feat(pool): add a Singleton pool type (#226)

This creates a `Singleton` pool service which wraps an inner make-service, and queues up all calls while waiting for the one instance to be created, and then hands out clones. This pattern fits HTTP/2 well.

The current implementation mirrors much of how the legacy pool handles connecting with HTTP/2 prior knowledge.

Closes hyperium/hyper#3953

Author: seanmonstar

Cargo.toml

@@ -42,6 +42,7 @@ futures-util = { version = "0.3.16", default-features = false, features = ["allo
 http-body-util = "0.1.0"
 tokio = { version = "1", features = ["macros", "test-util", "signal"] }
 tokio-test = "0.4"
+tower-test = "0.4"
 pretty_env_logger = "0.5"
 
 [target.'cfg(any(target_os = "linux", target_os = "macos"))'.dev-dependencies]
@@ -60,6 +61,7 @@ default = []
 full = [
     "client",
     "client-legacy",
+    "client-pool",
     "client-proxy",
     "client-proxy-system",
     "server",
@@ -74,6 +76,7 @@ full = [
 
 client = ["hyper/client", "tokio/net", "dep:tracing", "dep:futures-channel", "dep:tower-service"]
 client-legacy = ["client", "dep:socket2", "tokio/sync", "dep:libc", "dep:futures-util"]
+client-pool = []
 client-proxy = ["client", "dep:base64", "dep:ipnet", "dep:percent-encoding"]
 client-proxy-system = ["dep:system-configuration", "dep:windows-registry"]
 

src/client/mod.rs

@@ -4,5 +4,8 @@
 #[cfg(feature = "client-legacy")]
 pub mod legacy;
 
+#[cfg(feature = "client-pool")]
+pub mod pool;
+
 #[cfg(feature = "client-proxy")]
 pub mod proxy;

src/client/pool/mod.rs

@@ -0,0 +1,3 @@
+//! Composable pool services
+
+pub mod singleton;