
-pub struct ConfigBuilder {
+pub struct ConfigBuilder<St: BuilderState> {
     defaults: HashMap<Expression, Value>,
     overrides: HashMap<Expression, Value>,
+    state: St,
+}
+
+/// Represents [`ConfigBuilder`] state.
+pub trait BuilderState {}
+
+/// Represents data specific to builder in default, sychronous state, without support for async.
+#[derive(Debug, Default)]
+pub struct DefaultState {
     sources: Vec<Box<dyn Source + Send + Sync>>,
 }
 
-impl ConfigBuilder {
+/// The asynchronous configuration builder.
+///
+/// Similar to a [`ConfigBuilder`] it maintains a set of defaults, a set of sources, and overrides.
+///
+/// Defaults do not override anything, sources override defaults, and overrides override anything else.
+/// Within those three groups order of adding them at call site matters - entities added later take precedence.
+///
+/// For more detailed description and examples see [`ConfigBuilder`].
+/// [`AsyncConfigBuilder`] is just an extension of it that takes async functions into account.
+///
+/// To obtain a [`Config`] call [`build`](AsyncConfigBuilder::build) or [`build_cloned`](AsyncConfigBuilder::build_cloned)
+///
+/// # Example
+/// Since this library does not implement any [`AsyncSource`] an example in rustdocs cannot be given.
+/// Detailed explanation about why such a source is not implemented is in [`AsyncSource`]'s documentation.
+///
+/// Refer to [`ConfigBuilder`] for similar API sample usage or to the examples folder of the crate, where such a source is implemented.
+#[derive(Debug, Clone, Default)]
+pub struct AsyncConfigBuilder {
+    defaults: HashMap<Expression, Value>,
+    overrides: HashMap<Expression, Value>,
+    sources: Vec<SourceType>,
+}
+
+/// Represents data specific to builder in asychronous state, with support for async.
+#[derive(Debug, Default)]
+pub struct AsyncState {
+    sources: Vec<SourceType>,
+}
+
+#[derive(Debug, Clone)]
+enum SourceType {
+    Sync(Box<dyn Source + Send + Sync>),
+    Async(Box<dyn AsyncSource + Send + Sync>),
+}
+
+impl BuilderState for DefaultState {}
+impl BuilderState for AsyncState {}
+
+impl<St: BuilderState> ConfigBuilder<St> {
+    // operations allowed in any state
+
     /// Set a default `value` at `key`
     ///
-    /// This value can be overwritten by any [`Source`] or override.
+    /// This value can be overwritten by any [`Source`], [`AsyncSource`] or override.
     ///
     /// # Errors
     ///
     /// Fails if `Expression::from_str(key)` fails.
-    pub fn set_default<S, T>(mut self, key: S, value: T) -> Result<ConfigBuilder>
+    pub fn set_default<S, T>(mut self, key: S, value: T) -> Result<ConfigBuilder<St>>
     where
         S: AsRef<str>,
         T: Into<Value>,
@@ -83,25 +160,14 @@ impl ConfigBuilder {
         Ok(self)
     }
 
-    /// Registers new [`Source`] in this builder.
-    ///
-    /// Calling this method does not invoke any I/O. [`Source`] is only saved in internal register for later use.
-    pub fn add_source<T>(mut self, source: T) -> Self
-    where
-        T: Source + Send + Sync + 'static,
-    {
-        self.sources.push(Box::new(source));
-        self
-    }
-
     /// Set an override
     ///
-    /// This function sets an overwrite value. It will not be altered by any default or [`Source`]
+    /// This function sets an overwrite value. It will not be altered by any default, [`Source`] nor [`AsyncSource`]
     ///
     /// # Errors
     ///
     /// Fails if `Expression::from_str(key)` fails.
-    pub fn set_override<S, T>(mut self, key: S, value: T) -> Result<ConfigBuilder>
+    pub fn set_override<S, T>(mut self, key: S, value: T) -> Result<ConfigBuilder<St>>
     where
         S: AsRef<str>,
         T: Into<Value>,
@@ -110,6 +176,44 @@ impl ConfigBuilder {
             .insert(Expression::from_str(key.as_ref())?, value.into());
         Ok(self)
     }
+}
+
+impl ConfigBuilder<DefaultState> {
+    // operations allowed in sync state
+
+    /// Registers new [`Source`] in this builder.
+    ///
+    /// Calling this method does not invoke any I/O. [`Source`] is only saved in internal register for later use.
+    pub fn add_source<T>(mut self, source: T) -> Self
+    where
+        T: Source + Send + Sync + 'static,
+    {
+        self.state.sources.push(Box::new(source));
+        self
+    }
+
+    /// Registers new [`AsyncSource`] in this builder and forces transition to [`AsyncState`].
+    ///
+    /// Calling this method does not invoke any I/O. [`AsyncSource`] is only saved in internal register for later use.
+    pub fn add_async_source<T>(self, source: T) -> ConfigBuilder<AsyncState>
+    where
+        T: AsyncSource + Send + Sync + 'static,
+    {
+        let async_state = ConfigBuilder {
+            state: AsyncState {
+                sources: self
+                    .state
+                    .sources
+                    .into_iter()
+                    .map(|s| SourceType::Sync(s))
+                    .collect(),
+            },
+            defaults: self.defaults,
+            overrides: self.overrides,
+        };
+
+        async_state.add_async_source(source)
+    }
 
