balance: Add a p2c Pool implementation (#2541)

Following #2540, which introduces a new PoolQueue and Pool interface,
this change introduces a P2cPool implementation that replaces
Tower's p2c balancer (using the same underlying ReadyCache and p2c
implementations).

This balancer implementation is currently unused. It will be integrated
in a follow-up change.

Cargo.lock

@@ -1677,20 +1677,27 @@ dependencies = [
 name = "linkerd-proxy-balance"
 version = "0.1.0"
 dependencies = [
+ "ahash",
  "futures",
  "futures-util",
  "indexmap",
  "linkerd-error",
  "linkerd-metrics",
  "linkerd-proxy-core",
+ "linkerd-proxy-pool",
  "linkerd-stack",
+ "linkerd-tracing",
+ "parking_lot",
  "pin-project",
+ "quickcheck",
  "rand",
  "thiserror",
  "tokio",
  "tokio-stream",
+ "tokio-test",
  "tokio-util",
  "tower",
+ "tower-test",
  "tracing",
 ]
 

linkerd/proxy/balance/Cargo.toml

@@ -6,13 +6,16 @@ license = "Apache-2.0"
 publish = false
 
 [dependencies]
+ahash = "0.8"
 futures = { version = "0.3", default-features = false }
 futures-util = "0.3"
 indexmap = "1"
 linkerd-error = { path = "../../error" }
 linkerd-metrics = { path = "../../metrics" }
 linkerd-proxy-core = { path = "../core" }
+linkerd-proxy-pool = { path = "../pool" }
 linkerd-stack = { path = "../../stack" }
+parking_lot = "0.12"
 pin-project = "1"
 rand = "0.8"
 thiserror = "1"
@@ -24,4 +27,10 @@ tracing = "0.1"
 [dependencies.tower]
 version = "0.4.13"
 default-features = false
-features = ["balance", "discover", "load"]
+features = ["balance", "load", "ready-cache"]
+
+[dev-dependencies]
+linkerd-tracing = { path = "../../tracing" }
+tokio-test = "0.4"
+tower-test = "0.4"
+quickcheck = { version = "1", default-features = false }

linkerd/proxy/balance/src/lib.rs

@@ -11,10 +11,12 @@ use tower::{
 
 mod discover;
 mod gauge_endpoints;
+mod pool;
 
 pub use self::{
     discover::DiscoveryStreamOverflow,
     gauge_endpoints::{EndpointsGauges, NewGaugeEndpoints},
+    pool::P2cPool,
 };
 pub use tower::load::peak_ewma::Handle;
 

linkerd/proxy/balance/src/pool.rs

@@ -0,0 +1,4 @@
+mod p2c;
+
+pub use self::p2c::P2cPool;
+pub use linkerd_proxy_pool::{Pool, Update};