     /// Reads all registered [`Source`]s.
     ///
@@ -120,7 +224,7 @@ impl ConfigBuilder {
     /// If source collection fails, be it technical reasons or related to inability to read data as `Config` for different reasons,
     /// this method returns error.
     pub fn build(self) -> Result<Config> {
-        Self::build_internal(self.defaults, self.overrides, &self.sources)
+        Self::build_internal(self.defaults, self.overrides, &self.state.sources)
     }
 
     /// Reads all registered [`Source`]s.
@@ -132,7 +236,11 @@ impl ConfigBuilder {
     /// If source collection fails, be it technical reasons or related to inability to read data as `Config` for different reasons,
     /// this method returns error.
     pub fn build_cloned(&self) -> Result<Config> {
-        Self::build_internal(self.defaults.clone(), self.overrides.clone(), &self.sources)
+        Self::build_internal(
+            self.defaults.clone(),
+            self.overrides.clone(),
+            &self.state.sources,
+        )
     }
 
     fn build_internal(
@@ -158,3 +266,85 @@ impl ConfigBuilder {
         Ok(Config::new(cache))
     }
 }
+
+impl ConfigBuilder<AsyncState> {
+    // operations allowed in async state
+
+    /// Registers new [`Source`] in this builder.
+    ///
+    /// Calling this method does not invoke any I/O. [`Source`] is only saved in internal register for later use.
+    pub fn add_source<T>(mut self, source: T) -> ConfigBuilder<AsyncState>
+    where
+        T: Source + Send + Sync + 'static,
+    {
+        self.state.sources.push(SourceType::Sync(Box::new(source)));
+        self
+    }
+
+    /// Registers new [`AsyncSource`] in this builder.
+    ///
+    /// Calling this method does not invoke any I/O. [`AsyncSource`] is only saved in internal register for later use.
+    pub fn add_async_source<T>(mut self, source: T) -> ConfigBuilder<AsyncState>
+    where
+        T: AsyncSource + Send + Sync + 'static,
+    {
+        self.state.sources.push(SourceType::Async(Box::new(source)));
+        self
+    }
+
+    /// Reads all registered defaults, [`Source`]s, [`AsyncSource`]s and overrides.
+    ///
+    /// This is the method that invokes all I/O operations.
+    /// For a non consuming alternative see [`build_cloned`](Self::build_cloned)
+    ///
+    /// # Errors
+    /// If source collection fails, be it technical reasons or related to inability to read data as `Config` for different reasons,
+    /// this method returns error.
+    pub async fn build(self) -> Result<Config> {
+        Self::build_internal(self.defaults, self.overrides, &self.state.sources).await
+    }
+
+    /// Reads all registered defaults, [`Source`]s, [`AsyncSource`]s and overrides.
+    ///
+    /// Similar to [`build`](Self::build), but it does not take ownership of `ConfigBuilder` to allow later reuse.
+    /// Internally it clones data to achieve it.
+    ///
+    /// # Errors
+    /// If source collection fails, be it technical reasons or related to inability to read data as `Config` for different reasons,
+    /// this method returns error.
+    pub async fn build_cloned(&self) -> Result<Config> {
+        Self::build_internal(
+            self.defaults.clone(),
+            self.overrides.clone(),
+            &self.state.sources,
+        )
+        .await
+    }
+
+    async fn build_internal(
+        defaults: HashMap<Expression, Value>,
+        overrides: HashMap<Expression, Value>,
+        sources: &[SourceType],
+    ) -> Result<Config> {
+        let mut cache: Value = HashMap::<String, Value>::new().into();
+
+        // Add defaults
+        for (key, val) in defaults.into_iter() {
+            key.set(&mut cache, val);
+        }
+
+        for source in sources.iter() {
+            match source {
+                SourceType::Sync(source) => source.collect_to(&mut cache)?,
+                SourceType::Async(source) => source.collect_to(&mut cache).await?,
+            }
+        }
+
+        // Add overrides
+        for (key, val) in overrides.into_iter() {
+            key.set(&mut cache, val);
+        }
+
+        Ok(Config::new(cache))
+    }
+}
diff --git a/src/config.rs b/src/config.rs
index 3d80aeb..b9c64b4 100644
--- a/src/config.rs
+++ b/src/config.rs
@@ -1,7 +1,7 @@
 use std::collections::HashMap;
 use std::fmt::Debug;
 
-use crate::builder::ConfigBuilder;
+use crate::builder::{ConfigBuilder, DefaultState};
 use serde::de::Deserialize;
 use serde::ser::Serialize;
 
@@ -44,8 +44,8 @@ impl Config {
     }
 
     /// Creates new [`ConfigBuilder`] instance
-    pub fn builder() -> ConfigBuilder {
-        ConfigBuilder::default()
+    pub fn builder() -> ConfigBuilder<DefaultState> {
+        ConfigBuilder::<DefaultState>::default()
     }
 
     /// Merge in a configuration property source.
diff --git a/src/lib.rs b/src/lib.rs
index 00526c4..87c82bc 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -53,7 +53,7 @@ extern crate ron;
 #[cfg(feature = "json5")]
 extern crate json5_rs;
 
-mod builder;
+pub mod builder;
 mod config;
 mod de;
 mod env;
@@ -64,6 +64,7 @@ mod ser;
 mod source;
 mod value;
 
+pub use crate::builder::AsyncConfigBuilder;
 pub use crate::builder::ConfigBuilder;
 pub use crate::config::Config;
 pub use crate::env::Environment;